[App_rpt-users] RTCM Simulcast and 9.6 MHz Phase locking

Bryan Fields Bryan at bryanfields.net
Mon May 8 20:17:11 EDT 2017


On 5/8/17 5:26 PM, Joe Leikhim wrote:
> I think it is a bit more complicated than that. If you dither the reference
> oscillator of a repeater, it will also have an effect on the demodulated
> PL/DPL from the receiver. Some half duplex transceivers indeed apply
> modulation compensation directly to the reference oscillators, and others do
> not, but employ alternative compensation techniques.

Well in this case it would only be applied to the transmitter.  The receiver
really doesn't need a reference to GPS.

This would be a cheap and dirty way to make it work while phase locked to a
GPSDO.

> This from a Quantar Manual
> 
>     Modulation
>     The active VCO receives an audio/data modulation signal from the Station
>     Control Module via two low-pass filters. This modulation signal modulates
>     the active VCO to produce a modulated low-level rf carrier signal.
> 
>     Low-frequency modulation signals (below the loop filter corner) tend to be
>     interpreted by the PLL as VCO frequency error. A modulation compensation
>     signal is added to the PLL _control voltage_ to cancel out this effect and
>     allow for low frequency modulation.

Specific to the Quantar it has an internal 2.1 MHz reference derived from it's
16.4 MHz clock or 5/10 MHz external inputs.  There is a separate reference
from the SCM DSP chip for TX and RX.

On the exciter the custom PLL chip has a dedicated REF_AUDIO input of which it
uses to modulate the divided down reference frequency.  The normal VCO_AUDIO
is injected post filter through a resistor divider into the mod input on the
VCO (UHF1-4) or into a parallel diode (VHF/UHF0/800/900).

I could be wrong on my PLL theory here, but as the chip is a black box, what I
know is based on logic analyzers and probing.  I'm pretty confident it's
modulating the reference frequency.  It's the same chip as what's in the Astro
Spectra.

> The trick I believe, is in the way the PL/DPL is presented to the station and
> if the modulation compensation adjustment is sufficient to track out the PLL's
> tendency to cancel the low frequency tones.

While you can get it close for PL or DPL, digital modes require two point
modulation.

-- 
Bryan Fields

727-409-1194 - Voice
http://bryanfields.net


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