[App_rpt-users] audio for direct VCO FM

[Scott Zimmerman]

> 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?
Simple - Sampling frequency.

While there are audio filters built into the audio chain of the DSP routines, this audio is in a 
digital form, and must be converted to analog before being applied to the modulator. This is done 
inside of the CM119's DAC/Codec. Unfortunately, along with the frequency/amplitude limited audio 
signals, the output of the CM119 contains the audio sampling frequency.

If you look at the output of a "naked" CM119, it's easy to see this distortion, it looks like stair 
steps. Since these stair steps are square waves, they contain harmonics. It's these harmonics and 
sampling frequency that the audio filtering found on the RIM, RIM-Lite, and URI remove. These 
filters roll off the top-end frequency response and therefore remove the stair steps. The tricky 
part is to place the "knee" of the filter at a frequency that sufficiently rolls off the sampling 
frequency and harmonics, yet lets enough high-frequency voice audio though to achieve good fidelity. 
In the case of the RIM, this frequency is set for 10KHz. In teh case of the URI, it's MUCH lower.

You may ask, "Why should I care if there are high-frequency audio components being fed directly to 
my modulator?" Well, *YOU* may not care, but your adjacent channel neighbor and other services sure 
will!! High frequency harmonics in your audio modulation chain create spurs and sideband noise in 
your transmitter. This is an issue even if you are deploying a "transmit only" site. While YOUR 
equipment at a particular site may be only a transmitter, someone else may be trying to use a 
receiver at that site.

There are MUCH better explanations for the necessity of transmitter audio bandwith limiting found 
elsewhere on the web. The purpose of this explanation was to illustrate that when feeding a 
modulator DIRECTLY, you should try to keep your modulating audio clean to be a good neighbor. You 
don't have to worry about this when feeding audio into a radio ahead of the mic/audio processor 
since it takes care of all of this filtering for you.

Scott

Scott Zimmerman
Amateur Radio Call N3XCC
474 Barnett Road
Boswell, PA 15531

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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