I'm replying to everyone because there is some question at to how the Omni C
produces the nasty spur on 17 meters. Here's how:
When operating on 17m, the output of the VFO runs between 9 and 9.5mhz.
This signal is then fed to the TX mixer where it is mixed with the 9mhz,
carrier from the BFO. Mathematically, we have the sum of the two
frequencies, 18mhz to 18.5mhz and the difference, zero to .5mhz.
This zero to .5mhz carrier mixes with the desired signal and as you can see,
the difference in the two frequencies is small. For example, when we operate
on 18.070mhz, the VFO supplies 9.035mhz which is mixed with the BFO to
produce both the desired frequency AND .035MHZ.
Now the trouble starts. Here we have a healthy .035MHZ carrier which mixes
with the 18.070mhz desired signal to produce 18.105mhz and 18.035mhz. So we
now have two carriers within the 17m band and one that isn't. Lets move the
dial on up to 18.150mhz and now our spurs are at 18.225mhz and 18.025MHZ.
So the whole problem here is due to the VFO and BFO being much too close in
frequency thus producing products too close for the transmitter filter to
remove when operating on 17 meters. It's a bad design to say the least. On
other bands, the undesired products are several Mhz away from the desired
frequency and easily filtered by the TX. Just think; If we could operate on
18.001mhz, we would QRM ourselves!
Steve
N4LQ
----- Original Message -----
From: "Archie J Lopez" <sylvestr@uswest.net>
To: <N4LQ@igLou.com>
Sent: Thursday, March 23, 2000 12:31 PM
Subject: Interesting frequency combination?
Steve,
I don't understand where the second frequency comes from? First
you have the Oscillator frequency, it is mixed with the 9.0 MHz vfo freq
and then it is filtered, mixed and suppressed, further Xtal filtered
and finally amplified. I don't understand where the second frequency"
just a few hundred hertz" apart comes from? It is not in the math or
the doubling or hetrodyning scheme that I can see? Could it have been a
cracked xtal perhaps?? Give me a clue please??
Archie K7ILZ sylvestr@sttl.uswest.net
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