EMRFD Message Archive 2451

Message Date From Subject
2451 2008-11-27 10:00:37 timshoppa Birdies... coming through the crystal filter, or radiation, or ???
I built a nice little receiver with a 4.433 MHz IF, and TUF-1 mixer
and product detector. But there are some birdies.

I can even see the math that leads to the birdies. For example, to
hear 3.546MHz, the LO is set to 7.979 MHz. The fifth harmonic of 7.979
is 39.897, and the seventh harmonic of 4.433 MHz is also 39.897.

On other bands there are other harmonics that show up, things like the
11th or 13th harmonic of 4.433 MHz. They are always related to odd
harmonics, is this a clue?

Many of these birdies when I tune through them sound like there's some
60Hz FM or buzz going on. Is this a clue?

What I think is happening is that in the mixer in the front end,
balance isn't perfect and a lot of the odd harmonics of the LO are
going into the crystal filter. These happen to lie on overtones of
crystals in the crystal filter, and leak through into the product
detector. Of course the product detector then demodulates them because
the input waveform from the BFO has a lot of odd harmonics.

But an alternate way, not through the crystal filter, might be
radiation of the LO from the front end, that somehow is showing up
back in the product detector, maybe picked up by the speaker wiring
connected to the audio amp. Maybe this is like radiation making
tuneable hum in a DC receiver?

I haven't throughly tried testing all these possibilities. Much of the
front end is not in RF-tight boxes and I usually have the lid off my
LO setup too. Is there always a "stock answer" for where birdies come
from and how to eliminate them? And is the 60Hz I hear on the birdies
a clue?

Most of these birdies are tuned through very very quickly because
they're usually quite high harmonics of the LO, so they aren't really
affecting use of the receiver much. But I want to at least understand
how they're getting through to the product detector.

Tim N3QE
2453 2008-11-27 10:33:46 Allison Parent Re: Birdies... coming through the crystal filter, or radiation, or ?
2472 2008-12-08 16:38:23 davidpnewkirk Re: Birdies... coming through the crystal filter, or radiation, or ?