AoE x-Chapters, High-Speed op-amps section, DRAFT

[Phil Hobbs]

On 7/13/19 5:02 PM, Winfield Hill wrote:

Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.

OK, dunno about the 100dB number, but preamps that respond
to radio signals are legend. My most recent experience, a
modest-gain preamp for electret microphones with relatively
high signal levels. Worked OK in my lab, but out on the
Institute's deck, where the bee hives were, picked up AM
radio stations, especially a Latin music station. Excuse
me, we were trying to listen to the bees! My first step,
change op-amp to a CMOS type. Arrggh, it was worse! Next
step, solder a cap right onto the 0.1-inch mic terminals.

Absolutely. I have ultralow noise laser drivers that won't work right
unless they're all buttoned up, and that's in the 150-mA drive current
class. So it isn't just input stages that are vulnerable.

It's just that ~80dB issue that I'm querying. (We're all in this
together, after all, so clarity is important.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Winfield Hill]

Phil Hobbs wrote...

I don't know, but perhaps the huge disparity
of transconductance between BJTs and FETs has
something to do with it.

There may be some generalizations that can be
made, but there's a huge variation to be found
from one op-amp to another.


--
Thanks,
- Win

 

[Jan Panteltje]

On a sunny day (13 Jul 2019 14:02:59 -0700) it happened Winfield Hill
<winfieldhill@yahoo.com> wrote in <qgdgu3025v@drn.newsguy.com>:

Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.

OK, dunno about the 100dB number, but preamps that respond
to radio signals are legend. My most recent experience, a
modest-gain preamp for electret microphones with relatively
high signal levels. Worked OK in my lab, but out on the
Institute's deck, where the bee hives were, picked up AM
radio stations, especially a Latin music station. Excuse
me, we were trying to listen to the bees! My first step,
change op-amp to a CMOS type. Arrggh, it was worse! Next
step, solder a cap right onto the 0.1-inch mic terminals.

I designed and build an audio mixer, used a very small inductor and cap at the inputs.
ferrite bead might work too (have not tried that) against cellphones,
there were none around back then.
But we got audio from the electric tram radios that drove past the hall.

Filter any line level output too, to prevent any RF coming into the box.
Metal box is a must.

 

[Joerg]

On 2019-07-13 14:02, Winfield Hill wrote:

Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.

That's exactly how it is. The noise attacks inside the chip at the first
BJT pair so the closed loop gain no longer applies, it's at full open
loop gain.

OK, dunno about the 100dB number, but preamps that respond
to radio signals are legend. My most recent experience, a
modest-gain preamp for electret microphones with relatively
high signal levels. Worked OK in my lab, but out on the
Institute's deck, where the bee hives were, picked up AM
radio stations, especially a Latin music station. Excuse
me, we were trying to listen to the bees! My first step,
change op-amp to a CMOS type. Arrggh, it was worse!

That is strange unless it had some sort of input protection that was
really close to the Vf region of its diodes, such as back to back diodes
between IN+ and IN=. Or if it was operated with an input very close to
ground. However, AM stations are in the 1MHz range which is within the
bandwidth of most opamps. Cell phones are >1GHz so the only way for that
to get in is by rectification right at the beginning, not somewhere later.

The other thing to check for is whether the input stage was truly CMOS.

... Next
step, solder a cap right onto the 0.1-inch mic terminals.

Yes, module level measures should always be first. It's just when they
don't lead to success or seem too onerous to the client that I start
changing components. Swapping in a CMOS opamp is usually an eye-popper
to their engineers because it often completely eliminates the EMI. To my
surprise none of the younger engineers ever knew this, or why. Often
after explaining it to them they said "Why did our professors never talk
about that?". That is a question I always had.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[John Larkin]

On Sun, 14 Jul 2019 06:59:09 -0700, Joerg <news@analogconsultants.com>
wrote:

On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the first
BJT pair so the closed loop gain no longer applies, it's at full open
loop gain.

