Re: [Flexradio] Ext. Reference clock kit
At 05:53 PM 9/6/2005, ecellison wrote: Jim I knew i could bait you! (smile). I am sort of gobbled up by this precision thing! I do have my GPS receiver and am ready for the 1 part to the -13 (give or take a couple of exponents!). Can we take the 200 mhz standard out of the SDR 1000 as is? Actually I visit WWV frequently with the phase display, from 20 meters, where I been operating, and am actually pretty pleased with the long term accuracy of the radio. Course I fall asleep and leave it on. Eric You can skin that cat a number of ways. One way: pick off a sample of the 200 MHz and run it into a suitable divider/counter widget (like the one Brooke Shera described a few years back). Adjust in software Second way: Get a 200 MHz source that has a steering input and use it, instead of the 10 MHz source in a Z8301 type unit (or Brooke Shera's board). You could drive a divide by 20 with the 200 MHz source and use it in a system designed for 10 MHz unchanged. Third way: Get a high quality 200 MHz phase locked source and lock it to your 10 MHz source. ( you might be able to do this with an HP 8640..and used 8640s are cheaper than brand new 200 MHz phase locked sources) Fourth way: Measure the DDS output frequency against the 1pps or the 10 MHz, and calculate from there. You could either calculate a correction, and retune the DDS (but that might screw up the spur minimization techniques), or feed that into the IF processing in the software. Fifth way: Generate a comb from your stable reference, making sure that the comb spans the frequency bands you'll tune over. In software, find the comb, subtract it out, and use it to calibrate the rest. This is like using a crystal marker generator to calibrate your analog dial. The latter is what I'm doing at work, and I'll have a publically releasable descriptionof the details in a month or so. Suffice it to say today that we calibrate an arbitrary number of free running SDR1Ks and their PC sound cards to several ppb, including phase, especially if temperatures are reasonably stable. 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] Ext. Reference clock kit
At 07:12 PM 9/6/2005, Tom Clark, W3IWI wrote: Eric wrote: Tom et al Youse guys are going to get Jim Lux into this again! (REALLY do love your comments Jim). This is probably a do-able topic for early next year. Hey! Why not long term aging, and Loran. Not kidding. Anything is possible in software with a hardware assist, including Internet NTS corrections. We have our GPS receivers poised. What we need is a designer! You game? We are NOT going for 10 mhz multiplied, but 1 cycle or so at 200 mhz! Try this as a mostly software solution: If you were to feed the 200 MHz xtal into a long counter (i.e. 32 bits, with a total count of 2^32=4.3e9), the counter will wrap every ~ 21.5 seconds (with LSB = 5 nsec). Every so often (frequent enough so that you don't lose track of which 21+ second ambiguity cycle you are on), strobe the count into a register. The idea will be to use the integrated count (including the n*21+ second cycles) to establish the average clock rate. To give you an idea about how good a cheap GPS receiver can be as a clock, surf to http://gpstime.com/ and take a look at my various GPS timing papers (ION 2000 the last VLBI workshop are good starting places) and at Rick's M12+ receiver evaluation. Averaging over as few as 5-10 cycles (a couple of minutes) will tell you the error in the 200 MHz clock frequency to 1:10e10. In the SDR you already have a handle to correct the number cranked into the DDS to achieve a desired signal frequency -- this simply changes the phase increment number cranked into the DDS chips integrators. With only the addition of a simple counter register driven by the 200 MHz xtal (the jitter of it sets the phase noise floor) and a good GPS receiver, you have the ability to steer the master oscillator without the need for D/A converters totally in $oftware. That's exactly what I think is the way to do it. I wouldn't even fool with driving the DDS phase increment. I'd either step the phase offset register, or, more desirable, change the software LO in the final demod/mod. Without a GPS receiver, with a good, stable NTP server, you should be able to keep you computer's clock accurate at the 10-20 msec level. Just using it to grab a counter sample, over 1 day you will have enough resolution to track the 200 MHz error at a level ~1:10e7 (i.e. 10 Hz @ 100 MHz) without even using a GPS receiver. Even the grubbiest GPS receiver, not designed at all for precision timing, will provide timing at the 1 usec level (even when navigating to give a 3D