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

[John Larkin]

On Tue, 16 Jul 2019 11:40:48 -0700, Joerg <news@analogconsultants.com>
wrote:

On 2019-07-16 11:22, John Larkin wrote:
On Tue, 16 Jul 2019 10:28:05 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-16 08:37, John Larkin wrote:
On Tue, 16 Jul 2019 07:25:23 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-16 06:56, John Larkin wrote:
On Tue, 16 Jul 2019 06:53:17 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

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.



Here's the street. That's 22 megawatts right above those houses.

https://www.dropbox.com/s/4fzsmipz0rar4ke/Panoramic_Drive.jpg?raw=1

I bet they could have wireless LED night lights.


And each would probably cost $2M+ on the market, leaky plumbing or not.
No 10 horses would drag me to live in S.F.

I love this place. Most visitors see the parts of the city that we
never go to.


S.F. has deteriorated a lot. Sometimes you have to step over people
sleeping on the sidewalk, occasionally over some barf, et cetera.

That's downtown. We don't go downtown.


It'll spread fast. When I visited you guys on Otis Street and we walked
to Zeitgeist there were several folks sleeping on the sidewalks.

That was just the edge of downtown. Our new business place is much
better. Where we live is a nice clean little village with a canyon
full of critters.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Lasse Langwadt Christense]

tirsdag den 16. juli 2019 kl. 23.30.18 UTC+2 skrev Jeff Liebermann:

On Tue, 16 Jul 2019 13:27:09 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

https://youtu.be/lMuJKsUjD_o

That's the base insulator from what appears to be an AM broadcast
station. The tower is the transmit antenna and is NOT grounded.
Judging by the size of the sparks, my guess(tm) is about 20,000 volts
across the base insulator (based on 30kV/cm). The weeds are full of
water and make a fairly good conductor. Usually, cleaning the weeds
and debris from around the tower is done with the transmitter turned
off, or at a greatly reduced power level. The "burns" on his hands
were caused either by the heating of the water in the weeds burning
his hands or by the RF arcing through the gloves, causing local spot
heating in the gloves.

you are probably not supposed to be anywhere near it at full power

here's another one

https://youtu.be/Mh8-idXgbUY

 

[Clifford Heath]

On 17/7/19 3:28 am, Joerg wrote:

AM in the US is usually vertically polarized because they use a very
tall tower fed at the bottom. In other countries that sort of expense
isn't always palatable so they sometimes use dipoles strung between a
couple of high points and then it's horizontal. Horizontal has the
downside of nulls in two directions, making the range non-circular and
that's bad for listener numbers and thus advertizing revenue.

A pair of dipoles fed in quadrature gives a turnstile antenna. A
reasonable solution if nulls are a problem.

 

[Clifford Heath]

On 17/7/19 12:29 am, Joerg wrote:

On 2019-07-15 20:06, Clifford Heath wrote:
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".

IME such small bypass cap values make no sense at all.

I probably mis-remembered the actual values. But good points about
placing the vias for them.

Definitely not in

the days of MLCC. 0.01uF to 0.1uf in 0603 is fine. It's the layout that
counts, plus the proper use of large planes as "free" bypass capacitors.
This is part of my daily bread, helping engineers route and place vias
properly. "But that's less than 1/10th of an inch!" ... "Yes, but it
does matter here".

 

[Jeff Liebermann]

On Tue, 16 Jul 2019 15:09:35 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

tirsdag den 16. juli 2019 kl. 23.30.18 UTC+2 skrev Jeff Liebermann:
On Tue, 16 Jul 2019 13:27:09 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

https://youtu.be/lMuJKsUjD_o

That's the base insulator from what appears to be an AM broadcast
station. The tower is the transmit antenna and is NOT grounded.
Judging by the size of the sparks, my guess(tm) is about 20,000 volts
across the base insulator (based on 30kV/cm). The weeds are full of
water and make a fairly good conductor. Usually, cleaning the weeds
and debris from around the tower is done with the transmitter turned
off, or at a greatly reduced power level. The "burns" on his hands
were caused either by the heating of the water in the weeds burning
his hands or by the RF arcing through the gloves, causing local spot
heating in the gloves.

you are probably not supposed to be anywhere near it at full power

True. That transmitter is somewhere in the Ukraine. I'm 6,000 miles
away in California.

