[Lowfer] GPS frequency lock (and ZL1BPU DDS exciter)

[Dave Brown]

Hi Charles & Lyle-
Something you may like to consider-

When I ran up my ZL1BPU source I had in mind I wanted to lock the symbol
clock back to the TCXO as well- this is stricly speaking NOT necessary as
either the actual RC oscillator frequency can be tweaked or the software can
be used to 'calibrate' out the error (difference between desired freq and
actual)
However- I run my beacon in KISS mode without the software driver- It just
sits in the corner and 'gurgles' away with no connection to the serial port
at all-normally.  So to have accurately timed symbols I chose to use an
additional divider chain to generate the symbol clock directly from the TCXO
output as well.

I use a 74HC390 as a divide by 100 stage driven from the 12.8MHz TCXO output
and it drives the 4060. You can then select a range of symbol clock rates
off the 4060 by selecting the required divider stage output. - I'm using 500
Hz-  and adjust the K parameter (or that for the corresponding beacon mode
command FE) to set the required output baud rate.  You have to do this
latter process anyway-its just that the examples that Murray gives in his
docs are for a symbol clock of 32 Hz, not 500, so the hex values are
different.

A key issue here is that the selected  4060 divider output or symbol clock
gets further divided down in software to generate the final output baud
rate. Depending on the mode etc, the software divisor can range up to 64k,
as specified (in hex) by the K or FE command, so there is a minimum output
baud rate that can be achieved with any selected symbol clock. 120 second
dots can just be 'reached' with a 500 Hz symbol clock but if you wanted to
go for longer dot lengths you would need to select an output further down
the divider chain in the 4060, eg the 256 Hz output.
Depending on what divider outputs you want the 4020 could be used instead of
the 4060. They are both long chain dividers but have different available
outputs down the bottom of the divider chain.

73
Dave
 ZL3FJ

----- Original Message -----
From: "Charles" <cvest@hit.net>
To: <lowfer@mailman.qth.net>
Sent: Tuesday, November 12, 2002 7:55 AM
Subject: Re: [Lowfer] GPS frequency lock (and ZL1BPU DDS exciter)


> Hi Lyle,
>
>    I wish I had waited a bit. My parts should be here today. I have
> purchased the code from Murray too.He said I was the first person in the
US
> to contact him.
>    I am glad there will be others to help me out when I get in over my
> head.I will be looking forward to you getting your parts and I will
probably
> wait until you do to get started building other than the programming
> interface.
> 73...Charlie...COV...
>
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