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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Hi, just trying to calibrate this finicky beast of a frequency counter.
It was last calibrated in 1996 to +/- 0.5 ppm, but as you can see its well out compared to a known reference. I checked it against the (probably correct) 28.8 MHz oscillator which it read at 27.798 MHz but according to its 10MHz output its OCXO is oscillating at around 10 MHz. After calibrating (used Island FM as precise frequency source) and adjusting capacitor on the SDR to centre it the adjustment appears to have worked although it is noticeably RF noisy due to lack of shielding. The trace taken was from across the room, with the stock antenna I got from Arcadia Express. Note, had to use a harmonic as the signal on the SDR is non existent below 28 MHz due to an apparent clocking issue.
I probably do need to shield this, just putting my phone near it sends the frequency count all over the place. On the flip side at least the calibration is better than it was, reading 27.997 MHz on the SDR now.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
For LF, the RTL boards must be altered to do direct sampling, or use an up-converter. (a modified $10 dongle is usually a better choice, but note its own oscillator is usually a normal 10ppm xtal)
The LimeSDR scans down to 100kHz, but note the non-linearity of the front end still needs profiled. (I may write a review of the board in a few months, as I do intend to test if it can indeed be used as a wide-band VNA)
Frequency counters typically drift as they age, as most early models do not contain an auto-calibration mode.
For calibrated time references, most people will end up using GPS disciplined 10.000000MHz OCXO. (pre-made Trimble based GPSDO with 50ohm SMA outputs are about $200 on ebay)
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Callibration of frequencey sources interrested me, I had an ex-cellular radio network rubidium oscillator with 10 MHz output. (there were a lot of FE5680s cheap on eBay) Comparing the output to that of a friends gps-disciplined 10 MHz oscillator using Lissajous figures on a 'scope. From memory, an ex-service unit was better than 0.02 ppm without re-callibration.
My next best reference was 198 kHz Droitwitch, a.m. audio but also p.m. data at times, it averages out.
If you have a quartz crystal clock or watch that you have noticed to be accurate, i.e.needs little adjusting, it is surprisingly accurate. e.g. 1 ppm = 31.6 seconds/year if desperate for a reference you could use a quartz clock oscillator and 'callibrate' it using a time signal, and patience.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Heh, thanks. How come you got rid of it? I was actually after one of these for my time experiments. EDIT: managed to solve the problem, can actually get a Rb module for under £100. Or the other method is simply use the GSM frequency to centre a high precision OCXO as this is apparently atomic locked.
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Calibrate OCXO?