here's another one
https://youtu.be/Mh8-idXgbUY

Also in the Ukraine and a different problem. That's a 150 kW short
wave broadcast tower. The cut cables are the tower grounds across the
concrete tower leg supports. They're for lightning protection by
providing a bypass route around the concrete. Without such ground
wires, lightning would go through the concrete which would then
explode in a rather messy steam explosion. Concrete is about 15 - 20%
water. Such ground cables are usually made from copper and are a
prime target for copper thieves.
<https://www.google.com/search?q=copper+ground+wire+theft&tbm=isch>





--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Joerg]

On 2019-07-16 16:21, Clifford Heath wrote:

On 17/7/19 3:28 am, Joerg wrote:
AM in the US is usually vertically polarized because they use a very
tall tower fed at the bottom. In other countries that sort of expense
isn't always palatable so they sometimes use dipoles strung between a
couple of high points and then it's horizontal. Horizontal has the
downside of nulls in two directions, making the range non-circular and
that's bad for listener numbers and thus advertizing revenue.

A pair of dipoles fed in quadrature gives a turnstile antenna. A
reasonable solution if nulls are a problem.

That's what used to be on most apartment buildings in Germany for AM
reception, combined with FM/VHF/UHF, amplified and then fed into the
radio and TV lines to each apartment. I never saw it for a transmitter
though.

This has fallen by the wayside a long time ago and now I think they
discontinued AM band transmissions. Not a smart move IMHO.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Phil Hobbs]

On 7/16/19 10:21 AM, Joerg wrote:

On 2019-07-15 16:31, Phil Hobbs wrote:
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.


Theoretically yes but it cannot handle the transients at the onset. When
that stuff is on a die it can take uncontrolled paths, and from what I
experienced seems to do so. Large output amplitudes.

Not trying to be difficult, just to understand. (I have profound
respect for your RF chops, as you know.) Do you have any data about
"cannot handle the transients at the onset?" ISTM that it's mostly a
well-grounded impression that RFI crap can show up anywhere at seriously
inconvenient amplitudes if one is't knowledgeable as well as careful.
Amirite?

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

 

[Jan Panteltje]

On a sunny day (Tue, 16 Jul 2019 13:27:09 -0700 (PDT)) it happened Lasse
Langwadt Christensen <langwadt@fonz.dk> wrote in
<138c5184-e31b-4805-8f79-e6126b38a5c4@googlegroups.com>:

tirsdag den 16. juli 2019 kl. 22.01.45 UTC+2 skrev Phil Hobbs:
On 7/16/19 11:26 AM, John Larkin wrote:
On Tue, 16 Jul 2019 14:18:41 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Tue, 16 Jul 2019 06:56:47 -0700) it happened John Larkin
jjlarkin@highlandtechnology.com> wrote in
eqlrielfeeeovifsku3olimru2635svnld@4ax.com>:

On Tue, 16 Jul 2019 06:53:17 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

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.



Here's the street. That's 22 megawatts right above those houses.

https://www.dropbox.com/s/4fzsmipz0rar4ke/Panoramic_Drive.jpg?raw=1

Mega Watt? not kW?

Megawatts.

What frequency is that?

Everything. On a spectrum analyzer, the AM, FM, and TV bands are about
full. And the tower is used for police and fire comms too. It
overlooks the entire city, and then some.


Just one nickel-plated connector could cause some havoc.


https://youtu.be/lMuJKsUjD_o

Would not like to stand there.
OTOH his camera has no problems it seems,
is just RF HV being rectified, AM station?

 

[Michael Terrell]

On Tuesday, July 16, 2019 at 1:28:07 PM UTC-4, Joerg wrote:

FM is also vertical but that gets bent along the path, especially when
looking at car antennas.

You know as little about broadcasting as you do about CATV.

FM is circular polarized, not vertical. Vertical would favor car radios and portables with a whip. Home receivers with an outdoor antenna are Horizontal, which was originally used for FM broadcasting. They switched to circular to serve both markets.

 

On Wednesday, 17 July 2019 08:51:09 UTC+1, Michael Terrell wrote:

On Tuesday, July 16, 2019 at 1:28:07 PM UTC-4, Joerg wrote:

FM is also vertical but that gets bent along the path, especially when
looking at car antennas.

