[App_rpt-users] audio for direct VCO FM

[David McGough]

Steve,

Are you wanting to start from scratch building a transmitter, or start
from an existing radio, trimming away unneeded modules?  For VHF-LO FM,
there is a overflow of dirt cheap surplus commercial hardware available
worldwide. I highly recommend using surplus radios for several reasons.  
One of those reasons is if you have a hardware failure, you can have 10
spare 100 watt transmitters on the shelf waiting, rather than just being
down while waiting for repairs.

As for directly modulating a VCO, that is a complex subject, assuming you
need to stay within frequency and bandwidth tolerances, at is typically
required. If you have a lab full of test equipment available, and are
willing to put in the time designing a proper PLL filter, pulling this off
is very do-able, and many have.

Only once the homebrew exciter is working properly with signals from a
clean audio signal generator should you at attach it to AllStar.  The
output spectrum directly from a CM1xy, running at 48KHz sampling rate is
pretty dirty. When you plug a headset into the CM1xy and listen to music,
etc., you'll never notice this noise--the human ear isn't good enough!

However, this noise will modulate a VCO, generating spurs and wide-band
trash;  hence the reason that URI and RIM hardware include audio LP
filters at their outputs. For a review of the CM1xy output spectrum
WITHOUT filtering, take a look at this review. Of particular interest are
the SMPTE and CCIF IMD performance tests, where you'll see loud DAC
"image" related spurs between 30KHz and 96KHz. There are also more 
harmonically-related spurs even further out, at higher frequencies, which 
can't be seen on these Spectrum Analyzer plots:

http://nwavguy.blogspot.com/2011/11/c-media-cm119-dac.html

Okay, now that the basic transmitter is working, fast forward to AllStar.  
You'll have to use the usbradio channel driver, since simpleusb doesn't
implement an output level limiter and perhaps other needed features (like
CTCSS tone generation). Once all the levels are set and checked out, and
the exciter spectrum is clean, you should be ready.

So, from my perspective, this is a basic take on what's needed. All 
do-able, if you've got hours and hours of time, determination, test 
equipment and the engineering skills needed.  This is what the ham radio 
homebrewing spirit is about, of course, and I'm sure it would be a fun 
project.


73, David KB4FXC




On Tue, 31 May 2016, 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
>