RF Automatic Level Control

Those two diodes in the amplitude modulator are possibly PIN devices (conceptually can be viewed as current controlled variable resistances) rather than clippers?

piglet

Thanks, Clifford; thought I was in some alternative universe there for a while. ;-)

Neither of those are in the signal path, though, Piglet!

They are not in the *RF* signal path but they are in the amplitude error and demand signal paths. The amplitude leveling signal is thus subject to the vagaries and drift of those zeners - I haven't perused the circuit but first glance suggests they are intended to approximately cancel each out?

piglet

I *think* they constitute some kind of elementary temperature compensation arrangement, but I'm not sure. CR6 is nowhere near the signal, it forms part of the fixed reference voltage network to the non-inverting input. I think you're correct about everything else though - as usual!

Analog Devices does wideband product detectors which are a whole lot better.

This has been mentioned in previous threads.

You can roll your own at up to 500MHz by using an AD834 analog multiplier as a squarer, but AD has special purpose parts that go faster.

That diode seems fine. I've scoped it at E10 and I'm getting exactly the expected DC readings as shown on that node when I turn the RF output control pot (not shown) between 0 and 10dBm. However, it's not fluctuating at all as I expected given it's detecting the RF out which most certainly is.

The ALC portion is perfectly reasonable in 2020. There are chips to do it, but if all you want is to keep the level stable, you don't have to pay $$$--a 15-cent BAT15 is much better than good enough.

BITD HP sold very good RF diodes--I remember their P/N 5082-2835 fondly, and still have a few.

Cheers

Phil Hobbs

Thanks. Please note that it's really difficult to build a network analyzer without a signal source.

HP8754A user manual: Pg 2-3 Block Diagram and description showing ALC loop.

HP8754A user and service manual:

I could not find the reference designator or part number for the ALC diode. The parts list does show 12 diodes, but without a reference designator, I can't tell which diode is the ALC diode. Anything printed on the PCB?

More:

Amazing. Since the detector diode is an envelope detector, the harmonic and spurious signal content of the signal will have an effect on the ALC circuit operation. When you looked at a low frequency signal on your 350MHz scope (model number omitted again), did it look like a sine wave, or did it exhibit some distortion? I'll be really impressed if you can see and measure any level of distortion with an analog oscilloscope.

In that case, you will have some difficulties improving on the circuit. It's easy to design something that works at one specific temperature. It's not so easy to design something that works over a specified temperature range. It's also possible to improve on something that works as the working model will act as a reference to determine if anything has actually been improved. You can't do that if the reference model is non-functional.

Actually, it has quite a bit to do with the problem. Quoting myself:

I'm inclined to agree with the others, who suggest that something is probably fried in the circuit. I can't add my guess(tm) to the others, because you seem reluctant to provide any clue as to what you're viewing on your unspecified model 350MHz scope over at low freq part of the frequency range, or any other measurements. The quality of the answers you receive is proportional to the information you provide.

Also:

You probably didn't transmit into your network analyzer, but discharging a BFC (big fat capacitor) might have zapped the detector diode.

Lastly:

I changed my mind. No amount of luck will help you redesign an instrument that only needs some troubleshooting and repair.

Thanks for that bill of clean health, Phil. I only wondered because that particular model was designed to appeal to small start-up companies that couldn't afford the other VNAs that were produced by HP and others BITD. Consequently they nickle 'n' dimed it where they could to keep the cost down. It still came to as much as three new family cars back in '79, but I guess that was a worthwhile improvement on six. :)

Okay, Jeff. You've very generously devoted a lot of time to your carefully-considered remarks and they deserve a proper response which owing to the lateness of the our here I'm not able to provide right now, having wasted time earlier in pointless arguing with one of our resident trolls who shall remain nameless. I'll answer in full tomorrow and all will be revealed...... ;)

Or all that Cursitor Doom understands about the problem, which isn't likely to be all that useful.

Quite possibly but not necessarily, they are operating in the knee region, with the control current pushing them closer to full-on (which means full attenuation). But the net effect of their non-linearity (assuming they're matched) is progressive clipping leading to harmonic distortion - which at >7GHz is easy to lose/filter. If not matched, mostly the same anyhow, just assymetric. A poor man's variable attenuator, but quite good enough for the task.

Thanks to the OP, I know that these broad-range generators used this dual-VCO topology, but it's really good to see more detail.

CH.

I thought the whole point of PIN diode attenuators is that at sufficiently high frequency the PIN diode really does look like a resistor not a diode and therefore doesn't clip. The charge carrier lifetime in the intrinsic region needs to be many times longer than the period of the lowest frequency to be attenuated for this to work. Having dual VCOs and attenuating one of them before mixing neatly makes this condition true.

John

The carrier lifetime is part of the story but not all of it. The recombination rate goes as 1/(doping density), but it's hard to get that below about a nanosecond in silicon, even at 1E21 doping. (Gold-doped diodes such as 1N914s have reverse recovery times of a few nanoseconds.)

Pure (intrinsic) silicon has a minority carrier lifetime of a bit below a millisecond at room temperature, iirc (dim memory says 250 us). However, you can make good RF diodes out of material with long carrier lifetime--you just make the junction thin enough that all the charge gets swept out in a small fraction of a cycle.

A PIN diode has a thick region of low doping in the junction, so it stores a lot of charge there. That's what makes the series resistance nearly constant over a cycle, which gives PIN diode attenuators their lowish distortion.

Cheers

Phil Hobbs

I'm not sure which one you're referring to here, Jeff. If it's the signal pick-off one, the detector at the RF out stage, then it's internal to that amplifier module which is monolithic and not accessible without a can-opener. Consequently it wouldn't appear on the parts list AFAIAA.

Tek 2465A (is that really relevant?) Oh, you mean does it have a 50 ohm input? Yes, it does but you have to alter the input mode otherwise it's just 15pf||1Meg IIRC. Come to think of it I'd best just check I did have that on the right setting; not sure I did that.

I wasn't looking for distortion but certainly there was none visible, no. No clipping either.

OKay, I think we've established there's nothing to be gained from a re-design; I'll just focus now on fixing it.

It's kind of hard to provide that sort of detail on a text only bulletin board, though.

Nope. No BFCs involved and proper static protection precautions observed, too.

Yup, that's the goal now.