While I was working on my power sensor thing, I wanted to check the spectrum of some signal generator by using my 8568B spectrum analyzer.
About one year ago, I had the problem that the analyzer had some intermittent noise, i.e. it was mostly working fine, but for short periods, its noise rose quite a bit. I was not able to determine the source of the problem, but after a while, it somehow fixed itself and didn't appear anymore.
UNTIL NOW! ARGH! I need to fix this.

I made a video (it is quite loud, so lower the volume a bit! :-)):

The signal shown on that screen is an ultra low phase noise OCXO which is powered up since a week or so, so it has definitely settled in frequency. One can clearly see that most of the time, the signal is displayed nicely, but for a short time, the noise rises quite a bit and there are some artifacts visible.
I have already checked the power supplies, and they seem to be fine and within spec. The spectrum analyzer is also powered since about two weeks or so, so its internal OCXO is definitely also settled.
What else could cause this type of problem? has anyone ever seen something like that? I was also thinking whether it is some sort of ground loop or so, so I removed my OCXO signal and connected the calibrator signal. Same behaviour, so I assume it's something generated within the analyzer.

Any ideas?

Best
Tobias HB9FSX

Graeme, at one time I had one of these attenuators (NOS, if I recall correctly).? It's been years since I've seen it, but it might still be squirreled away somewhere in the shack.? And I don't recall if it was for the 606A or 606B.

I'm currently out of town, but I can take a look for it next week sometime.

- Jeff

...and clearly I should have checked if Dick had replied while I was composing my own reply.? Apologies for the duplication of effort!

- Jeff

Please correct me if I'm wrong, but looking at Dick Benson's description of his MATLAB application (as described in the PDF included with his MATLAB files) and comparing his features with those in John Miles' SSM.EXE, it looks to me as though SSM.EXE is quite a different beast from Benson's MATLAB application.?

Benson's application mimics the controls of a basic spectrum analyzer (and includes bells and whistles such as the ability to do Harmonic and IMD analysis, as well as trace memory), while Miles' program seems to me to be much more of a spectrum surveillance tool.

- Jeff

Yes, I am aware of John's excellent work and use it often.
This offering complements his work.

Dick

开云体育

Greetings,

?There appears to be a continuous wave of decommissioned Agilent E4406A "VSA Series Transmitter Tester" boxes hitting eBay.?
?They have a lot of bang-per-buck but have some limitations when being used as general purpose spectrum analyzers.
? A MATLAB application was created to address a few of them:


?Some of you folks might find this useful as well.

Best regards,
Dick (w1qg)

Hey Jeff,
I didn't see your message, it was not in the other thread!
But thanks anyway, I had the same idea as you suggested - I will soon receive some sensors with a blown thermocouple, and there I probably can replace the thermocouple with my own circuit.

As for now, autozero seems to work, and the sensor provides more or less accurate readings, but it is not terribly accurate. I hope to improve that soon, probably by adding the shaping network HP mentioned.

Best
Tobias HB9FSX

Hi all,
I made some further progress. I discarded my previous prototype and made a new one using better JFETs, this time the 2N4856A. According to its datasheet it is especially designed for chopper applications (well, I don't quite understand in which parameters a chopper JFET should differ from an ordinary JFET).
And with the new JFETs, indeed, the waveforms at the chopper output look much cleaner and nicer. However it is difficult to measure because my scope (Agilent 54832B) is very noisy with those small voltages.

I was able to get more stable and reliable readings, and the zeroing of the sensor also works more reliable. However, the linearity is still quite bad; at low input power, it is okay but towards increasing output power, linearity degrades quickly and at the worst point (0 dBm input into the detector) it is almost 3dB off. My detector can measure -30dBm to 0dBm with usable accuracy. Above it becomes a bit nonlinear - could be due to the diode no longer operating in the square law region. But I will further investigate in the subject. Probably I also need to add the shaping network mentioned in the 8484D service manual.

@ Dave NR1DX
I am testing the diode detector with the HP 436A calibration output. And I also test at various frequencies down to 1MHz and up to 2.6GHz with the 8663A signal generator. However, I think later it would be desirable that my detector works to, say, 6 to 10 GHz. I think otherwise it is not very useful. On the other hand it would also be good if it works to below 1MHz.

@ Rainer DF6NA
Thanks for the links. I will probably order some of those connectors when I am ready for a 'real' prototype! looks not toooo expensive.