position); combine this with a 15 minute average (~1000 seconds) gets you 1:10e8. I don't know that the 200 MHz clock on the SDR1K is that stable, unless it's in a nice box. It's pretty temperature sensitive (blowing on it is good for a several ppm change). Integrating over long times will take out the aging, but not the short term (few seconds) effects. Switching back and forth from Tx to Rx also changes the frequency over time (possibly due to power supply voltage changes or thermal distribution).. All this only leaves the sound card's sample clock as an undefined local oscillator in the chain and that remains as a fixed frequency offset that you take out as a constant by listening to WWV. This will continue to be an annoyance until someone comes up with an external sampling clock input for the sound card. And that clock is really, really bad and drifts A LOT. My measurements show that the sampling clock variation is about 100-1000 times worse than the 200 MHz DDS reference (for an on-mobo sound interface). Dividing down the 200 MHz to generate a pilot tone into the sound card might not be a bad idea. The other thing is that the sample clock error is a multiplicative effect and the DDS error is an additive frequency error. Consider two tones at, say, 1100 and 2300 Hz. If the sample clock is wrong, they might look like 1000 and 2170 Hz. (the spacing changes) If the DDS clock is wrong, they might look like 1000 and 2200 Hz. (i.e. the spacing is still the same) If all you're doing is pushing it back out to speakers at the same sample rate, the sample rate errors aren't a big problem (everything scales), but if you're doing hertz accurate filtering or demodulation, it's an issue. 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] Ext. Reference clock kit
Tim Ellison wrote: Reflock II info: http://gref.cfn.ist.utl.pt/cupido/reflock.html Cost is going to be near $1000 http://www.tapr.org/products.php You caught us... the Reflock II is going to fund the next TAPR board retreat in the Bahamas. :-) Actually, the prices are placeholders because we don't have them quite finalized yet (and we haven't officially announced availability); in particular, we don't have a final quote from the assembly house for the partially assembled and fully assembled versions. The Reflock has one fine pitch IC that's beyond what most hobbyists will be willing to tackle, so we'll be offering bare board only, complete kit with the IC soldered down, and assembled and tested versions. Think more in the $100-200 range (low end of that for the kit, higher end for assembled and tested). John
Re: [Flexradio] Ext. Reference clock kit
At 07:16 AM 9/7/2005, Tim Ellison wrote: Reflock II info: http://gref.cfn.ist.utl.pt/cupido/reflock.html Cost is going to be near $1000 Whoa, dudes... Are you sure that's not $100? Or is the FPGA some sort of exotic expensive one or is there some proprietary IP used that has a steep license fee? Or is that packaged in a box with a good quality oscillator, etc.? A decent low noise oscillator from Wenzel will set you back a couple hundred bucks at least. http://www.tapr.org/products.php -Tim --- Integrated Technical Services -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Ackermann N8UR Sent: Wednesday, September 07, 2005 9:08 AM To: Jim Lux Cc: FlexRadio@flex-radio.biz; [EMAIL PROTECTED] Subject: Re: [Flexradio] Ext. Reference clock kit All -- Just FYI, TAPR is working not only on the Reflock II, which can phase lock virtually any oscillator with a DC control voltage to either another frequency source, or to a 1pps from a GPS, but also an auxiliary board specifically designed to work with the SDR1000. It will provide a low-phse-noise 100MHz oscillator as well as an optional 10MHz TCXO to serve as reference (if you have a 1pps source, or other frequency standard, you can use that instead). The Reflock II will be shipping within a few weeks (we hope to have them for sale at the ARRL/TAPR Digital Communications Conference in Santa Ana, CA on Sep. 23-25). The SDR-1000 accessory board is just going into prototype but it's a pretty straightforward project so hopefully won't take too long to get ready for production. 