Electronics > RF, Microwave, Ham Radio

Purchasing HP 432A (was: eBay HP 478A thermistors. Any good?)

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Long story short: I want to calibrate/adjust a bunch of vintage CROs, because I think repair is a good way to get skill in electronics, but over all because I have fun from it.

So I purchased a vintage military version of HP 8640B and, of course, now i want to check output amplitude, levelling, etc. Considering now to get a power meter. It could be either a 432A or one of those that work with 848x series sensors, probably a 435A or 436A. I'm leaning towards one of those since they cover the 20dBm output range of signal generator and I think they are probably good enough to adjust a vintage o'scope

However, before pulling the trigger, I see that 478A thermistor sensors are easily found on eBay and even cheaper. Moreover, it looks 432A would be more precise and easier to calibrate/adjust, so I think that could be advantageous. But I wonder if these eBay sensors are any good. See https://www.ebay.es/itm/166500473128

Any pointer/hint, not just about the sensors but about using a signal generator to adjust an old CRO, would be greatly appreciated. Apart from that signal generator, I just have a TinySA and some cheap chinese attenuators/adaptors/dummy loads

Listing states it's working, therefore if it isn't any good you should be able to return it, note the sensors (all types) are very easily damaged and often advertised as untested for this reason. You will probably notice that there are many more meters out there, than working sensors.


This is just my opinion but I think the HP432A (with the 478A thermistor sensor) is now best used for metrology purposes rather than as a general purpose power meter. The stock HP478A sensor is rated from 10MHz to about 10GHz but in my experience the input VSWR is poor below about 30MHz and poor above a few GHz. It is best suited for metrology in the range of about 50MHz to about 1GHz in this respect. I typically only use mine for checking the 1mW reference from my Anritsu power meter using DC substitution and a decent Keithley DVM.

The 478A sensor is also very fragile and the sensor quotes 30mW as the maximum input power level. This means it could be damaged with the 8640B sig gen (in theory at least). One mistake and it will be cooked and then it's game over.

The thermocouple sensor based meters that replaced the 432A are more suited to general RF benchwork. They are easier to use, have a more stable zero performance vs time and temperature and it's possible to get sensors that work down to about 100kHz. They can also withstand 20dBm input power levels without damage although I would recommend using them up to about 15dBm maximum if you want the best performance in terms of sensor linearity.

For checking old CR scopes for frequency response you ideally want to use a sig gen with low harmonic distortion. The HP 8640B should be generally good here as long as you keep the power output fairly low. I haven't used one for many years but I'd expect it to be good (in terms of low distortion) at levels up to 0dBm.

The ideal setup would involve a good sig gen and an external levelling amplifier. I made my own levelling amplifier a few years ago as described here:


The benefit is accurate levelling and a well defined 50R source impedance. This is probably too good for just checking a scope for frequency response though.

One issue with using the 8640B will be tuning it across the frequency range of the scope in octave ranges. This could become quite tedious to do by hand. Modern synthesised sig gens can do this via remote control or via the front panel with a rotary control. I'd recommend something like the Agilent ESG series as these use a BFO/HET system up to about 250MHz. The harmonics are usually much lower on the BFO/HET range as long as the power is kept below about 0dBm. The IFR 341x series of sig gens have a BFO/HET range up to about 370MHz and typically produce low harmonic distortion up to this frequency as long as the output power level is kept below about 0dBm. The IFR sig gens tend to be very expensive though.

The analogue versions of the Agilent ESG sig gens sometimes appear at quite low prices. Probably not much more than a healthy 8640B sig gen especially for the 1GHz variants. The RF levelling is typically excellent with the ESG sig gens as well. The harmonic distortion does degrade at frequencies above the BFO/HET range but not by that much. An external LPF followed by a 10dB attenuator could be used on these ranges.

The really cheap way to check the 8640B for flatness would be to use a decent low barrier Schottky diode detector with its own internal 50R termination. This won't be as good as a HP power meter, but it should be good enough for the stuff you are doing.

Thought I might just add some data to what Jeremy was saying - Here are some images that show the cal section for one of my 478A's:

In comparison, here is the VSWR table for one of my 8481A sensors:

You can see that the passing spec for the 478A at higher frequencies is larger than the 8400 series sensor.

Hope this is interesting to you.


Anecdotes aren't evidence, but I got my hands on a number of thermistors from a lab that was shutting down. Every single one was bad - there were about 15 sensors, with both type-N and WR-28 inputs.

By contrast, I also received from the same lab a couple of 8482s, including one W8482 (WR-10!) and they're all in good shape. The 435s, especially the B suffix, are much nicer instruments to use than the 432s, and both can be found with HPIB ports that work with the calibration processes and external leveling of some of the HP synthesizers of the 8673 vintage (though I haven't done this myself, I've seen it mentioned in the user manuals).


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