--- Glenn Little WB4UIV <[EMAIL PROTECTED]>
wrote:
> Paul
>
> 910 Kc is twice the 455 Kc IF. Possibly there is a
> clue here. Motorola had
> problems with spurs in the Metrum VHF ham
> transceiver. It also used one
> crystal for both transmit and receive. I guess that
> this is a good data
> point as to why mobiles should not be used as
> repeaters. The repeater and
> base station is a complete redesign and uses
> separate crystals for transmit
> and receive. Probably they saw the problem and made
> the intelligent
> decision to keep the spurs at ground level in the
> mobiles and use a clean
> transmitter for base stations and repeaters.
I have used lots of Micor mobiles as repeaters and
never seen this problem. How far down is the spur from
the TX carrier? The Micor does not use a 455KHz IF. It
is a single conversion RX with IF at 11.7MHz.
My thought is that the 910KHz is too co-incidental.
Look at the RX that you are using to see if it has a
455 KHz IF, if it does, it's probably not a spur, but
an image of the 455 IF.
Follow me for a moment. If you have a standard
10.7/455 IF system in the radio you are using to see
this spur, you will have an oscillator at 10.245 (the
same would occur with any high IF, be it 21.4 or 31.2,
etc; the oscillator will be 455KHz removed from the
high IF. 10.7 is only used as an example). This means
that a signal coming through the 10.7 (or whatever)
filter that is 910KHz lower will also be demodulated
at the 455KHz IF (10.245-.455= 9.79). The only
attenuation will be whatever the high IF filter has at
that freq, maybe around 60-80 dB or so.
Now, if you are listening on your radio at 444.910, an
incoming signal on this freq will produce a 10.7 MHz
signal, which is what is desired. But a signal coming
into the RX at 444.00 (your RPT TX) will produce an
output on 9.79MHz, which will also be heard by your
455 demod, although at a much lower level.
Joe
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