73, John Jim Lux wrote: At 05:53 PM 9/6/2005, ecellison wrote: Jim I knew i could bait you! (smile). I am sort of 'gobbled up' by this precision thing! I do have my GPS receiver and am ready for the 1 part to the -13 (give or take a couple of exponents!). Can we take the 200 mhz standard out of the SDR 1000 as is? Actually I visit WWV frequently with the phase display, from 20 meters, where I been operating, and am actually pretty pleased with the long term accuracy of the radio. Course I fall asleep and leave it on. Eric You can skin that cat a number of ways. One way: pick off a sample of the 200 MHz and run it into a suitable divider/counter widget (like the one Brooke Shera described a few years back). Adjust in software Second way: Get a 200 MHz source that has a steering input and use it, instead of the 10 MHz source in a Z8301 type unit (or Brooke Shera's board). You could drive a divide by 20 with the 200 MHz source and use it in a system designed for 10 MHz unchanged. Third way: Get a high quality 200 MHz phase locked source and lock it to your 10 MHz source. ( you might be able to do this with an HP 8640..and used 8640s are cheaper than brand new 200 MHz phase locked sources) Fourth way: Measure the DDS output frequency against the 1pps or the 10 MHz, and calculate from there. You could either calculate a correction, and retune the DDS (but that might screw up the spur minimization techniques), or feed that into the IF processing in the software. Fifth way: Generate a comb from your stable reference, making sure that the comb spans the frequency bands you'll tune over. In software, find the comb, subtract it out, and use it to calibrate the rest. This is like using a crystal marker generator to calibrate your analog dial. The latter is what I'm doing at work, and I'll have a publically releasable descriptionof the details in a month or so. Suffice it to say today that we calibrate an arbitrary number of free running SDR1Ks and their PC sound cards to several ppb, including phase, especially if temperatures are reasonably stable. 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 ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz 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] Ext. Reference clock kit
Jim Lux escribió: No.. you really don't want an oscillator that can be adjusted... What you want is a very finely programmable divider that gets adjusted so that it tracks the oscillator's frequency. For instance, if you have a DDS that puts out 10 MHz from a 200 MHz output, the notional division ratio is 20. If you measure the 200 MHz and it goes up to 200.1, you change the division ratio to 20.01, so the output is still at 10 MHz. You can put a real narrow band filter on the output to clean up the spurs. In fact, you could use a crystal filter (as long as it doesn't drift too far). Of course, if you have a software oscillator to adjust things (like in the SDR1K, with an offset of 11 kHz), you leave the DDS the same, and just change the 11kHz LO in the software downconversion. This kind of thing is done in deep space transponders for radio science, where you want to measure the RF propagation delay to and from the spacecraft very precisely (usually, at two different frequencies, to measure the dispersion). In traditional designs, the crystal oscillator in the transponder is locked to the uplink from the earth (usually derived from a hydrogen maser at DSN), so the downlink signal has the spectral purity from the hydrogen maser, not the XO's basic purity. (obviously, there's some noise added too.. nothing's perfect). So you have a microwave oscillator (usually a DRO, but nowadays, InGaP MMIC VCOs look interesting) that's locked to a nice quiet XO, which in turn is locked to the uplink, which is a hydrogen maser. The Allan deviation from in to out might be 1E-13 integrated over 100 seconds. In new transponder designs (i.e. Electra, as used on recently launched Mars Reconaissance Orbiter, and in the Advanced Deep Space Transponder, still on paper and benchtop breadboards), the tracking of the uplink is done in software with digital NCOs. 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 ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Jim, you make me to remember the good old days at DSN, starting with with Pioneer 6-9 and the follow ons. -- 73 de Ignacio, EB4APL Ignacio Cembreros ___ Este mensaje, incluyendo los ficheros que pudiera llevar como anexos, puede contener información confidencial. Si usted ha recibido este mensaje por error, queda advertido que cualquier uso, difusión o copia están prohibidos legalmente. Le rogamos que nos comunique esta incidencia por la misma vía y proceda a destruir el mensaje inmediatamente. Disculpe las molestias y gracias por su cooperación _
Re: [Flexradio] Ext. Reference clock kit
