Re: Is there any chance of a GPS locked "Si5351A Synthesizer"
Hans,toggle quoted messageShow quoted text
Agreed on all points.
I did figure out what was causing that periodicity in the frequency - there was a dehumidifier which was turning on and off causing minor changes in air flow and temperature. I thought it was of but I was mistaken.
I still don't understand why all the bumps and squiggles. So, I took a step backwards and started to work forward and be more careful in my tests.
I am using a Vectron 7mm x 5mm oscillator (not TCXO) CDTGCP-27MHz 3.3v 50ppm in lieu of the crystal on the si5351a synthesizer board. 50ppm is a pretty common stability value for these devices. There are 25 and 30 ppm parts available and also quite common and inexpensive, these just happened to be what was easily available at the time. Inexpensive TCXO's are pretty common and typically spec 2.5ppm (a 20x improvement over 50ppm)
First test was to do a reset of the Progrock to return it to a known state and then just applying power and monitor the 27MHz of the oscillator in order to better understand what it is doing (see attached). Started about 30Hz high which quickly decreased as the device (progrock, synthesizer, oscillator) warmed up until stabilizing about 55Hz low. It took about 45 minutes to reach that point and another 25 minutes or so to stabilize even further. After that the frequency remained stable to about +/- 1.5 Hz, remember, this is just the oscillator and is not the programmed output frequency.
This was encouraging so I continued on to step two.
I wanted the synthesizer board to use a 5v supply. A LP2950-3.3v regulator was used in place of the lm317 and supplied with 5v. Pin 18 of the synthesizer was removed to isolate the 3.5V from the progrock board, and the lm317 on the progrock was replaced with a 78l05.
Also, I repogrammed the attiny84 with the latest firmware 1.01a so that I would have the benefit of any improvements in the latest firmware and also on start up would be in a known default condition.
New firmware, regulator mods and all set to go. Programmed to 7400000Hz (a local quite frequency) and using GPS disciplining, monitoring on my GPSDO disciplined Icom R75 receiver and using Spectrum lab to measure and record, the test began.
Start up and skewing to frequency was pretty quick. About 40 minutes after startup, the progrock is seen to make and adjustment, and from then on frequency has slowing increased for a total of about 1.1Hz over 3 hours. 5 hours after startup the frequency is within about 0.2Hz (low) of the target frequency.
Quite impressive actually (well done Hans). Not GPSDO reference frequency stable but very good non the less. And this for a Progrock sitting on my workbench in the basement with a half inch or so of fiberglass insulation over top of the device (dehumidifier has been turned off!).
Test will continue overnight and probably for a couple of days.
You can see the spectrum lab screen grabs here:
there is also a text file with frequency measurements with more detail that the images.
My need for a LO to monitor CHU or WWV carrier doppler requires stability over accuracy. Typically my receiver is offset 1kHz but even that is not critical.
More tests to come. I am going to get some 25 and 27 MHz TCXO's (inexpensive types of about 2.5ppm) and give those a try. I did find on that was 1 ppb (billion) but those small tiny devices are $100+ each.
cheers, Graham ve3gtc
On 2018-10-05 05:11, Hans Summers wrote: