Hi Geoff,
>> Not to forget that in a design such as the K3, it's the first 20dB to
>> 30dB of stop band attenuation which are really important.
>
> Maybe, assuming that the IMD performance of the roofer and ALL stages
> which follow the roofer is good enough at those spacings.
It also depends on whether you are talking about signals within the pass
band of the roofing filter or signals in the stop band. I would argue
that within the roofing pass band the K3 does have weaknesses, but if
the roofing filter bandwidth is close to the DSP bandwidth, then these
become secondary as other aspects such as TX and RX phase noise will
start to become limiting factors, especially at narrow bandwidths.
> Something to bear in mind is that the passband insertion loss of a
> quadrature type roofer using good crystals is less than that of most
> roofing filters of the same bandwidth, which will increase signal
> levels at the IF's input all other things being equal. The difference
> can be as much as 6 - 8db in the case of a 500 Hz roofer.
Which could be taken care of by using a 6dB resistive pad, should the
lower loss become a problem. I rather doubt that a quadrature type will
fit into the space which Elecraft has "alloted" per filter. It could be
the largest challenge if I decide to go the path which you and Martein have.
> disc ceramic capacitors - bad for IMD!
Full Ack.
very 73 de toby
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