Peter Chadwick wrote:
>I would be very inclined to measure the IMD with W8JI's 3 tone test,
>with 2 of the tones about 40 or 50 Hz apart - that will show up any
>problems with the time constants in the SG circuit as well as in the HV
>PSU.
>
To simulate the whole spectral content of speech, you'd have to go all
the way down to about 1Hz (the word or syllable rate) for the third
'tone' because this is where many poorly-regulated anode and screen
supplies start to fail.
There is an interesting circuit for a pulsed two-tone audio tester
(which is essentially the same thing as a three-tone source) in the 1994
RSGB Radio Communication Handbook. It's a normal two-tone audio source,
with the combined outputs going through a MC3340 voltage-controlled
attenuator. The control input of the 3340 is driven from a 555 square-
wave oscillator through a shaping circuit. As designed, the pulse
frequency is fixed at 10Hz, but it would be very easy to make this
variable or even swept.
>The alternative IMD measurement is to use white noise.
>
Or just talk to it!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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