[App_rpt-users] audio for direct VCO FM

[Joe Leikhim]

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.

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?


On 5/31/2016 3:40 AM, Steve Wright wrote:
> Gurus,
>
> What are the limitations with audio-driving a 50MHz FM Transmit VCO 
> directly?
>
> 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.
>
> 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?
>
> 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?
>
> 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.
>
>
> Cheers,
> Steve
>
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-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM

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