You know as little about broadcasting as you do about CATV.

FM is circular polarized, not vertical. Vertical would favor car radios and portables with a whip. Home receivers with an outdoor antenna are Horizontal, which was originally used for FM broadcasting. They switched to circular to serve both markets.

Here for main TV transmitters, yagis are placed horizontal. To get the smaller repeater sites used to fill in the gaps, end user's yagi needs to have its elements vertical.


NT

 

[Winfield Hill]

Joerg wrote...

You could have said that more politely. ...

Polite haa gone out of style here in the U.S.


--
Thanks,
- Win

 

[Joerg]

On 2019-07-17 00:51, Michael Terrell wrote:

On Tuesday, July 16, 2019 at 1:28:07 PM UTC-4, Joerg wrote:

FM is also vertical but that gets bent along the path, especially
when looking at car antennas.

You know as little about broadcasting as you do about CATV.

You could have said that more politely. I do (or did) know CATV quite
well. FM and stuff is back from Europe.

FM is circular polarized, not vertical. Vertical would favor car
radios and portables with a whip. Home receivers with an outdoor
antenna are Horizontal, which was originally used for FM
broadcasting. They switched to circular to serve both markets.

You are correct. I mixed that up with AM. FM is or at least was
horizontal, that's what the turnstyles were for and AM used the vertical
whip.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

On 2019-07-16 20:03, Phil Hobbs wrote:

On 7/16/19 10:21 AM, Joerg wrote:
On 2019-07-15 16:31, Phil Hobbs wrote:
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.


Theoretically yes but it cannot handle the transients at the onset.
When that stuff is on a die it can take uncontrolled paths, and from
what I experienced seems to do so. Large output amplitudes.


Not trying to be difficult, just to understand. (I have profound
respect for your RF chops, as you know.) Do you have any data about
"cannot handle the transients at the onset?" ISTM that it's mostly a
well-grounded impression that RFI crap can show up anywhere at seriously
inconvenient amplitudes if one is't knowledgeable as well as careful.
Amirite?

I don't know of any datasheet information that's useful. Not 100% sure
about the mechanism either, just that it's really bad with BJT opamps.
What I think happens is this:

The first stage in an opamp is a diff pair with a wimpy current source
in the common emitter leg. A few microamps in slow opamps. This does not
allow the first stage to react really fast upon a regular input signal.
At the same time this current source has capacitance. The capacitance
does not matter in normal operation.

When RF sneaks in and rectifies at the BE junctions of that first stage
it often gets into only one of the BJT, or into both and then with
opposite polarity because PCB traces form a small dipole antenna.
Rectification is instant and hard because at a GHz the tiny capacitance
of the current source does matter. IOW lots of 1st stage non-diff gain
-> wham-bam. The feedback loop can only react to that after this
unwanted pulse has reached the output which is too late to muffle the
onset of this rectified signal.

IOW, it's the usual voodoo

--
Regards, Joerg

http://www.analogconsultants.com/

 

[John Larkin]

On 17 Jul 2019 07:31:48 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Joerg wrote...

You could have said that more politely. ...

Polite haa gone out of style here in the U.S.

I find most people to be polite and cheerful in person [1]. And lately
I see a lot of very considerate driving, with people waving or
something to acknowledge courtesy. Of course, there are still a lot of
rude jerks who cheat to gain small advantages.

Without physical proximity to another's face and body, it's easy to
abstract a car or a usenet post as not representing a person.

A few of the posters here are basically 100% personal insult
generators. I wonder what they are like in real life. I'd guess that
they are very self-centered, which explains why they are bad with
objective things, like electronics.

[1] Not to change the subject, but we went to lunch on Saturday at The
Old Clam House and afterwards Mo put her backpack into the rear seat
of the Audi and slammed the door. As we were about to drive away, we
heard a tinkling sound in the back. The rear side window was slowly,
progressively shattering itself into a thousand pieces, still stuck
together. When we got home, there was a mobile car-window truck
working just across the street. The guy hand-wrote their phone number
on a dirty scrap of paper. So I called and they were super nice, and
another guy showed up at 6:30 PM with the replacement glass. He did a
wonderful job, even swept and vacuumed the sidewalk of little glass
bits. He didn't speak any English but we got along great. I gave him
$40 extra.

