[AMRadio] infinite impedance detector vs diode


Brett Gazdzinski Brett.Gazdzinski at verizon.net
Fri Sep 5 22:08:52 EDT 2008


Since we are talking about receivers, I am trying to design and build a CW 
rig using my basic receiver design, and a low power (50 watt) transmitter in 
a DX100 size box.

I have a good product detector I tested, its basicly a 2nd mixer, with IF 
and BFO frequencies injected, it works real well over a wide range of signal 
levels.
It will allow me to have a working AGC off the IF before the bfo is 
injected.

The questions I have are about the filter and single signal reception.
455 KHz IF, say a 1000 cycle wide IF filter, the BFO I want to be a crystal 
oscilator for stability, and I was thinking about a crystal say 700 hz off 
the IF frequency.
Does that sound like it would work well?
Cay you get a crystal at 455,700 cycles?
Or should it be 454,300 cycles?
Or should I have both and a switch between them for above and below the 
signal, or would it make no difference???
Or could it be 455KHz with single signal reception? (I tend to think not).

And the filter...what would be a good filter around 1000 cycles at a 455 KHz 
IF?
Mechanical filters are lossy and are tricky to terminate well, and tend to 
be expensive.
Do filters for say an old FT102 work in tube circuits?

In trying out experiments using the homebrew receiver and the product 
detector circuit, it works really well on CW, despite the wide (5.5KHz) IF 
filter in it, and the 455KHz bfo, and the lack of the isolated agc circuit.
If I can get it built, it should work real well!

If anyone can help out with these questions, thanks,
Brett
N2DTS



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