Re: GPSDO ubitx ?

Rafael Diniz

So it is worth trying to connect the NEO-7M GPS module (48MHz divided by
4) to the Si5351A and see what happens, as it is already there in the
Raduino. Hopefully it will be good enough to provide a precise and
jitter-free frequency synthesis!


On 11/17/20 12:07 AM, Jerry Gaffke via wrote:
Here's an interesting note regarding a 10mhz reference output 
from a cheap NEO-7M GPS module:
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.

That discussion continues in a couple other threads:

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.

Join to automatically receive all group messages.