Re: [Flexradio] ARRL FMT: one more win for the SDR-1000
At 05:58 AM 1/19/2006, John Ackermann N8UR wrote: Jim Lux wrote: Of course... the SDR1000 does have an advantage because it's a very simple receiver with fewer stages to screw things up. While the SDR-1000 can do a credible job on its own, I just want to reinforce that anyone with a signal generator that's locked to a decent reference, and a sound card, can measure to small fractions of a Hertz using the delta reference method I describe at http://www.febo.com/time-freq/fmt/technique.html. You don't need a lab full of expensive gear to do it. Indeed.. The two accuracy limiting things in John's scheme that I see are: 1) Group delay variations across the band in the presence of any drift. If the unknown and the reference move across the passband, and there's some group delay ripple, the relative phase will change between the tones. If you're integrating for 10 seconds, a phase change of 360 degrees would be a 0.1 Hz error. THis might be an area where older receivers using a long string of tuned LC networks for selectivity would be at disadvantage (because each section flips the phase 180 over the passband, and they all add up). You also clearly don't want that exotic 10 Hz CW crystal filter in the circuit. 2) Uncertainty in the sampling clock of the audio card. I suppose that one can calculate this by measuring the frequency of the known reference at two frequencies. A lot of signal generators have a way to put an AM modulation on the signal with precisely controlled frequency, and that would give you two marks, say, 400 Hz apart, with which to calibrate the FFT box. Remember that frequency measurement in the HF range doesn't really require a super-accurate source -- 1x10e-8 gets you down to 0.1Hz at 10MHz. That level of accuracy can easily be met with a $100 surplus ovenized oscillator (like an HP 10811A or the older 10544), a GPS receiver, and a $100 surplus counter like the HP 5334 (my personal low-cost favorite)*. And that's assuming you're fairly fumble-fingered. With a bit of care, you can set to at least one, and maybe two, orders of magnitude better than that (though the aging rate will limit how long that accuracy holds). The GPS receiver needs a 1pps output of some sort. (I tried this with a serial output only once, and while the timing of the messages was reasonably stable, it wasn't great: about 1ms uncertainty) Jim
Re: [Flexradio] ARRL FMT: one more win for the SDR-1000
Jim Lux wrote: At 05:58 AM 1/19/2006, John Ackermann N8UR wrote: Jim Lux wrote: Of course... the SDR1000 does have an advantage because it's a very simple receiver with fewer stages to screw things up. While the SDR-1000 can do a credible job on its own, I just want to reinforce that anyone with a signal generator that's locked to a decent reference, and a sound card, can measure to small fractions of a Hertz using the delta reference method I describe at http://www.febo.com/time-freq/fmt/technique.html. You don't need a lab full of expensive gear to do it. Indeed.. The two accuracy limiting things in John's scheme that I see are: 1) Group delay variations across the band in the presence of any drift. If the unknown and the reference move across the passband, and there's some group delay ripple, the relative phase will change between the tones. If you're integrating for 10 seconds, a phase change of 360 degrees would be a 0.1 Hz error. THis might be an area where older receivers using a long string of tuned LC networks for selectivity would be at disadvantage (because each section flips the phase 180 over the passband, and they all add up). You also clearly don't want that exotic 10 Hz CW crystal filter in the circuit. Yup. I normally use a fairly wide bandwidth; in fact, for the last test, I used the widest 3100Hz bandwidth in the 3586C selective level meter; good thing I did, too, as I mistuned on a broadcast hetrodyne on 40M, and only after the fact did I find W1AW 1kHz away, but still in the bandpass. 2) Uncertainty in the sampling clock of the audio card. I suppose that one can calculate this by measuring the frequency of the known reference at two frequencies. A lot of signal generators have a way to put an AM modulation on the signal with precisely controlled frequency, and that would give you two marks, say, 400 Hz apart, with which to calibrate the FFT box. We've been around the soundcard clock issue several times on the list. An advantage of the delta reference method is that you don't care about the absolute frequency, only the delta between two closely spaced frequencies. So the clock error is applied to a smaller number (the delta between the two tones) and thus has less absolute impact than if you were measuring absolute frequency. My measurements of the Delta 44 card I use shows that its clock error is in the noise for this test. Remember that frequency measurement in the HF range doesn't really require a super-accurate source -- 1x10e-8 gets you down to 0.1Hz at 10MHz. That level of accuracy can easily be met with a $100 surplus ovenized oscillator (like an HP 10811A or the older 10544), a GPS receiver, and a $100 surplus counter like the HP 5334 (my personal low-cost favorite)*. And that's assuming you're fairly fumble-fingered. With a bit of care, you can set to at least one, and maybe two, orders of magnitude better than that (though the aging rate will limit how long that accuracy holds). The GPS receiver needs a 1pps output of some sort. (I tried this with a serial output only once, and while the timing of the messages was reasonably stable, it wasn't great: about 1ms uncertainty) Yes, I should have said that. The good news is that there are lots of old Oncore receivers available for a pittance that will do the job. 73, John
[Flexradio] Fwd: Re: ARRL FMT: one more win for the SDR-1000
Date: Thu, 19 Jan 2006 10:24:46 -0800 From: Jan Tarsala [EMAIL PROTECTED] Subject: Fwd: Re: [Flexradio] ARRL FMT: one more win for the SDR-1000 X-Sender: [EMAIL PROTECTED] To: [EMAIL PROTECTED] Cc: FlexRadio@flex-radio.biz X-Source-IP: jtarsala.jpl.nasa.gov [137.78.41.62] X-Source-Sender: [EMAIL PROTECTED] X-AUTH: Internal IP Original-recipient: rfc822;[EMAIL PROTECTED] 2) Uncertainty in the sampling clock of the audio card. I suppose that one can calculate this by measuring the frequency of the known reference at two frequencies. A lot of signal generators have a way to put an AM modulation on the signal with precisely controlled frequency, and that would give you two marks, say, 400 Hz apart, with which to calibrate the FFT box. RMK, The soundcard program I used, Argo, permits a frequency calibration coefficient to be entered. I used the 1800 Hz third harmonic of the WWV/WWVH 600 Hz tone as demodulated by the TS-870S in its AM mode as a calibration standard. When ionospheric fades null the carrier, the demodulator distortion peaks and makes the audio third harmonic visible in the Argo spectrogram. In the end, as you have said, you have to be sensitive to ionospheric fluctuations and to snapshot the data during those intervals when propagation conditions are most stable. This takes good judgement. In the past I used Digipan and watched the vectorscope display as an indicator of path stability. However, Digipan only displays its tracking loop NCO to 100 mHz, so this year I wanted to evaluate my technique with Argo reading out its FFT to 10 mHz. My results from previous FMTs on each of two bands were -28.4 ppb and +14.2 ppb in 2002, and -55.8 ppb and +28.4 ppb in 2003. This year my result was +17.5 ppb on the only signal I could hear which was on 75 m. All these errors are commensurate with the frequency granularity of the NCOs used in the TS-870S. Since ionospheric Doppler fundamentally limits the quality of your measurement, a better FMT handicap amongst stations would be based on number of hops (i.e. distance) from Newington. 73, de VRN 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
Re: [Flexradio] Fwd: Re: ARRL FMT: one more win for the SDR-1000
Jim Lux wrote: The soundcard program I used, Argo, permits a frequency calibration coefficient to be entered. I used the 1800 Hz third harmonic of the WWV/WWVH 600 Hz tone as demodulated by the TS-870S in its AM mode as a calibration standard. When ionospheric fades null the carrier, the demodulator distortion peaks and makes the audio third harmonic visible in the Argo spectrogram. Argo is a really excellent tool; one of the guys who does the multi-multi FMT with me uses it and it has some very cool features for this application. And cool idea about using the distortion to get a calibration source! John
Re: [Flexradio] Fwd: Re: ARRL FMT: one more win for the SDR-1000
John Probably need to incorporate all these excellent ideas on your Febo! Eric -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Ackermann N8UR Sent: Thursday, January 19, 2006 3:20 PM To: Jim Lux Cc: FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Fwd: Re: ARRL FMT: one more win for the SDR-1000 Jim Lux wrote: The soundcard program I used, Argo, permits a frequency calibration coefficient to be entered. I used the 1800 Hz third harmonic of the WWV/WWVH 600 Hz tone as demodulated by the TS-870S in its AM mode as a calibration standard. When ionospheric fades null the carrier, the demodulator distortion peaks and makes the audio third harmonic visible in the Argo spectrogram. Argo is a really excellent tool; one of the guys who does the multi-multi FMT with me uses it and it has some very cool features for this application. And cool idea about using the distortion to get a calibration source! John ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
