RF Automatic Level Control

[Cursitor Doom]

Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4




--

"The BEST Deal is NO DEAL"

 

[Michael Terrell]

On Monday, January 6, 2020 at 1:05:46 PM UTC-5, Cursitor Doom wrote:

Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

What is the HP model number? All that link gave me was a snippet of the schematic that was too small to read.

 

[Jeff Liebermann]

On Mon, 06 Jan 2020 18:05:39 +0000, Cursitor Doom <cd@notformail.com>
wrote:

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

1. Any good reason for not bothering to disclose the model number of
the HP signal generator? It just might be interesting to lookup the
specs for the generator.

2. How did you determine that the RF signal level varies across the
frequency range? Translation: What instrument do you have that can
accurately measure to within +/-1dBm from 0 to 1300MHz?

3. If you want it stable and/or accurate over an unspecified
temperature range, are you prepared to test it in an environmental
oven?

4. If the output leveling is really that bad, perhaps something is
fried in either the amplifier/detector section, or in whatever you're
using for an output attenuator. I can just barely see the output
range at J1 as 0 to 1300MHz and 0 to +10dBm. That's a fairly useless
output range, so you'll certainly need an output attenuator. My
favorite mistake is transmitting into the signal generator. Please
don't ask how often I've done this or how much it has cost.

5. The amplifier/detector looks very much like the HP custom EECL (1)
hybrid used in the HP8640 generator and other generators. At least in
the HP8640, it likes to blow up with or without my assistance. The
symptom is very low output level, failure of the PLL to lock on a
frequency, or both. If this is the case, I've built my own
replacement hybrid using an MCL MAV-11:
<https://www.ve7ca.net/TstH86.htm#HK5>
While this is not a drop in replacement, others may have done
something similar with your unspecified model HP generator.

6. Good luck.


Note 1: Yes, it really is EECL, not ECL.
<https://www.ve7ca.net/TST/H86/n2gx.pdf>

--
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

 

[John Larkin]

On Mon, 06 Jan 2020 18:05:39 +0000, Cursitor Doom <cd@notformail.com>
wrote:

Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

That Yandex site is too picky about showing the image.

Dropbox is free.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Phil Hobbs]

On 2020-01-06 13:05, Cursitor Doom wrote:

Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

A diode isn't a terribly dumb detector for an ALC circuit. What's wrong
with the one you have?

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

 

[Cursitor Doom]

On Mon, 6 Jan 2020 11:06:52 -0800 (PST), Michael Terrell
<terrell.michael.a@gmail.com> wrote:


>What is the HP model number? All that link gave me was a snippet of the schematic that was too small to read.

8754A. I've checked the link and the image blows up massively
maintaining its clarity so you must be doing something wrong. Try
right clicking over the image and see what options come up.
--

"The BEST Deal is NO DEAL"

 

[Cursitor Doom]

On Mon, 06 Jan 2020 11:09:35 -0800, Jeff Liebermann <jeffl@cruzio.com>
wrote:

1. Any good reason for not bothering to disclose the model number of
the HP signal generator? It just might be interesting to lookup the
specs for the generator.

8754A and it's not a sig gen; it's a network analyser.

2. How did you determine that the RF signal level varies across the
frequency range? Translation: What instrument do you have that can
accurately measure to within +/-1dBm from 0 to 1300MHz?

I have a spectrum analyser (2 actually) which both go up to 22Ghz.
However, on this occasion I had my 350Mhz Tek scope right next on the
bench so used that for the sake of convenience. It was readily
apparent the amplitude variations were probably north of 4dB, even
within the first 150Mhz.

3. If you want it stable and/or accurate over an unspecified
temperature range, are you prepared to test it in an environmental
oven?

I don't have one. I'm snipping the rest of your post as it relates to
signal generators which this isn't.

--

"The BEST Deal is NO DEAL"

 

[Cursitor Doom]

On Mon, 6 Jan 2020 16:45:26 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

A diode isn't a terribly dumb detector for an ALC circuit. What's wrong
with the one you have?

Sorry Phil, no idea what your question relates to.
--

"The BEST Deal is NO DEAL"

 

[Cursitor Doom]

On Mon, 06 Jan 2020 12:28:07 -0800, John Larkin
<jlarkin@highland_atwork_technology.com> wrote:

On Mon, 06 Jan 2020 18:05:39 +0000, Cursitor Doom <cd@notformail.com
wrote:

Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

That Yandex site is too picky about showing the image.

Dropbox is free.