America is great. So way are so many people rude and angry?

Well, a few of them are Australian. Are Australians especially rude?




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Wed, 17 Jul 2019 07:06:32 -0700, Joerg <news@analogconsultants.com>
wrote:

On 2019-07-16 20:03, Phil Hobbs wrote:
On 7/16/19 10:21 AM, Joerg wrote:
On 2019-07-15 16:31, Phil Hobbs wrote:
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.


Theoretically yes but it cannot handle the transients at the onset.
When that stuff is on a die it can take uncontrolled paths, and from
what I experienced seems to do so. Large output amplitudes.


Not trying to be difficult, just to understand. (I have profound
respect for your RF chops, as you know.) Do you have any data about
"cannot handle the transients at the onset?" ISTM that it's mostly a
well-grounded impression that RFI crap can show up anywhere at seriously
inconvenient amplitudes if one is't knowledgeable as well as careful.
Amirite?


I don't know of any datasheet information that's useful. Not 100% sure
about the mechanism either, just that it's really bad with BJT opamps.
What I think happens is this:

The first stage in an opamp is a diff pair with a wimpy current source
in the common emitter leg. A few microamps in slow opamps. This does not
allow the first stage to react really fast upon a regular input signal.
At the same time this current source has capacitance. The capacitance
does not matter in normal operation.

When RF sneaks in and rectifies at the BE junctions of that first stage
it often gets into only one of the BJT, or into both and then with
opposite polarity because PCB traces form a small dipole antenna.
Rectification is instant and hard because at a GHz the tiny capacitance
of the current source does matter. IOW lots of 1st stage non-diff gain
-> wham-bam. The feedback loop can only react to that after this
unwanted pulse has reached the output which is too late to muffle the
onset of this rectified signal.

IOW, it's the usual voodoo

One way to avoid front-end RF rectification is to use a chip that just
amplifies anything that shows up. Specifically, one can sometime use
multi-GHz MMICS for AC or pulse gain.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Tue, 16 Jul 2019 14:09:39 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Tue, 16 Jul 2019 06:56:47 -0700, John Larkin
jjlarkin@highlandtechnology.com> wrote:

Here's the street. That's 22 megawatts right above those houses.
https://www.dropbox.com/s/4fzsmipz0rar4ke/Panoramic_Drive.jpg?raw=1

Nope. The 22 MegaWatts might be the EIRP (Equivalent Isotropically
Radiated Power), which is the transmitter RF output power, minus any
coax cable or waveguide loss, times the antenna gain.

I read the 22MW number in some article about the tower. It may be ERP
in some plane.

In my office, a pair of 3' banana leads from my Fluke to a schottky
diode makes about 80 mV. Some may be 60 Hz, but a lot seems to be
rectified RF.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Jan Panteltje]

On a sunny day (Wed, 17 Jul 2019 10:10:54 -0700) it happened John Larkin
<jjlarkin@highland_snip_technology.com> wrote in
<b8luiehsuf58kdh26s7jh6vqn6s5ia5m8t@4ax.com>:

On Tue, 16 Jul 2019 14:09:39 -0700, Jeff Liebermann <jeffl@cruzio.com
wrote:

On Tue, 16 Jul 2019 06:56:47 -0700, John Larkin
jjlarkin@highlandtechnology.com> wrote:

Here's the street. That's 22 megawatts right above those houses.
https://www.dropbox.com/s/4fzsmipz0rar4ke/Panoramic_Drive.jpg?raw=1

Nope. The 22 MegaWatts might be the EIRP (Equivalent Isotropically
Radiated Power), which is the transmitter RF output power, minus any
coax cable or waveguide loss, times the antenna gain.

I read the 22MW number in some article about the tower. It may be ERP
in some plane.

In my office, a pair of 3' banana leads from my Fluke to a schottky
diode makes about 80 mV. Some may be 60 Hz, but a lot seems to be
rectified RF.

80 mV is enough (if some current) to start a JFET oscillator with step up transformer
to light a LED,
http://panteltje.com/pub/lighting_a_LED_with_a_candle_IMG_3604.GIF
here from a thermocouple heated with a lighter
https://www.youtube.com/watch?v=hjsgwQs0LTw

Depends bit on the Zi of your ciruit.

