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