Recall that the purpose of the circuit
as a diplexer is to terminate the mixer with the correct impedance
(50R) at all frequencies, especially those to be expected from the
At 15kHz, the 'difference frequency' and the wanted 2nd IF, it is not
bad, just before it heads off for some rather high value. At the 33kHz
image it is en-route for something big.
Easily seen above is the same pattern as predicted by the simulations
with the load impedance of the original 'diplexer' (purple trace)
dropping off the scale before 2MHz. This negates the presence of the HF
termination arm and shorts the 2nd mixer output at higher frequencies,
exactly as predicted.
So much for maintaining proper termination at the
important frequencies of 1st IF (10.7MHz), 2nd LO (10.7MHz) and the sum
frequency (21.4MHz). Rather than 50R (actually the 47R R601) it is
practically zero. Ooops!
The lack of attenuation at the first aliased frequency (33kHz) is also
clearly visible on the transmission trace (upper blue).
Recall that the only purpose of this circuit as a filter is to block
any aliasing frequencies from reaching the ADC. If they get to the ADC,
they will be down-shifted
by sampling into the normal IF region and subsequently
indistinguishable from genuine IF signals. In STAR the first alias
occurs at 33kHz since the ADC sampling frequency is 48kHz (alias is at
48kHz - 15kHz). Oh dear. Not any significant attenuation, maybe a dB or
It is painfully obvious that the original diplexer/filter in the
PICASTAR 2nd IF design does not do either of the things it
should. It is neither a
diplexer nor a filter
In contrast, the revised diplexer/filter shows good cutoff at 33kHz
(pink trace) and the expected 47R termination impedance at high
frequencies (up to 11MHz shown). The higher insertion loss is
fortuitously more than compensated for in later stages by using
proper-value coupling capacitors in the modification.
The marker 'Mkr 5' at 30kHz is at the lowest possible frequency
component for aliased signal. Anything below this is rejected by the
DSP algorithm. See here for more on this
. The LO is shifted between USB and LSB, to keep SSB
straddling the 15kHz IF, so that the wanted SSB is 15kHz ±1.5kHz
the first image, which will alias down to the 15kHz DSP passband, would
be at 33kHz ±1.5kHz.