At 11:30 AM 11/26/2005, [EMAIL PROTECTED] wrote:
After my post a few days ago I decided to install a TCXO in my SDR1000.
Three days since my last cal, I just checked WWV on 10MHz. I'm seeing one
rotation on the 'Phase' display every 2 seconds or 1/2 Hz. Hard to tell
if it's WWV or me.
Could be either.. ionospheric effects are down in the 1 Hz area... is it a
steady <1Hz offset (i.e. always the same direction) or is it random? DDSes
DO have finite precision.
The classic way to look at this is to use two identical oscillators and
beat them against each other.
I thought this was fantastic until I read Alberto's post. He's not
satisfied with 2 to 3 in 10^11. That's 0.0002 to 0.0003 Hz at 10MHz.
Whew, Alberto what are you doing that needs that accuracy?
If you're working at microwave frequencies, 1 part in 10^10 is 1 Hz at
10GHz, and that might be unacceptable.. I've got a system that measures
the phases of 4 signals (it's a form of interferometer to measure direction
of arrival) and when we were testing, even having the sources and receivers
both locked to the same 10 MHz source, we discovered that there was a
relative offset of a few millihertz, so our phases would steadily drift
around (every 100 minutes or so), and since the original measurement
process assumed it would be static, there was a problem. (This was,
however, an example of bad experiment design)
The TCXO I'm using is a 10MHz unit from a piece of HP test equipment. I
padded the output for 1Vpp and am very happy with the results. Then
again, I don't plan to use it for submarine positioning at this time. :)
Lots of TCXOs on EBay for < $25.
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875
