Hewlett Packard 431C Power Meter

  I was given one of these meters some time ago in a very sorry state. The problem was probably due to storage in damp conditions and the markings on the dial had started to peel off. The dial was printed by a transfer method whereby a very thin film of material is stuck to the surface. The film had decomposed and broken up especially where it was blank and the peeling was completely obstructing the pointer which was stuck at zero.

Below is an image of one in good condition and here's a technical manual. There are two main variants: if yours has a serial number other than those shown use the first link.

 Serial number prefixed with 548

 Serial number prefixed with 707 & 747


 The instrument requires a specific lead and thermocouple probe which may be plugged in at the front or alternatively at the back. As with all sensitive power meters the probe has a relatively small tolerance in terms of input power. It can measure (at full scale deflection) 10 microwatts to 10 milliwatts and is accurate (with appropriate probe) from 10MHz to 40 GHz. If you would like to know this in voltage terms it will give you a reading of about 0.1 on its most sensitive scale when the probe sees 7 millivolts RMS. The switch on the lower right is set for the specific probe connected to the Thermistor Mount socket.



  The maximum power reading for this 478A probe, printed in red is expressed as 30MW average power which actually means 30mW RMS. The black ring is part of the N-connector input socket.

For some very odd reason Hewlett Packard designers (or more likely their Drawing Office?) have used a capital "M" which normally means "Mega" instead of lower case "m" which is "milli". They also inadvertently used upper case "DBM" when they should have used "dBm". The documentation accompanying the equipment and written by more knowledgeable Technical Authors is correct.

The probe is calibrated on the reverse side from 0.01GHz to 10GHz, showing a curve from which a correction factor can be selected on the front panel.

In case you're not familiar with dBs the designers calibrated the meter and switch in both dBm and mW. Again there's a clash between the front panel and meter markings.. the former half right but the latter 100% wrong.


 In the picture above you can see the degraded meter scale on my example. It's just about usable.


  Above is a picture showing the construction of the instrument. The big space is for a NiCad battery.

Just as British made stuff is obviously British, US-made stuff is obviously American, often using duralumin metalwork and always with coarse-threaded screws.

Now a point on practical measurements. You'll see that the power meter has a range switch and a meter scale marked in dBm and mW. These scales are equivalent so 0dBm = 1milliwatt.

As the probe efficiency is frequency dependent you can check the curve on the back of the probe and set the centre knob to correspond with the frequency at which you're working.

Now, for power readings to be fully meaningful, you need to know the resistances associated with the scale markings. In this meter the input impedance is 50 ohms so 0dBm means 1 milliwatt dissipated in 50 ohms and that means there's should be a voltage, V across the 50 ohm input (where V=square root of the product 50 x 1mW) of 0.2236 volt to give you a display of 0dBm. As we're using RMS or "root mean square" measurements this means that a little under a quarter of a volt RMS present across the load resistor makes the pointer show a reading of 0dBm.

Say we wish to connect a typical signal generator to the power meter with a coax cable. The signal generator will have an output impedance which is likely to be about 50 ohms. Setting the output level to 0dBm on the generator places a voltage of 0.2236 at its output socket. If we measure this voltage with a suitable high impedance voltmeter you should see a little under quarter of a volt AC, however if we now connect the power meter to this socket we place 50 ohms across the output. As power depends on voltage and current (Power=volts x amps) we have to consider any changes in current flow. If the generator output has a 50 ohm matching resistor in series with its output connector the current will pass through this and the 50 ohm input resistance of the power meter, thus half the output power will be dissipated in the generator and half in the power meter. Similarly if the output resistance of the generator is shunting its output the output voltage will now see 25 ohms and the power will be split 50/50 between the two 50 ohm impedances.

This is really annoying. One sees the generator output indicated as 0dBm and you see -3dBm displayed on the power meter scale. To get around this you can use a series resistor between the generator and the power meter (the resistor needs to be reactance-free at the frequency being used so you should not use a wirewound type or a carbon film type using a circular track). If you calculate the attenuation due to the resistor you can take account of this in your measurements. If you place 450 ohms between the generator and power meter you increase the load from 50 ohms to 500 ohms. A tenth of the power indicated at the signal generator is being passed to the power meter (this is -10dB) so a reading of 0dBm on the power meter means you have +10dBm registered on the generator. This is actually an approximation because the generator actually sees a 45 ohm load instead of 50 ohms. The output voltage is likely to be 90% of that indicated so the power will be less than indicated. Increase the attenuation to 20dB by using a larger series resistor 1531 ohms and you get a big improvement or 97% of the indicated power at the generator output. A larger series resistor of 4950 ohms gives you a load of 49.5 ohms which is even better if you can tolerate an attenuation of 30dB.

It's important to know lots of seemingly minor points about a measuring instrument if you're interested in really accurate numbers. For example, if you connect a voltmeter to the terminal on the rear of this power meter to see what's driving the display the operating manual says you need to make adjustments to the readings if your voltmeter has an input impedance of less than 10 Mohms (that's 10 Megohms not 10 milliohms).

I found this tester to be very useful for measuring small amounts of power (for example when testing my home-brew noise source) but unfortunately the meter let me down twice when peeling paint jammed the needle. I dismantled it and fixed it before deciding to replace the meter scale. I unscrewed the scale from the meter and scanned it. Using PhotoShop I managed to, not only clean it up and replace missing numbers, but to change the irksome lettering. Not "DBM" but dBm and certainly not "MW" but mW. I also removed the old numbers and swapped to a cleaner font plus taking off the decimal point from the mW readings and using "10" instead of 1 so that the voltage scale now reads 0 to 10. I made a slight error without thinking and failed to give the mW scale a red background. I'd vaguely imagined that yellow was correct but later changed the drawing to correct this. It's not perfect but does at least show some age...

Click the picture below to see the full size bitmap file.

 How did I change the scale? A little tricky as the scale is glued very firmly to the mirror-finish backplate. I laid the metal plate, scale uppermost, on a cooker hotplate. It got pretty hot and the scale started to bubble, after which it just peeled off. I printed the new scale on a standard laser printer and cut it to shape. I used a small bladed scalpel to cut the curved slot for the mirrorand lightly spayed the rear surface with a mounting adhesive. The paper wetted but after about 30 seconds it dried and I was able to fit the paper to the metal plate.



 Using this meter can be slightly puzzling because the range switch doesn't immediately line up wth the meter scales. The mW scale is OK (better since I changed the max reading to 10 rather than 1) but the dBm scale is calibrated 0 to -10 when the range switch is marked +10, +5, 0 etc. Of course you interpret the +10 marking as 10-0, +5 as 5 to -5, and 0 as 0 to -10.

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