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Tapping the discriminator outputs on the Icom IC-R20
The R20 uses a pair of Toshiba TA31136FN chips to detect FM. Toshiba describe this chip as "FM IF Detector IC for Cordless Telephone" (sic) which gives us an idea of its market origins. Having, as it does, two separate receive paths, two detector chips are used. These are IC2 and IC17 for 'A Band' and 'B Band respectively. Fortunately, it is not necessary to solder directly onto pin 9 of either detector chip as two output points are provided towards the bottom of the RF board. These test points are labelled CP1 and CP11 for 'A Band' and 'B Band respectively. Note that the 'A Band' and 'B Band have different frequency coverage and so you will have to decide which side will suit your required coverage. See Dualwatch
Toshiba TA31136FN Demodulated audio is produced at the quadrature detector and is then passed through a low pass filter. It then comes out of the IC on pin 9. From there, it usually goes though a de-emphasis filter
and an audio band pass filter. The purpose of a discriminator tap is to avoid
this filtering to give an audio signal which has no de-emphasis and does not
roll off at subaudiable frequencies. Note that the application schematic shown above is as suggested by Toshiba does vary slightly to that used by the R20. In particular, the final BPF will roll off lower as the R20 has wideband FM receive capability. Furthermore, provision will be made to feed the CTCSS / DCS decoder without the <300Hz roll-off.
You might notice some other things going on here too. The TA31136 demodulates both wide and narrow FM and the bandidth is controlled by A_WFMC. AM and SSB are demodulated not within the chip, but with a separate AM detector and in the case of SSB, a BFO output is generated by the logic circuit and mixed with the IF. This of course means that a pin 9 discriminator tap will not be able to provide audio for AM, CW or SSB modes.
Visit http://www.discriminator.nl for more general info on discriminator / decoding techniques
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