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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 14th Sep 2020, 10:00 pm   #1
RobGT390
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Default HP 8901B can someone hold my hand !

I have just acquired a HP 8901B modulation analyzer and know nothing about it, I want to use it to check FM deviation and AM modulation % in amatuer / CB radios. I had been using my scope for AM % and a heathkit deviation meter for FM until now. I have read the 8901b has a max input of 1w. I have no formal training in electronics or RF and have self taught myself from reading books or online articles, I have made a dummy load with a RF sniffer and have previously been using it to see modulation on the scope but I have not idea how much attenuation my sniffer has and am a bit afraid to hook up the 8901b in the same way in case I damage it. The radios I am working with will range from 5w to 20w output and occasionally possibly as much as 100w output. Could someone maybe suggest how I should proceed, what I need and what precautions I should take before I start connecting anything. Any advice would be much appreciated.
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Robert
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Old 14th Sep 2020, 11:14 pm   #2
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Default Re: HP 8901B can someone hold my hand !

I know the 8901A which doesn't have the head with the power sensor.

8901s are fairly expensive instruments, even second hand, even today. They are also very expensive to repair.

The easiest and most popular way od damaging them is to put in too much power.

A dummy load and a sniffer may be OK in general terms, but with so much at stake, you really should be using a good quality attenuator.

I've used 8901As a lot over the past dozen years measuring aviation transceivers wit powere from about 4W to 60W pep. A 20dB RF attenuator would allow the 8901 to work to 100W much of the time I was using a 30dB attenuator because that was what we had.

Go for a good make of 20 or 30dB RF attenuator with N-type connections.

Be aware that almost all attenuators are asymmetric. They may give the same attenuation in either direction, but they don't have the same power rating each way. Usually the shunt resistive element is beefier at one end than the other. So a 100W 20dB attenuator is sized to run 100W at one end and not necessarily much more than 1W at the other end. So pay great attention to which end is the input.

On its own, in default mode, the 8901 will detect some RF power which will trigger a search to find its frequency and lock on. Then you will start to get modulation readings.

You can circumvent the search time and get speedier results if you type in the carrier frequency, it will then lock much faster.

There are lots and lots of special function commands from the front panel which do things you will never need, and some you might like. One of them selects better counter resolution, for example. There should be pull-out plastic cards with listings.

David

Oh, people also blow up attenuators by transmitting into the wrong end of them. They seem to treat RF like software hacking and just give it a go.... Sadly this winds up being expensive. When you buy an attenuator, if you go to one of those "We don't have the equipment to test it, but it sure looks nice!" places, be a bit suspicious. You can test them with a DVM and a bit of calculation, so that sort of statement means that they haven't a clue at best.
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Old 15th Sep 2020, 11:13 pm   #3
RobGT390
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Default Re: HP 8901B can someone hold my hand !

Thank you very much for taking the time to reply David, I have found a Bird Tenuline 20dB 25w attenuator quite cheap on ebay which is supposed to be rated from DC to 54Mhz so hopefully that will get me started with the radios I am currently working on. This 8901b seems to be well spec`d with option 001,002,030,035 and 037, fingers crossed its in working order !
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Old 16th Sep 2020, 9:51 pm   #4
IanG3XYV
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Default Re: HP 8901B can someone hold my hand !

I have two 8901As which is one more than I really need. They are superb instruments and do a lot more than just AM and FM modulation percentages, including digital frequency and RF power measurement. Your B model does audio AC RMS voltage and has a built in modulation calibrator. They have a host of special functions that help check out operation but they are very easy to operate in automatic mode for the most needed measurements.

Being HP there is excellent documentation and you should easily find online operation and service manuals if you don't already have them. You'll have a lot of fun with it.

Bird Tenuline power attenuators are good quality and conservatively rated. It will do fine for your lower power measurements but if you plan on testing at an RF output of 100W you really need an attenuator rated to handle that and 30dB would be a better attenuation.

I note one of your options puts the main connectors on the rear panel. This is not so convenient for normal bench use but I'm sure you can work around that.

Enjoy your instrument.

Last edited by IanG3XYV; 16th Sep 2020 at 10:15 pm.
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Old 16th Sep 2020, 10:42 pm   #5
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Default Re: HP 8901B can someone hold my hand !

Search the HP Journal archives, there was a group of articles on the 8901 and the calibrator. It's interesting reading.

David
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