AFAICS, there's nothing wrong with that image, which I've been able to
blow up massively whilst its resolution is preserved. Try a bit
harder, John. You'll get it pretty quick I'm sure, bloke of your
calibre.
--

"The BEST Deal is NO DEAL"

 

[Michael Terrell]

On Monday, January 6, 2020 at 5:13:21 PM UTC-5, Cursitor Doom wrote:

On Mon, 6 Jan 2020 11:06:52 -0800 (PST), Michael Terrell wrote:


What is the HP model number? All that link gave me was a snippet of the schematic that was too small to read.

8754A. I've checked the link and the image blows up massively
maintaining its clarity so you must be doing something wrong. Try
right clicking over the image and see what options come up.

I did, before I posted. No menu opened up.

 

[Cursitor Doom]

On Mon, 6 Jan 2020 14:24:52 -0800 (PST), Michael Terrell
<terrell.michael.a@gmail.com> wrote:

On Monday, January 6, 2020 at 5:13:21 PM UTC-5, Cursitor Doom wrote:
On Mon, 6 Jan 2020 11:06:52 -0800 (PST), Michael Terrell wrote:


What is the HP model number? All that link gave me was a snippet of the schematic that was too small to read.

8754A. I've checked the link and the image blows up massively
maintaining its clarity so you must be doing something wrong. Try
right clicking over the image and see what options come up.

I did, before I posted. No menu opened up.

Have you got JavaScript turned off or something? If you right click
over the image, you should get half a dozen options; the first of
which is "view image" - click on that and you should get the
magnification options come up.

--

"The BEST Deal is NO DEAL"

 

[Cursitor Doom]

On Mon, 06 Jan 2020 12:28:07 -0800, John Larkin
<jlarkin@highland_atwork_technology.com> wrote:

That Yandex site is too picky about showing the image.

Dropbox is free.

Sigh....
Try this one: https://imgur.com/a/MzbIf8O


--

"The BEST Deal is NO DEAL"

 

[John Larkin]

On Mon, 06 Jan 2020 22:23:12 +0000, Cursitor Doom <cd@notformail.com>
wrote:

On Mon, 06 Jan 2020 12:28:07 -0800, John Larkin
jlarkin@highland_atwork_technology.com> wrote:

On Mon, 06 Jan 2020 18:05:39 +0000, Cursitor Doom <cd@notformail.com
wrote:

Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

That Yandex site is too picky about showing the image.

Dropbox is free.

AFAICS, there's nothing wrong with that image, which I've been able to
blow up massively whilst its resolution is preserved. Try a bit
harder, John. You'll get it pretty quick I'm sure, bloke of your
calibre.

Too much work. I tried to blow it up, and now it insists that I
register or do a capcha or something silly.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

 

[Phil Hobbs]

On 2020-01-06 17:21, Cursitor Doom wrote:

On Mon, 6 Jan 2020 16:45:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

A diode isn't a terribly dumb detector for an ALC circuit. What's wrong
with the one you have?

Sorry Phil, no idea what your question relates to.

The circuit you posted uses a half-wave diode detector, which isn't a
terrible method at all. Sounds like the ALC loop is open.

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

 

[Cursitor Doom]

On Mon, 6 Jan 2020 18:56:00 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-01-06 17:21, Cursitor Doom wrote:
On Mon, 6 Jan 2020 16:45:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

A diode isn't a terribly dumb detector for an ALC circuit. What's wrong
with the one you have?

Sorry Phil, no idea what your question relates to.

The circuit you posted uses a half-wave diode detector, which isn't a
terrible method at all. Sounds like the ALC loop is open.

Er, yes. I rather suspect so. However, the original question concerned
the el cheapo design and how it might be improved with modern-day ICs.
After all, they've used an op-amp as an error amplifier and op-amps
are not suited to use as comparators. And no, I don't know what the
number of that op-amp is, 'cos they've erased it and put their own
part number on it so no one can tell.
--

"The BEST Deal is NO DEAL"

 

[Phil Hobbs]

On 2020-01-06 19:02, Cursitor Doom wrote:

On Mon, 6 Jan 2020 18:56:00 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-01-06 17:21, Cursitor Doom wrote:
On Mon, 6 Jan 2020 16:45:26 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

A diode isn't a terribly dumb detector for an ALC circuit. What's wrong
with the one you have?

Sorry Phil, no idea what your question relates to.

The circuit you posted uses a half-wave diode detector, which isn't a
terrible method at all. Sounds like the ALC loop is open.


Er, yes. I rather suspect so. However, the original question concerned
the el cheapo design and how it might be improved with modern-day ICs.
After all, they've used an op-amp as an error amplifier and op-amps
are not suited to use as comparators. And no, I don't know what the
number of that op-amp is, 'cos they've erased it and put their own
part number on it so no one can tell.

