>G3SEK says:
>
>>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 only problem with this is that in order to get repeatable
>measurements, you would have to use gated spectrum analyser techniques.
>However, a lot can be determined from the screen supply time constants.
>Al though the variations in voice are, as Ian says, at very low
>frequencies, the screen voltage in the G2DAF is intended to follow these
>variations. The intent was not to rectify the grid drive to produce a DC
>screen voltage, but rather a unipolar voltage following the input
>modulation envelope, such that the larger the instanataneous input
>voltage, the larger the screen voltage and thus the larger the plate
>current. The linearity of the amplifier is dependent upon the transfer
>funsction of the screen grid and the transfer function of the control
>grid: it is possible that suitable adjustment could get to a square law
>which would be very good for linearity, although it would be depedent
>upon the stability or otherwise of the tube characteristics.
>
>A similar idea using a switching regulator to vary Vcc was used in a
>Collins solid state HF tx back in the late 60' - early 70's.
>
>73
>
>Peter G3RZP
>
This is interesting! Now we are learning something. This is what I
have been waiting for. A (for me) new approach on how to measure
IMD dynamically.
Earlier this autumn, I did some experimental measurements of dynamic IMD
using speech an a HP spectrumanalyzer. Compared the graph, with and without
the G2DAF amplifier on, and plotted them together. The plot is taken at full
output on 80m (maximum drive) and the graph shows abt 16dB gain and VERY
LOW add of distortion.
I´m not sure this measurement gives me the whole truth,
but at least its an interesting hint!
If you find it interesting I can mail you a scan of the plot as .tif
image abt 300Kb in size.
73 de Lars /SM3BDZ
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Lars Harlin Private: lars.harlin@pocab.se
Gräftågränd 22 Hobby: sm3bdz@pobox.com
S-831 71 Östersund Work: http://www.pocab.se
Sweden
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