richard allen wrote: Message (snip) It provides accuracy of better that 1 part in 10**(-10) or 1 Hz at 10 GHz. Its 10 MHz output will drive the little conversion kit for the sdr1k available from FlexRadio. One word of caution. When 10 MHz is fed into the same socket as the present 200 MHz xtal oscillator on the DDS, the DDS is then re-programed to have its internal PLL make the 200 MHz signal. This PLL has poorer short-term stability than the Xtal, which in turn means that the close-in phase noise will increase. This phase noise in turn decreases the dynamic range, and it will probably be unacceptable in a multi-multi contest station! A much better option is to replace the 200 MHz xtal with a clean 100 or 200 MHz source. TAPR is working on a lock box that should work well which would allow you to transer the stability of the GPS or Rubidium clock into the SDR-1000 without causing undue phase noise. 73 de Tom, W3IWI
Re: [Flexradio] Ext. Reference clock kit
Title: Message Tom et al Youse guys are going to get Jim Lux into this again! (REALLY do love your comments Jim). This is probably a do-able topic for early next year. Hey! Why not long term aging, and Loran. Not kidding. Anything is possible in software with a hardware assist, including Internet NTS corrections. We have our GPS receivers poised. What we need is a designer! You game? We are NOT going for 10 mhz multiplied, but 1 cycle or so at 200 mhz! Eric2 AA4SW From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Tom Clark, W3IWI Sent: Tuesday, September 06, 2005 7:46 PM To: FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Ext. Reference clock kit richard allen wrote: (snip) It provides accuracy of better that 1 part in 10**(-10) or 1 Hz at 10 GHz. Its 10 MHz output will drive the little conversion kit for the sdr1k available from FlexRadio. One word of caution. When 10 MHz is fed into the same socket as the present 200 MHz xtal oscillator on the DDS, the DDS is then re-programed to have its internal PLL make the 200 MHz signal. This PLL has poorer short-term stability than the Xtal, which in turn means that the close-in phase noise will increase. This phase noise in turn decreases the dynamic range, and it will probably be unacceptable in a multi-multi contest station! A much better option is to replace the 200 MHz xtal with a clean 100 or 200 MHz source. TAPR is working on a lock box that should work well which would allow you to transer the stability of the GPS or Rubidium clock into the SDR-1000 without causing undue phase noise. 73 de Tom, W3IWI
Re: [Flexradio] Ext. Reference clock kit
I would just echo Tom's comments... you're much better off using the GPS to discipline a 200 MHz source. An even easier way is to put your (quiet) 200 MHz oscillator in a separate box (pref ovenized, but not necessarily), and then count it against the GPS tick (or the 10MHz from your Z3801). Use the counts to adjust the frequency you tune the radio to. My measurements on a box-stock early SDR1K showed frequency changes on the order of 0.1 ppm per second, and if you take out the linear trend, it's about 1% of that. In Hertz, 0.1ppm is 20 Hz on a 200 MHz source. Most of the drift is thermal, so just getting the oscillator away from the radio would help a lot. Philosophically, if you can avoid needing to steer your oscillator, that has two advantages: 1) The tuning pin doesn't provide an additional noise-frequency change path 2) The crystal can be higher Q, because it doesn't need to be adjustable. Nice quiet SC cut crystal is probably what you want, and put it in a reasonably temperature stable box, and then let it age and just track out the aging. Jim At 05:21 PM 9/6/2005, ecellison wrote: Tom et al Youse guys are going to get Jim Lux into this again! (REALLY do love your comments Jim). This is probably a do-able topic for early next year. Hey! Why not long term aging, and Loran. Not kidding. Anything is possible in software with a hardware assist, including Internet NTS corrections. We have our GPS receivers poised. What we need is a designer! You game? We are NOT going for 10 mhz multiplied, but 1 cycle or so at 200 mhz! Eric2 AA4SW From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Tom Clark, W3IWI Sent: Tuesday, September 06, 2005 7:46 PM To: FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Ext. Reference clock kit richard allen wrote: (snip) It provides accuracy of better that 1 part in 10**(-10) or 1 Hz at 10 GHz. Its 10 MHz output will drive the little conversion kit for the sdr1k available from FlexRadio. One word of caution. When 10 MHz is fed into the same socket as the present 200 MHz xtal oscillator on the DDS, the DDS is then re-programed to have its internal PLL make the 200 MHz signal. This PLL has poorer short-term stability than the Xtal, which in turn means that the close-in phase noise will increase. This phase noise in turn decreases the dynamic range, and it will probably be unacceptable in a multi-multi contest station! A much better option is to replace the 200 MHz xtal with a clean 100 or 200 MHz source. TAPR is working on a lock box that should work well which would allow you to transer the stability of the GPS or Rubidium clock into the SDR-1000 without causing undue phase noise. 73 de Tom, W3IWI ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz 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] Ext. Reference clock kit