For ALC, a diode isn't a bad idea in 2020. All you care about is that
the response be flat with frequency, so a very quick Schottky in a
low-inductance layout is the bee's knees.

You appear to be a bit confused about the op amp idea--an error amp
isn't a comparator, it's a linear thing, just the sort of application op
amps are best at. Given the speed requirement, an LM358A would probably
be just ducky.

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

 

[Clifford Heath]

On 7/1/20 10:18 am, Cursitor Doom wrote:

On Mon, 06 Jan 2020 12:28:07 -0800, John Larkin
jlarkin@highland_atwork_technology.com> wrote:

That Yandex site is too picky about showing the image.

Dropbox is free.

Sigh....
Try this one: https://imgur.com/a/MzbIf8O

The original Yandex image is still here, trivially accessible (as you
said) to anyone with a modicum of nous:

<https://avatars.mds.yandex.net/get-pdb/2846114/394be906-be9c-474a-b262-f63b8da1d8bf/s1200>

 

[Clifford Heath]

On 7/1/20 5:05 am, Cursitor Doom wrote:

Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

It looks like a perfectly acceptable circuit. The generator makes
4MHz->1.3GHz by mixing two VCO outputs (looks like 3.6-4.5 and
3.6-3.8GHz) so only the low product survives (no deliberate LPF needed.

The ALC uses a diode to derive a DC level to measure the output
amplitude, which is immediately RC filtered. It won't steal much energy
or distort the signal much (the RC charges up until the diode stops
stealing energy), and it works the same way across the whole frequency
range.

That level signal drives an error amp that feeds a two-diode modulator
which robs energy from the 3.6-3.8GHz signal before the low pass filter.
The process flat-tops symmetrically because of the two diodes, and the
2nd harmonic distortion is discarded by the following filter.

I reckon that either your error amp is faulty or one of the two
modulator diodes is busted.

Clifford Heath.

 

[Cursitor Doom]

On Mon, 6 Jan 2020 20:17:26 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

For ALC, a diode isn't a bad idea in 2020. All you care about is that
the response be flat with frequency, so a very quick Schottky in a
low-inductance layout is the bee's knees.

Are you referring to the detector diode? The manual doesn't state what
kind of diode it is but I'm sure they'll have used something flat with
a fast recovery. It's not that aspect that's bothering me.

You appear to be a bit confused about the op amp idea--an error amp
isn't a comparator, it's a linear thing, just the sort of application op
amps are best at. Given the speed requirement, an LM358A would probably
be just ducky.

The op amp is only dealing with relatively slowly undulating DC, so
I'd have thought just about any jellybean device would suffice.

But - and here's the thing - no one has thus far suggested how the
posted design (from February 1979!!) could be improved!
--

"The BEST Deal is NO DEAL"

 

[Clifford Heath]

On 7/1/20 8:32 pm, Clifford Heath wrote:

On 7/1/20 5:05 am, Cursitor Doom wrote:
Greetings gentlemen,

I'm trying to improve on this 40 year old design by HP. The ALC
section is supposed to maintain the RF source output level across a
range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to
a combination of component ageing and a restricted design budget at
that time, it doesn't meet this spec; not even close. There is
"considerable room for improvement" as they say.
I'm inclined to believe there are better solutions available today
with better frequency and temperature compensation than that afforded
by the handful of discretes shown; possibly something at least partly
monolithic?
Suggestions welcome...

Here's the schematic; you can see the relevant signal path is shown by
dashed lines:

https://tinyurl.com/yjzxtgn4

It looks like a perfectly acceptable circuit. The generator makes
4MHz->1.3GHz by mixing two VCO outputs (looks like 3.6-4.5 and
3.6-3.8GHz) so only the low product survives (no deliberate LPF needed.

The ALC uses a diode to derive a DC level to measure the output
amplitude, which is immediately RC filtered. It won't steal much energy
or distort the signal much (the RC charges up until the diode stops
stealing energy), and it works the same way across the whole frequency
range.

That level signal drives an error amp that feeds a two-diode modulator
which robs energy from the 3.6-3.8GHz signal before the low pass filter.
The process flat-tops symmetrically because of the two diodes, and the
2nd harmonic distortion is discarded by the following filter.

I reckon that either your error amp is faulty or one of the two
modulator diodes is busted.

Actually forget that. Your level detector diode has been blown by a
pulse coming in the output, and so the ALC is running open loop.

Clifford Heath.