Tag Archives: Li-SOCl2 battery

HP (Agilent Keysight) 8970A Noise Figure Meter: getting started, Error E26

Gain and noise figure are criticial performance parameters for amplifier, mixers, and so on. In my lab, these parameters have so far been determined mostly to a qualitative (i.e., approximate) degree of accuracy, by using either general purpose equipment, or specially build test setups, for the microwave region. Such approach comes with a lot of time lost, difficulty, and lack of absolute measure. Time to improve the noise figure measurement capabilities.

Which instrument to take, well, there are a few options, mainly HP 8970A/8970B and Eaton/Ailtech/Maury (the 2075 model). Remarkably, the 2075 is actually superior the 8970x units, and also had a lower list price, considering that the HP 8970x sold for about USD 10k.
The downside of the 2075, and in particular the more advanced B and C models, there doesn’t seem to be any documentation around, no schematics could be found (if you have a service manual of an Eaton 2075B, or Maury 2075C, please let me know); for the HP 8970A, full manuals and several application notes are around.

As luck would have it, I recently found a unit on xbay, a 8970A, non-working condition, error E26. This error indicated that the IF attentuator calibration has been lost, typically, it is associated with a low battery of the non-volatile memory.

8970a error E26

Note the Keysight cal seal – the first one I have ever seen. To proceed with repair, this will be the first thing to be removed. Never mind!
8970a cal seal

There are multiple versions of the 8970A controller boards, earlier boards use a 3.6 V NiCd rechargeable battery, but these tend to leak, and are now no longer available. The unit I am dealing with has a more recent control boards, using a BR2325 Lithium cell (3.0 V).

8970a 2325 holder

Rather than dealing with BR2325 cells, decision was made to replace it with a more permanent solution, which might last 10 years, or longer. A 3.6 V Li-SOCl2 cell, of prime quality, SAFT, Made in France. These cells are of excellent quality, and have very small self discharge.

This is the battery, with some (transparent) heat shrink tubing around the leads.
8970a ls14500 battery

…a few minutes later, the battery fitted to the board.
8970a battey fitted

Note the “BR” rather than “CR” coin cell! CR uses Mn-oxide (MnO2) vs Li, whereas the BR type is using a carbonmonofluoride compound, it typically has lower capacity but better long-term performance, which is important for memory back-up applications, where self-dicharge is often more limiting than actual current drain.

br2325 li coin battery

8970a ls14500 li-socl2 battery

The new cell has 2.6 Ah capacity, 15 times more than the coin cell!

Having the battery replaced, now the tricky part. The actual attenuator calibration, to get the proper cal coefficients back into the 8970A. The calibration itself is just a matter of 1 or 2 seconds (initiated by special function SP 33.0), however, it requires a 346B noise source, with 15 ENR output, unfortunately, I don’t own any of these wonderfully noisy things.
Well, there is a work-around. Just push the green PRESET button, and connect a 30 MHz generator (adjustable down to -100 dBm) to the 8970A. Then, we need to adjust power to approximately equal 15 ENR in a 4 MHz bandwidth, the bandwidth of the 8970A.
15 ENR, that’s about 174 dBm/Hz+15 dB+10*log(4 MHz)=-93 dBm on the 30 MHz generator, to get the same total power into the 8970A detector, roughly. Using special function SP 9.2, the 8970A can actually measure absolute noise power, and the generator can be adjusted to result in a reading of about 15-16 dB (which is dB above a -174 dBm/Hz noise floor). Adjusted the generator to -95.5 dBm, which gave just about 15 dB.
Then, execute SP 33.0 to calibrate the attenuators, and all should be fine. In fact, it worked, and the unit is back alive.

8970a working

The inner construction, it is a thing a beauty, and a pleasure to the eye.
8970a top view

Note the wire above the input section resistor – is this for adjusting their parasitic properties? This I will need to put to a test later, using a few resistors, and measuring the SWR of such PI-type attenuators, with and without wires….
8970a input section

8970a last converter detector

The 1st LO, a YIG oscillator. The same as used in the 8558B spectrum analyzer!
8970a yig 5086-7080

8970a 1st and 2nd converter
… the 2nd converter is idential, or nearly (fully?) identical to the 2nd converter of th 8565A, 8569A, 8569B spectrum analyzer units (P/N 08565-60216, the A35 assembly of these analyzers). Another example of HP cleverly re-using some of their most ingenious designs.

The shielding of the 8970A assemblies, this seems to have been developed for the 8970 series exclusively, a cast aluminum body, separate, spring-contacted aluminum covers, and a more rugged overall cover with an elastomer/metal mesh seal in the corners to prevent any of the waves from escaping, and to ensure that no spurious signals are getting into the chain of modules. In the end, this is a highly sensitive receiver, working at the thermal noise level. So we don’t want any high-level radio stations radiating into there.
8970a shielding

5372A Frequency and Time Interval Analyzer: out of sensitivity cal. error – easy fix

For most precise time interval counting, I use a trusty HP 5370A, which has no less than 20 ps single-shot resolution. The 5370A and its inner workings could be discussed for hours (well, pages) here, but this is not the topic of this entry. This is about the 5372A, a much more modern machine – it still measures time intervals, but has a pretty capable build in numerical analyzer. It has a CRT, excellent quality, magnetic deflection, and this provides a nice and sharp green display. It doesnt’t add much weight, because the 5372A is massively heavy anyway, due to all the shielding, and a huge linear transformer. Why did HP use a linear transformer? No idea! Even the most quiet synthesizers can run with some kind of high quality switchmode supplies. Well, in the end, these machines are not build to be carried around but rest, or work, in an adequately thermostated metrology lab.

The 5372A has some very useful functions, not available for many other counters, unless you spent a lot of time developing your on software, work via GPIB; the 5372A can do virtually all of the most exotic tests, just with the single box.

That’s the main CPU, a MC68020 (the first real 32 bit processor ever!!!), and a floating point co-processor, MC68881, and a lot of traces that let the bits flow around.
5372a main cpu

5372a cpu board traces

This one, managed to get it for very little money, with the build-in precision HP 10811 reference OCXO already worth 100, used. And it even has the very desirable Option 030, 2 GHz input. Ideal for measuring locking processes of PLLs (via a triggered frequency vs. time display – shown directly on the screen, as a diagram/graph!), etc.

With the 5372A there are two important things to consider:

(1) It is extremely heavy, and does not ship well, unless it is packages with utmost care. My unit arrived in a big box, and barely made it – don’t even consider international shipment without a lot of bubble wrap, heavy foam, and double-boxes.

5372a box

You can see the box already damaged; and there was just a single layer of bubble wrap; but I was lucky, the machine survived.

5372a unpacked

(2) Second item, never get desperate about the “160 out of sens cal error”. This error is well-known for these units, and the 5373A, and its 99% cause is a dead lithium battery on the CPU board. Just replace the battery, and follow the instructions to calibrate.

These are the main parts: a spare AA Li-SOCl2 cell, the CPU board with the dead battery (Tadiran is a really good quality Li-SOCl2 cell). And the new battery fitted (of presumably lesser, OmniCell, Made in China; but will be fine).

5372a lithium battery

5372a tadiran

5372a new battery

Note that this cell had been changed before, and some solder spilled!!5372a solder
It seems, it didn’t kill the board, but it is removed now.

The calibration, not very spectacular (using a 8904A Source):

5372a sens cal completed

With the calibration done – no error displayed any more.

More detail testing will follow, but according to the self test, all is fine!

5372a self test pass