Rectified RF is no different from input DC offset. Both happen at the
internal b-e junctions of the diff pair. But a unity-gain follower has
Voffset at its output, not open-loop gain times Voffset.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Phil Hobbs]

On 7/14/19 9:59 AM, Joerg wrote:

On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid.  What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood.  But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead.  That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the first
BJT pair so the closed loop gain no longer applies, it's at full open
loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 7/13/19 8:02 PM, Winfield Hill wrote:

Phil Hobbs wrote...

I don't know, but perhaps the huge disparity
of transconductance between BJTs and FETs has
something to do with it.

There may be some generalizations that can be
made, but there's a huge variation to be found
from one op-amp to another.

Well, a CFA with a FET front end would be much slower, other things
being equal, because it would have only a few percent of the
transconductance of a BJT.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com>
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.



RF either comes in directly

through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.

If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Joerg]

On 2019-07-14 09:56, John Larkin wrote:

On Sun, 14 Jul 2019 06:59:09 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the first
BJT pair so the closed loop gain no longer applies, it's at full open
loop gain.


Rectified RF is no different from input DC offset. Both happen at the
internal b-e junctions of the diff pair. But a unity-gain follower has
Voffset at its output, not open-loop gain times Voffset.

It usually gets is after the feedback, see answer to Phil. The other
problem is that the feedback is slow. DC and stuff is being regulated
out but not the steep onset transients of a pulsating RF source. In an
audio path it sonds lke "POCK-POCK-POCK".

The fix is easy but the downside is that CMOS opamps have much worse
input voltage noise and also often cost more.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

On 2019-07-14 10:32, Phil Hobbs wrote:

On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.

The RF is added after the feedback divider. RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

On 2019-07-15 07:10, John Larkin wrote:

On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.

Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.

RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.

Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Phil Hobbs]

On 7/15/19 11:03 AM, Joerg wrote:

On 2019-07-15 07:10, John Larkin wrote:
On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid.  What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood.  But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead.  That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply?  How exactly does that work?  It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.


Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.



  RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.


Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".

Okay, so you're talking about rectified components near or beyond the
loop bandwidth. I agree with that--there's not so much difference
between A_VOL and A_VCL up there. But that's because A_VOL isn't 100 dB
up there either.

Down at the 220 Hz GSM data rate, your average op amp has plenty of
speed to respond to rectified RF in the approved closed-loop manner.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Mon, 15 Jul 2019 08:03:45 -0700, Joerg <news@analogconsultants.com>
wrote:

On 2019-07-15 07:10, John Larkin wrote:
On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.


Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.



RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.


Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".

When I used to work on Judah Street, I drove to work over a road on
Twin Peaks, just under the Sutro tower. My old ratty Fiesta had a
cheap add-on radio. I could hear buzzing from the speakers with the
radio turned off.

People live up there.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Tom Gardner]

On 16/07/19 00:41, John Larkin wrote:

On Mon, 15 Jul 2019 08:03:45 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-15 07:10, John Larkin wrote:
On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.


Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.



RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.


Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".


When I used to work on Judah Street, I drove to work over a road on
Twin Peaks, just under the Sutro tower. My old ratty Fiesta had a
cheap add-on radio. I could hear buzzing from the speakers with the
radio turned off.

People live up there.

I was first "hit" almost literally by GSM RFI in ~1994.
Someone had a newfangled digital phone, and the ba-da-da
ba-da-da ba-da-da bzzzz was *very* loud in my deafaid.

I almost dropped my coffee cup, and if I had been holding
an infant, I could have dropped that

I did a little research and measurement, and found that
DCS1800 was much worse than GSM900, and that the audio
power dropped off as a function of distance somewhere
between r^4 and r^8, and was proportional to f^2(?)
up to at least ~3GHz.

That put me in an awkward position at work; I was lucky
that the company was a responsible company in many ways.

