Today I finished construction on the circuitry for the RMS-to-DC converter described on the October 1992 QST article by W0IYH on measuring receiver performance. This is an article that I found on the EMRFD CD when I was searching for information on measuring the noise figure of amplifiers and receivers. I didn’t find the article directly, but was directed to it because it was referenced in another paper (on the EMRFD CD) by W0IYH on homebrew noise sources. Thankfully, the authors were kind enough to include both papers on the CD, since the October ’92 article gave a lot of the background needed to fully grasp the noise source article. The detector is a fairly simple circuit, since all of the hard work is being done by the Analog Devices AD636JH RMS-to-DC converter. This device measures the true RMS value of DC and AC signals (up to 200 mVrms full scale) by converting the input signal RMS value to an equivalent DC voltage. The rest of the circuit basically consists of op-amps providing plenty of buffering and one stage of gain. The gain stage is present so that you can use a 10:1 scope probe on the input.
As you can see in the photo to the right, there are some external controls for the circuit. At the far left is a pot which controls the signal level from the first buffer into the second buffer (which preceeds the gain stage). This allows you to accurately calibrate the detector to an on-board 200 mV reference. The rotary switch to the upper-right of the pot is the input coupling selector. You can choose from DC, AC, or GND, just like an oscilloscope. In the middle of the photo, you can see the 10:1 Tektronix scope probe that I’m using with this detector. Finally, in the upper-right corner is a rotary switch that allows you to switch in three different levels of signal attenuation: 0, 3, and 10 dB. The 3 dB and 10 dB levels are set with the little blue trim pots to the right of the probe tip. By having this attenuation built-in to the detector, the process of making NF and S/N measurements is a lot simpler.
Unseen in these photos is the simple dual polarity power supply that I built for this detector. Wall current is fed into a 120 V:12 V transformer, which then is recitified by a 1N4007 and regulated by a 10 V zener for both the positive and negative rails. I’ve still got to get the thing in it’s enclosure, but of course I had to smoke test it first before I went any further. After trimming the offsets on the gain amp and the RMS-to-DC converter, the circuit performed pretty much exactly as I expected. No great surprises, fortunately. I’ve still got a bit of copper clad to trim off of the top edge of the board, then I should be able to get the whole thing mounted in the enclosure and ready to go by this weekend.