HP 8970A Noise Figure Meter: defective A7 voltmeter assembly – temporary fix

A broken noise figure meter, not really a good situation with so many tasks related to noise figures at hand, not only the noise source projects. So, another look at the A7 assembly. With the suspect TL072 opamp replaced by a less suitable, but known-working subsititute, the fault still comes and goes – well, maybe, in the end, the TL072 is not even at fault? There aren’t so many components around, so I checked for all the likely and unlikely things, and found – a defective integrating capacitor!

See the schematic – there are two of the same kind – C4 is the bad one (integrator cap; upper orange frame), C3 (auto-zero; lower orange frame) is fine.
8970a a7 assy schematic c3 c4 capacitors

0.15 µF, 100 V, Mylar, 1982 vintage, and after all these years, somehow, it has developed an intermittent fault (the first Mylar cap with such fault I have ever seen).

trw hew-446 0.15uF 100vdc

With no spare at hand in my tiny New Jersey workshop, I decided to swap the caps, using C3 as C4, and temporary mounted a 0.1 µF film capacitor (Wima FKM) as C3. For the auto-zero function, the exact value and leakage of the capacitor won’t matter so much, anyway.

a7 assy swapped cap

See, how nicely it works: red – integrator charged from input voltage; blue – integrator discharged by reference voltage; grey – auto-zero; this sequence repeats over and and over again, and the duration of the reference segment is determined, after applying the input voltage for a fixed time (all controlled by LS TTL logic on another board).

0.25 V input, 1.2 V reference.
a7 voltmeter 0.25 v input

1.0 V input, 1.2 V reference.
a7 voltmeter 1 v input

Some quick thoughts about the capacitor; typically, Mylar/PET/polyester caps aren’t the best for integrators, because of higher leakage current, and dielectric absorption, compared to, say, polypropylene caps. Maybe, at the time, HP engineers determined that the TL072 leakage current, and other leakage currents on the board would be much larger than any capacitor leakage current; or, they didn’t want to introduce specialized parts – these axial Mylar capacitors of TRW brand were quite common in 1970- early 1990 era HP gear. These are actually not metallized Mylar/PET, but film-foil capacitors (using discrete plastic and metal foil, similar to Wima FKS-3).

Look inside the dead cap – there actually are the plastic and metal foils.
mylar and metal film

For the next few weeks, this configuration will be sufficient; then I will check capacitor stock back at the main workshop; most likely there are some Wima/Epcos/TDK FPK or MKP (PP dielectric foil-foil or metallized PP foil) capacitors around; if not, then I will just fit a pair of good Mylar caps.

HP 8970A Noise Figure Meter: voltmeter assy (A7) defect

Not so good news today, after characterizing all kinds of noise sources, the 8970A stopped working. Can’t get it to calibrate properly, or to show any reasonable noise power values. A quick check revealed that the detector output (voltage proportional to the noise power measured) is good. But no proper display when activating the 8970A-internal volt meter (special functions 80, 81).

Checking various traces and signals – the issue seems to reside with the A7 assembly, voltmeter.
8907a a7 assy

Red – input voltage section; green – reference voltage section (about 1.2 V); blue – auto-zero section.
8970a a7 assy schematic

How it works, quite well-established dual-slope integration with autozero – a capacitor, initially at zero volts, is charged first from the input voltage, then from a (negative) reference voltage, until zero is reached again. The time it takes to do this directly relates to the input voltage.

See here, working example (sorry a bit dim- see the triangular shape in the lower left hand corner of the scope screen).
working trace

Here, non-working condition – integrator not working.
non-working trace

After checking various FETs, and timing signals – the TL072 integrator opamp appears to be the faulty device. It is a strange, intermittent fault – not triggered by vibration, but appears to be intermittent with no direct external cause – maybe a defective output stage of the opamp? Removed it from the circuit; unfortunately, all spare back at the main workshop in Germany, but fair enough will get some TL072s in soon.

tl072 defective

…. once repair is done, noise source project will continue asap!

