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

Joe Leikhim rhyolite at leikhim.com
Mon May 8 17:26:10 EDT 2017


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.

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.

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.

Joe Leikhim


On 5/8/2017 4:33 PM, Bryan Fields wrote:
> I've been following this with some interest.  One of the fundamental 
> issues with the PCGM was the master clock going to the radio was 
> unable to be modulated.  This made low frequency signals (PL) distort, 
> and digital PL was even worse.
>
> Typically most radios use two point modulation, with modulation on the 
> reference oscillator.  When you add an external reference you loose this.
>
> Some base stations are designed to do two point modulation even on an 
> external clock, the Quantar for example.  This station will even phase 
> lock it's PL and DPL generation if provided a 1 PPS in addition to a 
> clock.
>
> If we could inject modulation into the clock generator simulcast 
> performance would vastly improve.  I'd be interested to see your 
> design to see if this would be a possibility.
>
> Thoughts?
>
> --
> Bryan Fields
> M: +1-727-409-1194
>
> On May 8, 2017, at 14:15, Reid Brandon <rfpowerguy at yahoo.com 
> <mailto:rfpowerguy at yahoo.com>> wrote:
>
>> Hi Joe,
>> I got a page with a pic of a man that rotated  around saying DOWNLOAD 
>> NOT FOUND.
>> Try again!?
>> Tks  Reid
>>
>>
>> On Sunday, May 7, 2017 12:25 PM, Pierre Martel <petem001 at gmail.com 
>> <mailto:petem001 at gmail.com>> wrote:
>>
>>
>> I am very interrested in all this project. My email is VE2PF @ VE2MRC 
>> dot com!
>>
>> what radio did you used for the project?
>>
>> Thanks for sharing!
>>
>> Pierre
>> VE2PF
>>
>>
>> 2017-05-07 12:32 GMT-04:00 Kevin Custer <kuggie at kuggie.com 
>> <mailto:kuggie at kuggie.com>>:
>>
>>     Joe,
>>
>>     Why not consider do it as an article for Repeater Builder?
>>
>>     Thanks,
>>     Kevin
>>
>>
>>     On 5/7/2017 3:13 AM, Joe Moskalski wrote:
>>
>>         I wanted to throw this email out to the group for all those
>>         who want to simulcast with the RTCM. For those who don't
>>         know, in order to make the RTCM suitable for simulcast you
>>         must have a more accurate 9.6 MHz clock for the RTCM in order
>>         to ensure the audio sounds acceptable in the overlap areas. I
>>         did this a few months ago using 9.6 MHz OCXO's. This worked
>>         reasonably well. Not as well as I would have liked but the
>>         system was very usable, but of course I wanted it to be
>>         better. Since the PCGM (Programmable Clock Generation Module)
>>         is no longer available, I decided to roll my own as some say.
>>         I created a PLL circuit that can accept a 10 MHz, 1 MHz or
>>         100 KHz reference input and produce a phase locked 9.6 MHz
>>         out for the RTCM. I have been testing this over the past few
>>         days and it has made an great improvement in the audio
>>         quality in the overlap. The quality of the audio in the
>>         overlap areas sounds very, very good, it's very similar to
>>         what I expect to hear in commercial simulcast systems. My
>>         circuit is heavily based off of an old Motorola MSF5000 HSO
>>         PLL circuit that I was able to adapt and modify to my needs. 
>>         There are other uses for this circuit that make it attractive
>>         for simulcasting but I wont get into them here. I am will to
>>         share my schematic and bill of materials (digikey) with
>>         anyone. Please send me an email and I will be willing to
>>         share all of my information on the project.
>>
>>
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-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM

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