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

Joe Leikhim rhyolite at leikhim.com
Mon May 8 17:32:31 EDT 2017


    More on this:

    Exciter Modulation Signals
    Digitized audio/data intended to be transmitted from the station is
    output from the DSP circuitry to a D/A converter via the TX/Voice
    Audio signal (p/o the Serial Synchronous Interface bus, connected
    between the DSP and the DSP ASIC). The digitized signal is converted
    to analog, level shifted and amplified, and fed to a 0-6 kHz filter.
    The output of the filter is then fed to one of the inputs of a
    multiplexer. The output of the multiplexer is fed to two individual
    digitally controlled potentiometers (each of which is adjusted by
    the Host uP via the SPI Bus) and output to the Exciter Module as
    modulation signals VCO MOD AUDIO and REF MOD AUDIO.


On 5/8/2017 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.
>
> 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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-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM

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