[Elecraft] K3 Design

[Geoffrey Mackenzie-Kennedy]

Val  val at vip.bg wrote:


Brett N2DTS -- 
>> I wonder if, with today's computer control and other
>> fancy tricks like switched in band pass filters, wide range
>> vfo gizmos, etc, if it would be possible to build a single conversion
>> receiver with a 455 KHz IF frequency.
>>
>> With good filters and DSP, could it work?


Val -- 
> The DSP can't help for the image rejection.
>
> 73 Val LZ1VB

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An Image Reject mixer might be useful in this case Val, although I have not 
looked at the specific "numbers" for an IF of 455 kHz. If high performance 
is required, for sure large cored coils would be required in the preselector 
to avoid IMD being generated there, e.g. T-68-X size. Also "strong" mixers 
and all of the following stages including the filters and DSP must be able 
to handle big signals both within and outside of the IF filter passband.

Another option might be direct conversion using an Image Reject mixer 
feeding audio DSP, but you would be hard pushed I think to get more than 
50 - 60 db unwanted sideband suppression from the mixer and associated audio 
networks themselves, but no other images of consequence would have to be 
dealt with - assuming a clean LO and strong mixers. Collins used this 
approach to cover the 20 to 2000 MHz range in their 95S-1A receiver produced 
some years ago. Perhaps the DSP wizards know of a way to improve on the 
unwanted sideband suppression.

73,
Geoff
GM4ESD