Toby Deinhardt wrote on Thursday, November 27, 2008 3:33 PM:
Hi Toby,
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.
Putting aside the K3 and thinking in the context of receivers in general, if
you plot the Input IP3 of a ladder crystal filter vs frequency you will find
that in most cases the value of its IIP3 has not reached a maximum untill
the frequency is well into the upper and lower stopbands. For example the
IIP3 of the 500Hz quadrature ladder filter that I use starts to decrease
from + 55dbm at delta 6kHz from centre passband frequency, to a minimum of +
34 dbm at delta 500Hz, stays between +34dbm and +38dbm until reaching the
opposite delta 600 Hz, then increases back to near +54 dbm at delta 10kHz
before settling.
Now if the plot of a filter's IIP3 is superimposed onto the plot of its
frequency response it then becomes possible to determine relatively quickly,
in the context of Gain Distribution, what is the level of two or more
signals appearing in the skirt and stopband regions and their positions that
will result in noticeable IMD products generated by the filter appearing in
the passband. This is why I said "maybe" with ladder filters in mind, and is
one of the reasons why three and four tone IMD tests are used by some.
I agree with your comment about RX LO phase noise which must be suitably low
if one objective is good in-passband performance, which also requires a
bullet proof IF. Surprisingly TX phase noise has not been an issue here,
even when digging out a SSB DX station sitting between very strong 40m BC
stations.
> > 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.
A pity to lose any potential improvement in overall noise figure by doing
that, especially on the higher bands!
73,
Geoff
GM4ESD
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