Thursday, 10 September 2015

HP 437B power meter: new toy; traps for beginners

HP 437B power meter: new toy; traps for beginners

Introduction and traps for beginners

I have recently turned 60 and decided to get some new toys, in the form of stand-alone test equipment. For some odd reason, mainly curiosity, I decided a power meter would be useful, so I bought a 20 year old HP 437B power meter, with sensor and attenuators. Modern power meters of similar capability are about $10K or more!

As well as being expensive, the meters are not particularly intuitive to use, primarily because they use a range of power sensors that have very different features, discussed in this post, as it is a trap for beginners. In my case, the sensor was too sensitive for the unit's calibration signal; requiring attenuators just to get the device working.

Power measurement is not a simple task, other than for a unmodulated carrier; much less, a 7 MHz wide digital TV signal, especially DVB-T with 8000 carriers across its bandwidth.  Power measurement is not covered well in the literature. One of the best sources is a series of Agilent application notes: "Fundamental of RF and Microwave Power Measurements" (Part 1-4; Part 1: application notes 1449-1, literature number 5988-9213EN)

Recommissioning the parts

I bought the four main parts of the power meter from four sources to try and save money; "poor man pays twice". The parts are the meter, the cable, a sensor and an attenuator. After a quick glance at the operating manual, plugging it in and turning it on went ok, so it seemed to be working. However, when I went to zero the meter, it showed overload or would not zero.

The sensor I had was a common HP8484A thermistor sensor. It is useful from 10 MHz to 18 GHz, but has an input range of -70 to -20 dBm or 100 pW to 10 uW; a very low range. The problem is that the meter's calibration output, at the back of mine, but usually on the front, is 1 mW at 50 MHz; higher than the maximum for my sensor!

The solution is fairly easy, just add 40 dB or more of calibrated attenuators (not cheap) and carefully read the operating manual (especially Section 3-10: Simplified operation). Essentially, plug sensor (via attenuators) into calibration output and push "Zero" etc. Disconnect from the calibration port, then the instrument is ready to use, indicating a couple of thousandths nW (a very tiny amount of power and close enough to zero!). Each sensor has a table of calibration coefficients. These need to be keyed into the meter to get field calibration of the device.

Using the meter 

The power meter after zeroing and displaying signal generator output via 40 dBm of attenuators.


The HP 8484A thermistor type sensor (vs diode), a cheap 10 dBm attenuator, and a HP 1234 reference attenuator; 50 W 30 dBm, connected to the signal generator. The sensor's calibration table can be seen in the photo.


The HP 1234 RF signal generator generating a 1 GHz signal at 10 dBm (with some modulation), which is attenuated by 40 dBm before entering the sensor.


The power meter indicates -34.41 dBm, which is approximately correct. The sensor's correction factor had not been entered and the accuracy of the 10 dBm attenuator is not known. But it is working for the first time since obtaining the power meter.

The instruments are quite stable, a photo of the power meter about three hours latter, indicating 34.44 dBm, a tiny difference.


Conclusion

Power meters are not particularly simple devices to operate and are designed to give much more accurate measurements than the average amateur radio operator would want. Power measurement, for anything other than CW, is complex, especially for wide-band digital modes like TV, especially DVB-T, my interest.

Power measurement can be done to some extent with a spectrum analyzer, which is needed anyway, to ensure the amplifier is operating in its linear mode.


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