 

[John Larkin]

On Tue, 16 Jul 2019 01:25:04 +0100, Tom Gardner
<spamjunk@blueyonder.co.uk> wrote:

On 16/07/19 00:41, John Larkin wrote:
On Mon, 15 Jul 2019 08:03:45 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-15 07:10, John Larkin wrote:
On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.


Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.



RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.


Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".


When I used to work on Judah Street, I drove to work over a road on
Twin Peaks, just under the Sutro tower. My old ratty Fiesta had a
cheap add-on radio. I could hear buzzing from the speakers with the
radio turned off.

People live up there.

I was first "hit" almost literally by GSM RFI in ~1994.
Someone had a newfangled digital phone, and the ba-da-da
ba-da-da ba-da-da bzzzz was *very* loud in my deafaid.

I almost dropped my coffee cup, and if I had been holding
an infant, I could have dropped that

I did a little research and measurement, and found that
DCS1800 was much worse than GSM900, and that the audio
power dropped off as a function of distance somewhere
between r^4 and r^8, and was proportional to f^2(?)
up to at least ~3GHz.

That put me in an awkward position at work; I was lucky
that the company was a responsible company in many ways.

My experience with opamps rectifying RF on PC boards is that there are
usually several narrow resonances where it's very sensitive, in the
low 100s of MHz.

One of my (ex) competitors NMR temperature controllers, with a
thermocouple sensor, could be hard shut down with a GR signal
generator, from clear across the room.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

On Mon, 15 Jul 2019 19:21:48 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

On Tue, 16 Jul 2019 01:25:04 +0100, Tom Gardner
spamjunk@blueyonder.co.uk> wrote:

On 16/07/19 00:41, John Larkin wrote:
On Mon, 15 Jul 2019 08:03:45 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-15 07:10, John Larkin wrote:
On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.


Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.



RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.


Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".


When I used to work on Judah Street, I drove to work over a road on
Twin Peaks, just under the Sutro tower. My old ratty Fiesta had a
cheap add-on radio. I could hear buzzing from the speakers with the
radio turned off.

People live up there.

I was first "hit" almost literally by GSM RFI in ~1994.
Someone had a newfangled digital phone, and the ba-da-da
ba-da-da ba-da-da bzzzz was *very* loud in my deafaid.

I almost dropped my coffee cup, and if I had been holding
an infant, I could have dropped that

I did a little research and measurement, and found that
DCS1800 was much worse than GSM900, and that the audio
power dropped off as a function of distance somewhere
between r^4 and r^8, and was proportional to f^2(?)
up to at least ~3GHz.

That put me in an awkward position at work; I was lucky
that the company was a responsible company in many ways.


My experience with opamps rectifying RF on PC boards is that there are
usually several narrow resonances where it's very sensitive, in the
low 100s of MHz.

One of my (ex) competitors NMR temperature controllers, with a
thermocouple sensor, could be hard shut down with a GR signal
generator, from clear across the room.

In a previous job, we had a lot of interference from a 2.4GHz ISM band
frequency hopping transmitter with a 5ms TDMA slot. We were shipping
for about a year without any problems, then it hit the fan. Turns out
that the opamp vendor changed the opamp process and increased the
transistor Ft. We spent the better part of a year trying to get rid
of the 200Hz buzz reliably.

 

[Clifford Heath]

On 16/7/19 12:21 pm, John Larkin wrote:

My experience with opamps rectifying RF on PC boards is that there are
usually several narrow resonances where it's very sensitive, in the
low 100s of MHz.

One of my (ex) competitors NMR temperature controllers, with a
thermocouple sensor, could be hard shut down with a GR signal
generator, from clear across the room.

An RF design friend of mine has had issues with parallel capacitors
(like 0.1uF/100pF) adjacent on a supply line. He's had quite high-Q
resonance between the 100pF and the parasitic inductance at between 400
and 900MHz. Traps, and he's not a "young player".

