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Quadrature Sampling Detector (QSD), IC4
Next we can actually observe the audio frequency signals, with 700Hz frequency, at the four outputs of the Quadrature
Sampling Detector circuit around IC4. Note that from now on, I have changed the horizontal speed to 1ms/division, for
suitable observation of 700Hz audio frequency signals. The signals are a bit wide, because superimposed on the audio
frequency, is still a component of the local oscillator frequency. This is filtered out in the pre-amp stages by capacitors C4
and C7.
The following traces are taken from resistor R5 (Ch 1) and resistor R9 (Ch 2), at their junction with the Quadrature
Sampling Detector's sampling capacitors (these are C43-C46, refer to the circuit diagram). If you have assembled the
PCB in the recommended way, putting the body of the resistor on the side where the silkscreen has a circle drawn (refer
to layout diagram in the manual), then the side of the resistors R5, R6, R8 and R9 nearest the top of the board, will be the
long wire of the resistor; it makes a neat hook to attach the x10 'scope probe to!
