Electronics > Metrology

Homebrew GSPDO Questions

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Wallace Gasiewicz:
I kinda built this:[attach=1]

Only used a Navman Jupiter 12 and a Isotemp 118 12   OCXO (temp controlled frequency)
I had this stuff laying around.
I used an old Sony brick PS also (more junk)

It appears to work, the 10 KHz square waves from the Navman and the divided down 10 KHz from the OCXO line up.
It produces a lot of usable harmonics up to maybe 500 MHz also.

The square waves viewed on my scope were 12 uSec apart when it was cooler in the room and now they are 14 uSec apart. The alignment appears quite stable over days, I do not have time graphing capabilities at this time.
After they line up the output appears stable also.

What Phenomenon causes the Sq Waves to line up and stop moving relative to one another?
Is it "An odd kind of Sympathy?"
I confess I know just about enough to get into trouble.


--- Quote ---What Phenomenon causes the Sq Waves to line up and stop moving relative to one another?
--- End quote ---

See attached pict
The XOR Gate compares the 10KHz from the Jupiter to the 10KHz out from the PIC , and controls the OCXO's EFC


Wallace Gasiewicz:
Yea I know the XOR compares the two signals, but what makes it STOP.
Its not like the two signals are exactly on top of one another. If they were there would be no control voltage. No XOR output.

The short answer is: You've built a PLL, and it "locks" the VCXO to the GPS.

A slightly longer answer is that the XOR gate acts as a phase discriminator. It outputs a PWM that represents the phase difference between the two 10kHz signals. This PWM is then low-pass filtered (this is your loop filter) and applied to the EFC input of the VCXO. While the frequencies are not aligned, the EFC "wobbles", and so does the output frequency of the VCXO. When the frequencies are identical, the wobbling stops. The phases are now locked to each other, the phase angle is determined by the propagation delays of the XOR gate and the PLL filter.

EDIT: Strike that. The phase angle is determined by the tuning voltage required to bring the divided-down VCXO to the same frequency. Every change in phase (e.g. due to frequency difference) will cause a correction of the frequency, bringing it back into lock.

Wallace Gasiewicz:

I get that. I know that it is like a PLL.
So any minor change in the phase difference, which here is really a very minor change in freq difference, causes a different control voltage and hence changes the freq of the XO.  I get that.
I just cannot wrap my head around why it stops at equal freq. Is it because it really never completely stops but keeps readjusting at a undiscernable (at least to me) rate?
This is why I used a temp controlled oscillator, I figured the wobble or phase shifting would be less over the short term verses a varicap VCXO.


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