 

[Jan Panteltje]

On a sunny day (Mon, 15 Jul 2019 16:41:44 -0700) it happened John Larkin
<jjlarkin@highland_snip_technology.com> wrote in
<0e3qie9ivv3io0dcsf0uhihf61m4lbo028@4ax.com>:

When I used to work on Judah Street, I drove to work over a road on
Twin Peaks, just under the Sutro tower. My old ratty Fiesta had a
cheap add-on radio. I could hear buzzing from the speakers with the
radio turned off.

People live up there.

It is common, my Sony alarm clock FM radio also makes noise when my cellphone interacts with the tower,
even with that radio off, so do my PC speakers.

 

[Tom Gardner]

On 16/07/19 03:21, John Larkin wrote:

On Tue, 16 Jul 2019 01:25:04 +0100, Tom Gardner
spamjunk@blueyonder.co.uk> wrote:

On 16/07/19 00:41, John Larkin wrote:
On Mon, 15 Jul 2019 08:03:45 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-15 07:10, John Larkin wrote:
On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid. What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood. But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead. That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply? How exactly does that work? It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.


Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.



RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.


Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".


When I used to work on Judah Street, I drove to work over a road on
Twin Peaks, just under the Sutro tower. My old ratty Fiesta had a
cheap add-on radio. I could hear buzzing from the speakers with the
radio turned off.

People live up there.

I was first "hit" almost literally by GSM RFI in ~1994.
Someone had a newfangled digital phone, and the ba-da-da
ba-da-da ba-da-da bzzzz was *very* loud in my deafaid.

I almost dropped my coffee cup, and if I had been holding
an infant, I could have dropped that

I did a little research and measurement, and found that
DCS1800 was much worse than GSM900, and that the audio
power dropped off as a function of distance somewhere
between r^4 and r^8, and was proportional to f^2(?)
up to at least ~3GHz.

That put me in an awkward position at work; I was lucky
that the company was a responsible company in many ways.


My experience with opamps rectifying RF on PC boards is that there are
usually several narrow resonances where it's very sensitive, in the
low 100s of MHz.

I didn't spot any and doubt there were any, but
I wasn't looking for them. Other effects were
of more interest.

With deafaids
- there are minimal components so fewer
opportunities for resonance and
- the circuits are much smaller than typical PCBs,
so antennas are "short" and not operating
efficiently near a resonance.

People who have done EMI/EMC studies are confused
that the coupled RF power is theoretically proportional
to f^2. Their assumption is that the power falls with
increasing frequency. The difference is that they
are concerned with worst-case susceptibility with
the most efficient possible RF coupling. That
isn't relevant in this case.

Looking back at my notes...

Even in an anechoic chamber physical movements
caused 5dB changes in coupled RF power (corresponds
to 10dB audio)

The coupled RF power at 3GHz was 7dB more than
at 1GHz, corresponding to a f^1.2 dependency,
not the theoretical f^2 dependency. That was
supported by other ETSI studies.

The audio power is proportional to the square
of the RF power.

Overall the audio power was proportional to
- tx power ^2
- f ^2.4 (ish)
- 1/distance^4 to 1/distance^8

 

[Tom Gardner]

On 16/07/19 09:19, Tom Gardner wrote:

On 16/07/19 03:21, John Larkin wrote:
On Tue, 16 Jul 2019 01:25:04 +0100, Tom Gardner
spamjunk@blueyonder.co.uk> wrote:

On 16/07/19 00:41, John Larkin wrote:
On Mon, 15 Jul 2019 08:03:45 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-15 07:10, John Larkin wrote:
On Mon, 15 Jul 2019 06:58:58 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-14 10:32, Phil Hobbs wrote:
On 7/14/19 9:59 AM, Joerg wrote:
On 2019-07-13 14:02, Winfield Hill wrote:
Phil Hobbs wrote...

On 7/13/19 4:26 PM, Joerg wrote:
On 2019-07-13 13:10, Phil Hobbs wrote:

So colour me stupid.  What's so different about offsets
due to rectification vs. intrinsic imbalances, such that
they get multiplied by A_VOL and not A_VCL like all the
others?

