Re: GPSDO ubitx ?


Jerry Gaffke
 

Here's an interesting note regarding a 10mhz reference output 
from a cheap NEO-7M GPS module:   

https://groups.io/g/HBTE/topic/69599658#699
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It is worth noting that the module only produces a "nice" output at divisions of it's internal reference (48MHz). At any other The output is erratic. A frequency counter (which only counts edges) will show the correct frequency (e.g 10MHz) but a spectrum analyser will show the truth, and an oscilloscope will exhibit multiply triggering.
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That discussion continues in a couple other threads:
https://groups.io/g/HBTE/topic/69627276
https://groups.io/g/HBTE/topic/69910563

Programming the NEO-7M for 24mhz and 12mhz is done with an even divide by 4, and so creates a clean clock.
Other divide ratios use a dual modulus divider, and thus the clock has lots of jitter.
Another issue, that clock apparently comes from an internal crystal oscillator, it is occasionally disturbed
by corrections to keep the edge counts agreeing with what is coming in from the GPS satellite.
So edge counts over a second or so are very accurate, but as an RF source the NEO-7M is rather noisy.
That may or may not be suitable for use as an Si5351 reference, depends on what SiLabs did
with regard to the loop filter on their VCO.

The Si5351A in the MSOP10 would probably work fine with a 24mhz reference clock
into one of the crystal pins.  The datasheet says 25 to 27mhz, but 24mhz is pretty close.
And it actually does work with a reference of 10mhz, but the Si5351 outputs have more jitter.
The Si5351C does have a dedicated CLKIN pin that is spec'd for a much wider frequency range.

There are better GPS modules than the very cheap NEO-7M that do give a clean reference clock.

Jerry, KE7ER


uple other threads of interest (a continuation of the discussion


On Mon, Nov 16, 2020 at 06:01 PM, Rafael Diniz wrote:
That makes total sense. I have some affordable GPSDO with 10MHz output I
got from ebay. I feel your suggestion might be the best solution, of
course, at the cost of re-doing the Raduino and adding (may be in the
same Raduino board?) the GPSDO with 10MHz output already connected to
the refclk pin of the Si5351C, plus a new SMA connector for the GPS antenna.

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