The NEO-7M output is not great, but PLL's can be surprisingly forgiving.
Due to the divide ratio in the PLL, the phase detector is not getting updated very often.
And the low pass filter between the phase detector and the VCO smooths it out further.
So the Si5351 might work fine when driven from the NEO-7M's? 24mhz.
But attempting to filter that dirty 24mhz with an LC filter might make matters much worse,
as I believe the output would drop out entirely for awhile with the occasional
180 degree phase change.
Creating 30mhz using a 5x LC filter on a dirty 48/8=6mhz square wave would have?
occasional phase shifts in the 30mhz of 5*360/8 = 225 degrees, which could also
be problematic.
So more to explore, but this is turning into a major rabbit hole.
Maybe a 30mhz TCXO is good enough.
Or a GPS module with a clean 10mhz (or 30mhz) output.
Or an Si5351 disciplined from the pps signal from a GPS.
Or possibly, an Si5351 driven from the dirty 24mhz?
Jerry, KE7ER
On Mon, Jan 13, 2020 at 02:14 AM, <erik@...> wrote:
Some further analysis on high frequency the output of the NEO-7M
The NEO-7M creates its high frequency output by setting the count of the high and the low output in 48MHz ticks.
The first divider can thus be minimally be a divide by 2. (one 48MHz tick high and one tick low) and thus the 24MHz output is clean.
All other frequencies are created by dynamically adapting the high and low count as in a fractional divider to keep in sync with the requested frequency.
If this can be done with a stable high and low count you get little jitter (only the adapting to keep in sync with the GPS)
An example is 16MHz with one count high (or low) and 2 counts low (or high) or the 12MHz with 2 high and 2 low
If there is not stable high/low count the counts are adapted continuously to get to the required output.
And example is 10MHz which is between a divide by 4 and divide by 5 so every couple of output pulses the? pulse is stretched with one 48MHz count to get the divide by 4.8
This suggests that using the 24MHz output as reference for a SI5351 could work but if the XCO of the NEO-7M is a tiny bit high so you get from time to time stretching of the high or low phase output to keep in sync with the GPS.
But this pulling will knock the SI5351 PLL out of phase so it always will be very visible in the output of the SI5351
There is no way for the NEO-7M to keep the 24MHz in sync of the XCO is too low.
It is clear that the larger the high/low count the cleaner the output of the NEO-7M will be.