@ Mike N2MS
I think for the diode sensors (like 8484A, 8481D) it is not possible to replace the diode without a highly sophisticated workshop and tools. I have never seen one of these sensors from the inside, but I expect wire bonded black magic inside; the service manual even says that the position and shape of certain wires is important, otherwise the calibration goes nuts.

@ Dave NR1DX
The thermocouple has the advantage that it really measures power dissipation, i.e. RMS. Since it does the measurement via temperature, it is basically independent of the waveform and even independent of the frequency - as long as the mechanics is reliable and precise enough, you can measure each frequency with this, even DC. The HP sensors have an internal DC blocking capacitor, so this is why they don't work down to DC.

@ bownes
yes, like AD8307 or so would be nice as well. However, I am not sure whether it is possible to connect these somehow to a HP meter like the 436A. For instance, the AD8307 (or some other LogAmps) need a power supply, which is not available on the existing interface to the sensors, except if you abuse some signals, e.g. the chopper signals and then rectify them etc.
Besides that, you need to mimic the behaviour of the thermocouple/diode detector signals.
This is why I stuck to the 8484A/8481A design.


@Dave McGuire, AK4HZ
You said you have already done what bownes suggested? was it working? could you tell a bit more about this? :-)

Steffan...

Maybe I missed you saying so, but with the Keysight IOLibraries installed the 82357A/B should be essentially transparent to you. Connection Expert will let you assign it a name such as GPIB0 and then you just send commands to the 436's address.
I don't recall the exact details but there is a box to check to use 488.2 vintage protocol but that's about it. Unless there is something amiss with it, I doubt that the adapter is the problem. Since it appears to be at least sending commands, that doesn't seem likely.

From Tom Holmes, N8ZM

Thermocouple detectors are definitely more linear, RMS, and have been around very long time. Some of the peak power meters use the diode versions. These typically have lower dynamic range. The HP8900D uses diode detectors for peak reading. Analog Devices bought Linear Technology but they are still selling independent product lines so the power detector IC's are there. We designed a detector based on one of their log-amp based power detectors that reads linear voltage for dB RF. It was a mistake and so I found the ADL5511 which has both voltage for dB in and also envelope. It also has temperature compensation to first order. Envelope is perfect for peak reading. We have built over 60 of these in a small pod, that has a PIC to take care of curve fit and they track our Keysight and Boonton meters on single frequency (which is what we needed). There are numerous companies now making USB power heads that go direct into a PC, like R&S, LadyBug (ex HP), Keysight, MiniCircuits. They all use the new generation ICs.

A long time ago, I remember an eBay seller offering NOS HP replacement thermocouples, with calibration labels, at incredibly low starting bid prices.? I really don't think he knew what he was selling.? I can only imagine the seller's surprise at the ensuing bidding frenzy.? Those silly little pieces of bent wire were worth a lot of money!??

Update

I figured out a way to make the 8642B stop reporting A6 counter fault in FM mode when I go from internal to external dc. I turn on AM modulation and set it to internal then turn on FM and there is no problems switching from int to ext dc, weird. I have a A3 processor board on the way, from the troubleshooting I have done in the manual it suggests swapping out the A3 board after I already swapped out A4.

Hi Jeremy,
Thanks for your suggestions. I've searched without success so far. Can't find a non-working 606B at this time. I'll keep monitoring.
Hi Kim,
Thank you for the offer of the unit. I'd be most happy to try it in my 606B.
I'm unsure how to send you the postage costs and my postal address. Perhaps you could advise me.
With my thanks
Graeme

On 11/18/18 4:30 PM, wrote:

I’ve long thought about using one of the Linear Tech or Analog Device RF sensor ICs as a front end to an HP power sensor head with blown thermistor/diode.

They have a great frequency and sensitivity range, come in a tiny package that could fit in an existing head, and come in linear and log versions.

A local friend and I have done a lot of work on exactly that. The
idea has great potential, but there are some dragons hiding.

-Dave

--
Dave McGuire, AK4HZ
New Kensington, PA

I’ve long thought about using one of the Linear Tech or Analog Device RF sensor ICs as a front end to an HP power sensor head with blown thermistor/diode.

They have a great frequency and sensitivity range, come in a tiny package that could fit in an existing head, and come in linear and log versions.