Jim I knew i could bait you! (smile). I am sort of gobbled up by this precision thing! I do have my GPS receiver and am ready for the 1 part to the -13 (give or take a couple of exponents!). Can we take the 200 mhz standard out of the SDR 1000 as is? Actually I visit WWV frequently with the phase display, from 20 meters, where I been operating, and am actually pretty pleased with the long term accuracy of the radio. Course I fall asleep and leave it on. Eric From: Jim Lux [mailto:[EMAIL PROTECTED] Sent: Tuesday, September 06, 2005 8:38 PM To: ecellison; [EMAIL PROTECTED]; FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Ext. Reference clock kit I would just echo Tom's comments... you're much better off using the GPS to discipline a 200 MHz source. An even easier way is to put your (quiet) 200 MHz oscillator in a separate box (pref ovenized, but not necessarily), and then count it against the GPS tick (or the 10MHz from your Z3801). Use the counts to adjust the frequency you tune the radio to. My measurements on a box-stock early SDR1K showed frequency changes on the order of 0.1 ppm per second, and if you take out the linear trend, it's about 1% of that. In Hertz, 0.1ppm is 20 Hz on a 200 MHz source. Most of the drift is thermal, so just getting the oscillator away from the radio would help a lot. Philosophically, if you can avoid needing to steer your oscillator, that has two advantages: 1) The tuning pin doesn't provide an additional noise-frequency change path 2) The crystal can be higher Q, because it doesn't need to be adjustable. Nice quiet SC cut crystal is probably what you want, and put it in a reasonably temperature stable box, and then let it age and just track out the aging. Jim At 05:21 PM 9/6/2005, ecellison wrote: Tom et al Youse guys are going to get Jim Lux into this again! (REALLY do love your comments Jim). This is probably a do-able topic for early next year. Hey! Why not long term aging, and Loran. Not kidding. Anything is possible in software with a hardware assist, including Internet NTS corrections. We have our GPS receivers poised. What we need is a designer! You game? We are NOT going for 10 mhz multiplied, but 1 cycle or so at 200 mhz! Eric2 AA4SW From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Tom Clark, W3IWI Sent: Tuesday, September 06, 2005 7:46 PM To: FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Ext. Reference clock kit richard allen wrote: (snip) It provides accuracy of better that 1 part in 10**(-10) or 1 Hz at 10 GHz. Its 10 MHz output will drive the little conversion kit for the sdr1k available from FlexRadio. One word of caution. When 10 MHz is fed into the same socket as the present 200 MHz xtal oscillator on the DDS, the DDS is then re-programed to have its internal PLL make the 200 MHz signal. This PLL has poorer short-term stability than the Xtal, which in turn means that the close-in phase noise will increase. This phase noise in turn decreases the dynamic range, and it will probably be unacceptable in a multi-multi contest station! A much better option is to replace the 200 MHz xtal with a clean 100 or 200 MHz source. TAPR is working on a lock box that should work well which would allow you to transer the stability of the GPS or Rubidium clock into the SDR-1000 without causing undue phase noise. 73 de Tom, W3IWI ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz 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] Ext. Reference clock kit
Philosophically, if you can avoid needing to steer your oscillator, that has two advantages: 1) The tuning pin doesn't provide an additional noise-frequency change path 2) The crystal can be higher Q, because it doesn't need to be adjustable. Sounds like an application for the Huff'n'Puff stryleof stabilzer... 73, Lyle KK7P
Re: [Flexradio] Ext. Reference clock kit
Lyle Been out of it for a while. Maybe I'm being duped, but what the heck I s a Huff'n'Puff? !!! Eric -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Lyle Johnson Sent: Tuesday, September 06, 2005 8:55 PM To: FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Ext. Reference clock kit Philosophically, if you can avoid needing to steer your oscillator, that has two advantages: 1) The tuning pin doesn't provide an additional noise-frequency change path 2) The crystal can be higher Q, because it doesn't need to be adjustable. Sounds like an application for the Huff'n'Puff stryleof stabilzer... 73, Lyle KK7P ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz
Re: [Flexradio] Ext. Reference clock kit
Been out of it for a while. Maybe I'm being duped, but what the heck I s a Huff'n'Puff? !!! It is a means of gently stabilizing an oscillator. It requires the oscillator to be reasonably stable already. There are several examples of this sort of circuit, pioneered by PA0KSB back in the early 1970's. See http://www.hanssummers.com/radio/huffpuff/index.htm for some references and links. 73, Lyle KK7P
Re: [Flexradio] Ext. Reference clock kit
Lyle Tnanks ... Readin Eric2 -Original Message- From: Lyle Johnson [mailto:[EMAIL PROTECTED] Sent: Tuesday, September 06, 2005 9:11 PM To: ecellison Cc: FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Ext. Reference clock kit Been out of it for a while. Maybe I'm being duped, but what the heck I s a Huff'n'Puff? !!! It is a means of gently stabilizing an oscillator. It requires the oscillator to be reasonably stable already. There are several examples of this sort of circuit, pioneered by PA0KSB back in the early 1970's. See http://www.hanssummers.com/radio/huffpuff/index.htm for some references and links. 73, Lyle KK7P
Re: [Flexradio] Ext. Reference clock kit
Eric wrote: Message Tom et al Youse guys are going to get Jim Lux into this again! (REALLY do love your comments Jim). This is probably a do-able topic for early next year. Hey! Why not long term aging, and Loran. Not kidding. Anything is possible in software with a hardware assist, including Internet NTS corrections. We have our GPS receivers poised. What we need is a designer! You game? We are NOT going for 10 mhz multiplied, but 1 cycle or so at 200 mhz! Try this as a mostly software solution: If you were to feed the 200 MHz xtal into a long counter (i.e. 32 bits, with a total count of 2^32=4.3e9), the counter will "wrap" every ~ 21.5 seconds (with LSB = 5 nsec). Every so often (frequent enough so that you don't lose track of which 21+ second ambiguity cycle you are on), strobe the count into a register. The idea will be to use the integrated count (including the n*21+ second cycles) to establish the average clock rate. To give you an idea about how good a cheap GPS receiver can be as a clock, surf to http://gpstime.com/ and take a look at my various GPS timing papers (ION 2000 the last VLBI workshop are good starting places) and at Rick's M12+ receiver evaluation. Averaging over as few as 5-10 cycles (a couple of minutes) will tell you the error in the 200 MHz clock frequency to 1:10e10. In the SDR you already have a "handle" to correct the number cranked into the DDS to achieve a desired signal frequency -- this simply changes the phase increment number cranked into the DDS chips integrators. With only the addition of a simple counter register driven by the 200 MHz xtal (the jitter of it sets the phase noise floor) and a good GPS receiver, you have the ability to "steer" the master oscillator without the need for D/A converters totally in $oftware. Without a GPS receiver, with a good, stable NTP server, you should be able to keep you computer's clock accurate at the 10-20 msec level. Just using it to grab a counter sample, over 1 day you will have enough resolution to "track" the 200 MHz error at a level ~1:10e7 (i.e. 10 Hz @ 100 MHz) without even using a GPS receiver. Even the grubbiest GPS receiver, not designed at all for precision timing, will provide timing at the 1 usec level (even when navigating to give a 3D position); combine this with a 15 minute average (~1000 seconds) gets you 1:10e8. All this only leaves the sound card's sample clock as an undefined "local oscillator" in the chain and that remains as a fixed frequency offset that you take out as a constant by listening to WWV. This will continue to be an annoyance until someone comes up with an external sampling clock input for the sound card. Now for a really hair-brained extension: The sound card clock offset could be removed in the middle of the passband if, instead of using a ~11 kHz "IF" as the offset of the 1st LO, you were do a true "zero IF" receiver, and let the FFT handle suppression of the unwanted sideband. I get the impression that the Delta-44 is close enough to being an "ideal" A/D converter that the "DC" hole in the middle of the passband might not be such a problem. If you can go to a "zero IF" receiver, then any error in the sound card's sampling frequency will only effect the apparent offset signals away from the "DC" middle of the passband. It's fun to speculate on these ideas! 73, Tom
