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RF Automatic Level Control

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:

formatting link 

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"The BEST Deal is NO DEAL"
Reply to
Cursitor Doom
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What is the HP model number? All that link gave me was a snippet of the schematic that was too small to read.

Reply to
Michael Terrell

  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: 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.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

That Yandex site is too picky about showing the image.

Dropbox is free.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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
Reply to
Phil Hobbs

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. 
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"The BEST Deal is NO DEAL"
Reply to
Cursitor Doom

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

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.

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

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"The BEST Deal is NO DEAL"
Reply to
Cursitor Doom

Sorry Phil, no idea what your question relates to.

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"The BEST Deal is NO DEAL"
Reply to
Cursitor Doom

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"
Reply to
Cursitor Doom

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

Reply to
Michael Terrell

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"
Reply to
Cursitor Doom

Sigh.... Try this one:

formatting link 

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"The BEST Deal is NO DEAL"
Reply to
Cursitor Doom

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

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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
Reply to
Phil Hobbs

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"
Reply to
Cursitor Doom

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
Reply to
Phil Hobbs

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

Reply to
Clifford Heath

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.

Reply to
Clifford Heath

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.

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!

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"The BEST Deal is NO DEAL"
Reply to
Cursitor Doom

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.

Reply to
Clifford Heath

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