 

[Joerg]

On 2019-07-17 07:56, John Larkin wrote:

On Wed, 17 Jul 2019 07:06:32 -0700, Joerg <news@analogconsultants.com
wrote:

On 2019-07-16 20:03, Phil Hobbs wrote:
On 7/16/19 10:21 AM, Joerg wrote:
On 2019-07-15 16:31, Phil Hobbs wrote:
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.


Theoretically yes but it cannot handle the transients at the onset.
When that stuff is on a die it can take uncontrolled paths, and from
what I experienced seems to do so. Large output amplitudes.


Not trying to be difficult, just to understand. (I have profound
respect for your RF chops, as you know.) Do you have any data about
"cannot handle the transients at the onset?" ISTM that it's mostly a
well-grounded impression that RFI crap can show up anywhere at seriously
inconvenient amplitudes if one is't knowledgeable as well as careful.
Amirite?


I don't know of any datasheet information that's useful. Not 100% sure
about the mechanism either, just that it's really bad with BJT opamps.
What I think happens is this:

The first stage in an opamp is a diff pair with a wimpy current source
in the common emitter leg. A few microamps in slow opamps. This does not
allow the first stage to react really fast upon a regular input signal.
At the same time this current source has capacitance. The capacitance
does not matter in normal operation.

When RF sneaks in and rectifies at the BE junctions of that first stage
it often gets into only one of the BJT, or into both and then with
opposite polarity because PCB traces form a small dipole antenna.
Rectification is instant and hard because at a GHz the tiny capacitance
of the current source does matter. IOW lots of 1st stage non-diff gain
-> wham-bam. The feedback loop can only react to that after this
unwanted pulse has reached the output which is too late to muffle the
onset of this rectified signal.

IOW, it's the usual voodoo

One way to avoid front-end RF rectification is to use a chip that just
amplifies anything that shows up. Specifically, one can sometime use
multi-GHz MMICS for AC or pulse gain.

That wouldn't be very palatable in most of my situations. Mostly the
replacement part can't cost more than 10c and it can't draw more than
half a milliamp.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Michael Terrell]

On Wednesday, July 17, 2019 at 10:26:33 AM UTC-4, Joerg wrote:

On 2019-07-17 00:51, Michael Terrell wrote:
On Tuesday, July 16, 2019 at 1:28:07 PM UTC-4, Joerg wrote:

FM is also vertical but that gets bent along the path, especially
when looking at car antennas.

You know as little about broadcasting as you do about CATV.


You could have said that more politely. I do (or did) know CATV quite
well. FM and stuff is back from Europe.


FM is circular polarized, not vertical. Vertical would favor car
radios and portables with a whip. Home receivers with an outdoor
antenna are Horizontal, which was originally used for FM
broadcasting. They switched to circular to serve both markets.


You are correct. I mixed that up with AM. FM is or at least was
horizontal, that's what the turnstyles were for and AM used the vertical
whip.

You weren't the least bit polite when you claimed that FIOS was CATV. You've also claimed that you don't read anything posted from Google Groups.

 

[Joerg]

On 2019-07-17 11:32, Michael Terrell wrote:

On Wednesday, July 17, 2019 at 10:26:33 AM UTC-4, Joerg wrote:
On 2019-07-17 00:51, Michael Terrell wrote:
On Tuesday, July 16, 2019 at 1:28:07 PM UTC-4, Joerg wrote:

FM is also vertical but that gets bent along the path,
especially when looking at car antennas.

You know as little about broadcasting as you do about CATV.


You could have said that more politely. I do (or did) know CATV
quite well. FM and stuff is back from Europe.


FM is circular polarized, not vertical. Vertical would favor car
radios and portables with a whip. Home receivers with an outdoor
antenna are Horizontal, which was originally used for FM
broadcasting. They switched to circular to serve both markets.


You are correct. I mixed that up with AM. FM is or at least was
horizontal, that's what the turnstyles were for and AM used the
vertical whip.


You weren't the least bit polite when you claimed that FIOS was CATV.
You've also claimed that you don't read anything posted from Google
Groups.

That was years ago when Google spammed groups was and thus necessary.
What's that got to do with politeness?

--
Regards, Joerg

http://www.analogconsultants.com/