"And you can build this basic design with a single crystal at the expense of wider skirts on the IF filter. But you might be able to do RTTY with 170Hz shift."
?
OK a few things the filter in there can be alter for more bandwidth and RTTY would fit easily as
the existing filter was something like 500hz.
That style of filter making it wide/narrow is determined by the caps in the filter that serve to load it.
Using Varicap diode of the right value you can even get variable bandwidth.
FYI the number of crystals int eh filter do not directly determine bandwidth it determines how
steep the skirts are outside the bandwidth.
Allison