Imbalances are static, they do not show up as spectrally
significant noise. Rectified RF is not constant and that's
the problem. GSM phones ...

Understood.  But unless I misunderstand, you're claiming
that an op amp with A_VCL = 20 dB (say) multiplies the
rectified RF by A_VOL (100 dB) instead.  That's what I'm
wondering about.


That's exactly how it is. The noise attacks inside the chip at the
first BJT pair so the closed loop gain no longer applies, it's at full
open loop gain.

So feedback magically doesn't apply?  How exactly does that work?  It
works on every other source of offsets.


The RF is added after the feedback divider.

So is DC offset.


Yes, but it does not pulsate. The transitions are the problem. If it
were a constant RF carrier there would be not a problem other than maybe
an elevated noise level.



    RF either comes in directly
through a plastic enclosure or via power cables and the like and then
re-radiated inside the box. The traces to IN+ and IN- form an unwanted
dipole antenna, the RF couples onto those and then affects both BJTs in
the first pair. It's like a comparator, the feedback loop is nearly
powerless. While it does regulate out slower effects it cannot handle
the sharp onset and drop-off of, for example, a GSM phone that seeks to
establish communication with a cell tower. The pulses is what gets
through and annoys, not DC.


If the forward gain of the opamp is slow, it's slow for both the
RF-induced offset and for the feedback.

We could Spice that, but we'd have to decide where inside the loop to
inject the RF offset.


Immediately at (inside) the BE junctions of the first pair. It's hard to
do because RF also attacks other BE junctions in there.

The problem is that such signals develop inside the opamp and the
feedback loop only notices it after the fact. "Oh s..t! Where did that
come from? Let's fight it!".


When I used to work on Judah Street, I drove to work over a road on
Twin Peaks, just under the Sutro tower. My old ratty Fiesta had a
cheap add-on radio. I could hear buzzing from the speakers with the
radio turned off.

People live up there.

I was first "hit" almost literally by GSM RFI in ~1994.
Someone had a newfangled digital phone, and the ba-da-da
ba-da-da ba-da-da bzzzz was *very* loud in my deafaid.

I almost dropped my coffee cup, and if I had been holding
an infant, I could have dropped that

I did a little research and measurement, and found that
DCS1800 was much worse than GSM900, and that the audio
power dropped off as a function of distance somewhere
between r^4 and r^8, and was proportional to f^2(?)
up to at least ~3GHz.

That put me in an awkward position at work; I was lucky
that the company was a responsible company in many ways.


My experience with opamps rectifying RF on PC boards is that there are
usually several narrow resonances where it's very sensitive, in the
low 100s of MHz.

I didn't spot any and doubt there were any, but
I wasn't looking for them. Other effects were
of more interest.

With deafaids
 - there are minimal components so fewer
   opportunities for resonance and
 - the circuits are much smaller than typical PCBs,
   so antennas are "short" and not operating
   efficiently near a resonance.

People who have done EMI/EMC studies are confused
that the coupled RF power is theoretically proportional
to f^2. Their assumption is that the power falls with
increasing frequency. The difference is that they
are concerned with worst-case susceptibility with
the most efficient possible RF coupling. That
isn't relevant in this case.

Looking back at my notes...

Even in an anechoic chamber physical movements
caused 5dB changes in coupled RF power (corresponds
to 10dB audio)

The coupled RF power at 3GHz was 7dB more than
at 1GHz, corresponding to a f^1.2 dependency,
not the theoretical f^2 dependency. That was
supported by other ETSI studies.

The audio power is proportional to the square
of the RF power.

Overall the audio power was proportional to
 - tx power ^2
 - f ^2.4 (ish)
 - 1/distance^4 to 1/distance^8

I've just noticed I have another easily
accessible result...

For one type of deafaid, the audio power fell
off rapidly above 2.2GHz, for others it was
above 3.5GHz and not so rapid.

The cause was not investigated, but obvious
guesses could be made.