EMRFD Message Archive 2065

Message Date From Subject
2065 2008-09-14 15:38:49 Weddig, Henning-C... AW: [emrfd] BFO Feedthrough into IF
my first thought that Your problem when using a "high" level Schottky diode ring mixer as a product detector is the LO to RF isolation.
I just looked into the MCL databook, but they state for frequencies of 11 MHz that the L-R isolation is in the range of 69 dB meaning that for a +7 dBm LO the LO at the RF input is about -62 dBm, much less than e.g. my preferred MC1496 (running with -5 dBm of BFO power).
What is the gain of the IF stage???
How "good" is the decoupling of the BFO to IF amp on the supply lines?
Do You have test equipment ot measure the LO leakage at the RF port of the mixer?

Henning Weddig


Von: emrfd@yahoogroups.com im Auftrag von timshoppa
Gesendet: So 14.09.2008 14:35
An: emrfd@yahoogroups.com
Betreff: [emrfd] BFO Feedthrough into IF

I know that others have had this problem and conquered it, but I'm
wondering what steps were taken.

I've got my homebrew crystal filter in a can. No feedthrough problems

But I built my hybrid cascode (dead bug style) and on the other end of
the same board (which is only 5 inches away) I've got my BFO. I put the
mixer (a TUF-1) in between them because, well, the mixer goes in
between them :-).

No matter what attempts at shielding I make (see below), this
arrangement allows enough BFO leakage into the IF input, enough that it
is triggering some minimal AGC interaction.

Shielding I've tried so far: metal partition around BFO didn't help at
all. Feedthroughs and chokes didn't help at all. Putting my hand near
the board even with a metal partition seems to have some effect on
feedthrough, usually making it worse.

The leakage is the same whether or not the mixer is in place.

Should the mixer go behind the shielding? There's a short distance
(about half inch) of open wire from the BFO pad to the mixer, and I
don't think this is the problem because no matter where I bend it or
even if I remove it the problem seems to be unchanged.

Does the BFO need to go into it's own 100% shielded can? Does the mixer
need to go into it's own 100% shielded can? I was hoping to put the IF
amp and BFO and mixer and audio amp all on the same board.

I've built NE-602 product detector rigs with everything
deadbugged onto the same little board and never noticed any
feedthrough from the BFO there, but of course there the injection into
the NE-602 is way way less than what a TUF-1 takes. Is BFO shielding
going to be an issue on every single DBM product detector receiver
because the BFO has to put out at a higher level?

Tim N3QE

[Non-text portions of this message have been removed]
2074 2008-09-15 07:44:55 timshoppa Re: BFO Feedthrough into IF