Re: [Flexradio] Frequency stability and calibration
Tom First let me say thanks for this message. I have already spent many hours following and reading much of the stuff on the links, you have provided with great enjoyment. I really like the cursor follower clock! Where can I get it for my local machine! Also thanks to you, Rick Hamby, Bob N4HY TvB, Frank Brickle and quite a few others for giving of your richness of knowledge and skill! It is a thrill to read! I wish I had been there for your presentations! I especially enjoyed the inspired design of the pic slaved to the 10 mhz time source! In a word Elegant! I am having trouble with your 2. below. I cant locate the files to defeat the calibration signals on the Jupiter. Where are they on gpstime.com? Thanks Eric2 From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Tom Clark, W3IWI Sent: Thursday, November 24, 2005 1:30 AM To: Jim Lux Cc: [EMAIL PROTECTED]; FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Frequency stability and calibration Jim Lux wrote: There ARE actually sources with better close in phase noise than a quartz crystal, just in case you see one at a hamfest or surplus place (or, you're wealthy enough). A hydrogen maser, for instance (that's what we use at work, JPL, when we're concerned about such things.. but then we have an infrastructure to distribute the maser signal around, and a budget for the support staff). Actually, all H-Maser I know rely on a really high quality xtal for their short-term stability (and hence intrinsic phase noise); by high quality, I mean BVA xtal units costing in the $5k range. The transition from the BVA xtal to the maser is typically done at times ~30-100 seconds or so (see the AVARs in my tutorials I mention later, or http://www.leapsecond.com/museum/manyadev.gif to see that the BVA performance is better than the Maser up to ~30 seconds. The goal is to hand off from one oscillator to the next when their Allen deviation is equal). BTW -- we actually have a couple of amateurs that have both passive active H-Masers in their basements. One is Tom vanBaak (no call) whose efforts can be viewed at http://leapsecond.com/ and another is Jim Jaeger (K8RQ) (see http://www.clockvault.com/ if you can stand the music!). TvB offered a review paper on amateur timekeeping at the 2003 PTTI meeting, which can be fetched at http://tycho.usno.navy.mil/ptti/ptti2003/paper35.pdf. Also be sure to note TvB's Most Accurate WristWatch when you log onto leapsecond.com. I've said it before, and I'll repeat it now -- you are better off thinking in the frequency (and phase noise) domain when you are considering oscillators on time scales shorter than tens of seconds, and in the time domain for minutes and longer. If you are interested in these topics, you might want to fetch one of my Timing for VLBI tutorials at http://gpstime.com/ . In my past incarnation I ran NASA's Geodetic VLBI program and was responsible for H-masers as time and frequency standards. While I am on here making comments on this thread, I note that Alberto, I2PHD is using a circuit similar to the one I developed for locking an xtal to the 10 kHz output from the Connexant/Navman Jupiter-T receiver. A couple of notes on what I found: My initial effort also used 74HC390 dividers as a ripple counter to get from 10 MHz - 10 kHz. But I found that the propagation delay thru these dividers varied strongly with temperature, amounting to a couple of hundred nsec in a day. I fixed this problem by using a simple, but elegant circuit developed by Tom van Baak (see http://www.leapsecond.com/tools/ppsdiv.zip) which uses a PIC with its clock input driven by the 10 MHz signal and a finite state machine that executes a fixed number of instructions to generate lower frequencies. Not only is it a very stable synchronous divider, but also it need only a couple of $$ worth of parts. I did a lot of work to optimize loop time constants to try to achieve performance at the couple of nsec levels. Most of the time, the Jupiter-T steered the oscillator very well, but about once per hour, the 10 kpps (and 1pps) output sawtooth goes thru a zero-beat, with a fixed bias error spanning intervals of 10s of seconds. You can see some of these sawtooth hanging bridges that really screw up the locking in my tutorials on gpstime.com. And you can see the fix that Rick (W2GPS) is using in his latest CNS clock using the M12+ in the latest of the gpstime.com tutorials. 73, Tom
Re: [Flexradio] Frequency stability and calibration