TWS-N15 Noise Source: checking out some design alternatives

So far, we have mainly been discussing series type noise sources, i.e., noise sources where neither anode nor cathode are connected to ground. Another common design is shown here – the shunt configuration (one port of the noise generation element grounded).

noise source bfr93a shunt

The assembly, more or less just a little blob of solder with a few tiny parts inside… mostly, 0603 SMD format. The output attenuator (not shown) is a 14.5 dB(!), 18 GHz coaxial attenuator.

noise source bfr93a shunt assy

Some quick measurements, at bias currents of 2.5, 5 and 7 mA…. still, there seems to be a lot of 1/f noise (increase of noise power at lower frequencies). This is model #1, with a 22 nF capacitor (see schematic)

noise bfr93a shunt configuration 1

Don’t really see any advantage over the series variant of the noise source. But will test further.

…Progress on another front, ordered a set of PCBs – they can be used for various noise source configurations. Not yet a “prototype”, but need to see what kind of GHz performance is available from such design, and how reproducible it is. No current source yet on this PCB – will add later, or on a separate board – to limit shielding to the RF section.

noise source pcb 150827-2

TWS-N15 Noise Source: some RF transistors as noise generating devices

After testing some Zener diodes and regular transistors (see earlier posts), some attempts with high frequency transistors, to generate white noise (noise power constant with frequency).

So far we have found that Zener diodes generate high noise power, and are rather flat out to 1.5+ GHz (if proper package and mounting is chosen). However, there is appreciable 1/f noise (increase of noise power) below 100 MHz, and this is difficult the equilize with just plain R-C networks.

Another attempt, with regular tansistors – they don’t have enough noise power at high frequencies, past a few 100 MHz.

Now, finally, I have received some 6 GHz BFR93A and 22 GHz BFG410W transistors, from my stock of parts back home in Germany, and have put these to the test. Same circuit is used like before, with positive current fed into the emitter, and the base grounded via some resistors (transistor is run in emitter-base breakdown condition to generate noise).

These are the parts concerned, some general notes – the BRF93A is a very useful part for all kinds of RF applications, and available at low cost.

noise bfr93a

The BFG410W, it is also quite remarkable and I use it a lot for LNA (low-noise amplifier) designs – hard to beat at their cost, delivering considerable gain, at low power. Unbelievable what the semiconductor folks have been able to achieve, a 22 GHz transistor, for a few cents each!

noise bfg410w

Here, the ENR results, vs. bias current, in mA.

noise enr vs bias bfr93a
-note that the ENR increases at low bias current!

noise enr vs bias bfg410w

As can be seen, and don’t ask me why, the BFG410W generates much less noise. Some quick change of the attenuator pad – 4 dB less attenuation. Just to check if this has any effect (besides increasing output power) – all seems well behaved and power is increased without changing any of the general characteristics.

BFG410W – lower 3 traces are 390 ohms parallel, upper 3 traces are 130 ohms parallel output attentuator (390 ohm 0603 pad resistor, paralleled with 390 or 130 ohm 0603 resistor)
noise enr vs bias bfg410w 130 ohm pad

The BFG410W appears to have the best white noise characteristics so far, note that the measurements are still not too accurate, mainly for screening of parts. With proper bias current selection, flatness, 100 to 1000 MHz, <0.2 dB should be possible. Will do some more experimentation, and fine-tuning of the filter/equilization components; ideally, the noise power should be a bit higher, to be able to use a larger, well-matched attenuator, giving good output SWR. Also, I think it is now about time to fabricate some better HF boards (still using FR4, but precision made), to get a reproducible assembly, and to have several TWS-15N prototypes made and characterized.

TMS 2532 EPROM adapter: one byte every 50 ms…..

EPROM progammers seem like a thing of the past, still, they are very popular for test equipment repair, arcade games, and all kinds of other occasions where small amounts of data need to be stored in a bulky, fancy package.
Such programmers, mostly copies of the “Willem” design, are widely available, Made in China, and generally, these work pretty well. Well, as luck would have it, most of the ancient pieces of equipment use 2532 EPROMs, and just this kind is not supported by the common programmers, which support the 2732….. same capacity, different pin layout.

2732 pinout



To adapt the 2532/2532A (these only differ by their programming voltage, 25 V vs. 21 V – make sure the set it correctly) to the common 2732 programmers, the only thing you need is a small adapter, with a most complicated schematic (the only pins that change are 18, 20 and 21). Most of these EPROMs require programming pulse widths of about 50 ms, but often program OK with just 10 ms, or less.

2532 eprom adapter for programming schematic

2532 programming adapter view 2

2532 programming adapter view 1