On Wed, 2 Apr 1997 09:03:02 -0800 (PST) "David B. Ritchie"
<dbr@alumni.caltech.edu> writes:
>(an aside)
>How do you use the volt meter as an accurate measure of the signal
>above
>the noise level? Sounds like a neat trick. Where do you measure it,
You measure it right at the speaker terminals Dave. The Simpson 260 or
Tripplett 630 have good audio response at 1000Hz (I dont know what the
limits are). Set the range for a low range AC scale (0-2.5V on the 260)
and read from the dB scale.
Connect a signal generator to the antenna input and with the AGC OFF set
the audio gain for 0 dB. Make sure there is no generator signal
leakage...you just want noise. Some cheap generators leak like a sieve
and you will need either additional attenuation or a better generator.
Next, turn up the generator on say 14.000 MHz and use the RX tune control
to peak the signal on the 260 meter (not the S meter).
Now you can either start from zero signal/zero dB and go up or start
higher and go down. The ear responds better to the former.
I am assuming that the generator has a variable attenuator here. Older
tube type generators are easily found at hamfests in the $50-100 range
and are MORE than adequate for most receiver work. Besides, they have far
less phase noise than some of the SS junk that dealers are asking $1000
for. Some decent ones to look for are the HP-606C thru F and the military
AN/URM-25D or F.
After it is all said and done report back to us at what level out of the
noise you could hear the signal.
There are a multitude of other ways to generate a pee weak signal, I just
used a classic test bench illustration.
73....Carl KM1H
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