Thought I'd do a quick check of the S-Meter accuracy, and did the little calibration procedure from the alignment procedure shown in the attachment. I used the Agilent 8648A to inject the 10,000 uV signal at the antenna terminals. I suppose they liked to use that type of nomenclature back then but in effect it is close to -27 dBm at the ant. terminal (~50 ohm). Then you're supposed to adjust for 4.5 volts on a terminal ...
But, after doing the adjustments the way they say, the metering accuracy sucks, even knowing that many S-Meters aren't that great anyway. Everything seemed to be biased way too high. I know this because I compared it to the standard S-Meter set points in the attached document "Signal Level Strength Meter Calibration and IARU" and my Icom IC-7300, switching the same antenna back and forth.
So I said to myself "Why don't we just calibrate the actual S-Meter directly, using known set points to it's scale? That's what the scale on it is for." A crazy idea but it just might work.
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif.pagespeed.ce.5xOqKkq0Co.gif)
So what I did was tuned in WWV @ 10 MHz on the 7300 and noted the average S-Meter reading, and then tuned off a bit and read the noise level. I then calibrated the SPR-4 to those settings using the two pots in the instructions while it was tuned to WWV. Well, that resulted in pretty good S-Meter readings.
I then compared the S-Meter response to the set points in the document using the Agilent as the antenna input and it really compared quite well. S9, which is supposed to be -73 dBm, resulted in the meter reading just a tad above S9, and the readings below S9 tracked within reason. Readings above S9 were off a little but fell within the expectations of this type of simple circuit.
So, I don't know who wrote or transcribed that S-Meter cal procedure but it doesn't work very well as far as I'm concerned. I suspect I'll be running into more as I go on that will require a certain amount of common sense to be substituted for what is written.
![Shocked :o](https://www.eevblog.com/forum/Smileys/default/xshocked.gif.pagespeed.ic.CrwtrNUjLO.png)