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<p>Assuming the VCO is part of a PLL, the PLL will tend to "correct"
the modulation, especially those low frequency (example CTCSS and
DCS signalling ) components that fall within the loop filter. Most
modern land mobile transmitters have modulation compensation where
a small amount of modulation is fed to the Reference Oscillator in
order to fool it into ignoring the "error' it would see due to the
modulation of the VCO. So to answer your question, you should
insert modulation where it can feed both the VCO and the Reference
Oscillator Modulation Compensation circuitry other wise the result
will be distorted. <br>
</p>
<p>The matters of preemphasis/deemphasis and channel BW modulation
mask are another important set of topics and beg the question, why
do you want to drive the VCO directly without employing the
existing filtering?<br>
</p>
<br>
<div class="moz-cite-prefix">On 5/31/2016 3:40 AM, Steve Wright
wrote:<br>
</div>
<blockquote cite="mid:574D3FEF.5060204@meshnetworks.co.nz"
type="cite">Gurus,
<br>
<br>
What are the limitations with audio-driving a 50MHz FM Transmit
VCO directly?
<br>
<br>
Sorry that's a really open-ended question. I suppose it will vary
per each chan_ driver as well, as well as the emphasis settings
and so on.
<br>
<br>
So dealing with the issues of emphasis, compression,
(audio)bandwidth limiting and so on - how much can be taken out of
the radio and handled by the chan_ drivers, and how different are
they each?
<br>
<br>
It was suggested elsewhere, that everything over 12KHz be not
permitted anywhere near an FM modulator stage, but how much does
that actually happen with the CM119 chipset?
<br>
<br>
I ask because on a transmit-only site, there hardly seems to be
any point to having a "transceiver" there at all, when a clock
generator, buffer, PA, and filtering, is a fairly simple unit.
<br>
<br>
<br>
Cheers,
<br>
Steve
<br>
<br>
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<pre class="moz-signature" cols="72">--
Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
<a class="moz-txt-link-abbreviated" href="mailto:JLeikhim@Leikhim.com">JLeikhim@Leikhim.com</a>
407-982-0446
<a class="moz-txt-link-abbreviated" href="http://WWW.LEIKHIM.COM">WWW.LEIKHIM.COM</a></pre>
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