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

[Joe Leikhim]

Does the Quantar give you an option of running the receiver on its own 
reference oscillator? Reason I ask is that Motorola Simulcast don't 
dither the site reference oscillator of the TX to my knowledge. A multi 
channel simulcast system might have 20 trunked stations each with 
different low speed data streams at any moment.


On 5/8/2017 8:17 PM, Bryan Fields wrote:
> 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.
>

-- 
Joe Leikhim


Leikhim and Associates

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