Last night I took my Huff-n-Puff driven Master Clock for the Propeller Microprocessor to Jack's Homebrew Amateur Radio Meeting, to compare it with his GPS Disciplined 10MHz Standard (see previous post).
Jack had problem getting his equipment ready, the 10MHz Standard was questionable and he had not built the Phase Comparator/Detector as of yet. We proceeded anyway to look at his and my 10MHz signal on his Dual Trace Scope.
I had previously miss-adjusted my TCVCXO to a slightly lower frequency, so that we could watch the Huff-n-Puff circuit in action. Jack's 10MHz signal was connected to one Scope Channel (and driving the Sync), my signal was on the second channel.
With the Huff-n-Puff still turned OFF, it was obvious that my signal was marching across the Scope screen, right to left (indicating a lower freq). We waited a short period to allow my GPS to be acquire it's satellites, and then I turned ON the Huff-n-Puff function.
Like magic my Scope signal slowed down its march across the screen as the Huff-n-Puff derived TCVCXO correction voltage was being applied. The I2C POT displayed value went from 0 (wiper center) to +12 in about 45 seconds. After a little more settling, it stopped on +14, the two 10MHz signals were almost the same, But not perfect.
At this point Jack's Standard was still suspect, because we could not verify that it was working with the internal GPS receiver and in Sync with the satellites. We were not familiar with the computer program that provided information and therefore unsure. We were just going to assume that it was working.
Jack did not have his detector built, so we did the next best thing. We put the the scope in "Signal Add" mode, so I could count the "in and out" of phase condition by observing the screen.
With Jack's stop-watch, I counted 50 nulls in 141 seconds:
50 / 141 / 10M = 0.0000000354 => 35 ppb
At his point, the best that we could do was; state that we were close in Frequency, by 35 ppb (parts per billion) or 35 x 10^-9 (I think I have the math correct).
This morning I received an email from Jack, stating that he has learned more about his Standard and its operation, and now he thinks his 10MHz Standard was probably working with the GPS and therefore we can assume it to be correct, or within reported/stated accuracy In either case, we will do the experiment again, maybe next month.
If our initial results are correct, then my Huff-n-Puff has Out Performed my expectation by 15 ppb, as suggested on my previous post were I had calculated 50 ppb.
I am a happy camper :-)
I really should rename this circuit, because it is NOT a traditional active/passive Huff-n-Puff circuit, it is mostly in Software (with only a small computer controlled POT). Maybe, I should call it a: Software Defined Huff-n-Puff - or SDHnP !
With this configuration I should be able to provide very accurate WSPR and QRSS signals - That is the GOAL.
The method used to compare the standard with my TCVCXO did not take into account the direction (sign) of the drift between the two, and therefore the "absolute" frequency may be better than the reported 35 ppb. About half of the time my TCVCXO signal appeared to change directions relative to Jack's Standard. More experiments and some research is necessary to understand what was observed.