[App_rpt-users] RTCM Simulcast and 9.6 MHz Phase locking
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
> This from a Quantar Manual
> 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
> 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
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