Had a few hours to kill this afternoon. I added an 8-pole Chebyshev filter
between the Shewood PD-4 product detector and the AF amp. The new board
kills what's left of any BFO leakage through the Sherwood product detector.
Clean audio consistent with the SM0VPO audio board and there's no trace of
ringing owing to the Chebyshev design.
The filter comes from Elecraft. It's their model K3DF and is used to modify
early K3s that had an 8 kHz spurious leak on the DSP board. Two channels
are included, 4-poles each. I simply cascaded the sections to achieve
8-poles of filtering.
The K3DF board is very small and contains all SMD components. This helps to
minimize RF and other induced noise. A single low-noise quad op-amp is used
as an active low-pass filter. The only changes required were the addition
of a 78L08 regulator, decoupled and fed from the Sherwood power supply
board, and a pair of 50K metal film resistors to bias the input at 1/2 the
+8 Vcc voltage. The upper filter cutoff is about 4 kHz and the low-end is
nearly at DC. Since it is connected directly after the product detector, AM
reception is not affected.
Cost is $40 + shipping through Elecraft.
Paul, W9AC
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