Jim Lux wrote: There ARE actually sources with better close in phase noise than a quartz crystal, just in case you see one at a hamfest or surplus place (or, you're wealthy enough). A hydrogen maser, for instance (that's what we use at work, JPL, when we're concerned about such things.. but then we have an infrastructure to distribute the maser signal around, and a budget for the support staff). Actually, all H-Maser I know rely on a really high quality xtal for their short-term stability (and hence intrinsic phase noise); by high quality, I mean BVA xtal units costing in the $5k range. The transition from the BVA xtal to the maser is typically done at times ~30-100 seconds or so (see the AVARs in my tutorials I mention later, or http://www.leapsecond.com/museum/manyadev.gif to see that the BVA performance is better than the Maser up to ~30 seconds. The goal is to "hand off " from one oscillator to the next when their Allen deviation is equal). BTW -- we actually have a couple of "amateurs" that have both passive active H-Masers in their basements. One is Tom vanBaak (no call) whose efforts can be viewed at http://leapsecond.com/ and another is Jim Jaeger (K8RQ) (see http://www.clockvault.com/ if you can stand the music!). TvB offered a review paper on amateur timekeeping at the 2003 PTTI meeting, which can be fetched at http://tycho.usno.navy.mil/ptti/ptti2003/paper35.pdf. Also be sure to note TvB's "Most Accurate WristWatch" when you log onto leapsecond.com. I've said it before, and I'll repeat it now -- you are better off thinking in the frequency (and phase noise) domain when you are considering oscillators on time scales shorter than tens of seconds, and in the time domain for minutes and longer. If you are interested in these topics, you might want to fetch one of my "Timing for VLBI" tutorials at http://gpstime.com/ . In my past incarnation I ran NASA's Geodetic VLBI program and was responsible for H-masers as time and frequency standards. While I am on here making comments on this thread, I note that Alberto, I2PHD is using a circuit similar to the one I developed for locking an xtal to the 10 kHz output from the Connexant/Navman Jupiter-T receiver. A couple of notes on what I found: My initial effort also used 74HC390 dividers as a ripple counter to get from 10 MHz - 10 kHz. But I found that the propagation delay thru these dividers varied strongly with temperature, amounting to a couple of hundred nsec in a day. I fixed this problem by using a simple, but elegant circuit developed by Tom van Baak (see http://www.leapsecond.com/tools/ppsdiv.zip) which uses a PIC with its clock input driven by the 10 MHz signal and a finite state machine that executes a fixed number of instructions to generate lower frequencies. Not only is it a very stable synchronous divider, but also it need only a couple of $$ worth of parts. I did a lot of work to optimize loop time constants to try to achieve performance at the couple of nsec levels. Most of the time, the Jupiter-T steered the oscillator very well, but about once per hour, the 10 kpps (and 1pps) output sawtooth goes thru a zero-beat, with a fixed bias error spanning intervals of 10s of seconds. You can see some of these sawtooth "hanging bridges" that really screw up the locking in my tutorials on gpstime.com. And you can see the fix that Rick (W2GPS) is using in his latest CNS clock using the M12+ in the latest of the gpstime.com tutorials. 73, Tom
Re: [Flexradio] Frequency stability and calibration
Tom I ran across TvB on one of my time forays off the FlexRadio Forum last year. This guy is amazing! I probably spent 8 hours reading about his shack and what he has done to get many stabilized time sources, and wandering around these websites. Talk about dedication to one hobby and becoming a master! WOW. All of this is worth the read folks! If just to marvel at the work and dedication. Eric From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Tom Clark, W3IWI Sent: Thursday, November 24, 2005 1:30 AM To: Jim Lux Cc: [EMAIL PROTECTED]; FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Frequency stability and calibration Jim Lux wrote: There ARE actually sources with better close in phase noise than a quartz crystal, just in case you see one at a hamfest or surplus place (or, you're wealthy enough). A hydrogen maser, for instance (that's what we use at work, JPL, when we're concerned about such things.. but then we have an infrastructure to distribute the maser signal around, and a budget for the support staff). Actually, all H-Maser I know rely on a really high quality xtal for their short-term stability (and hence intrinsic phase noise); by high quality, I mean BVA xtal units costing in the $5k range. The transition from the BVA xtal to the maser is typically done at times ~30-100 seconds or so (see the AVARs in my tutorials I mention later, or http://www.leapsecond.com/museum/manyadev.gif to see that the BVA performance is better than the Maser up to ~30 seconds. The goal is to hand off from one oscillator to the next when their Allen deviation is equal). BTW -- we actually have a couple of amateurs that have both passive active H-Masers in their basements. One is Tom vanBaak (no call) whose efforts can be viewed at http://leapsecond.com/ and another is Jim Jaeger (K8RQ) (see http://www.clockvault.com/ if you can stand the music!). TvB offered a review paper on amateur timekeeping at the 2003 PTTI meeting, which can be fetched at http://tycho.usno.navy.mil/ptti/ptti2003/paper35.pdf. Also be sure to note TvB's Most Accurate WristWatch when you log onto leapsecond.com. I've said it before, and I'll repeat it now -- you are better off thinking in the frequency (and phase noise) domain when you are considering oscillators on time scales shorter than tens of seconds, and in the time domain for minutes and longer. If you are interested in these topics, you might want to fetch one of my Timing for VLBI tutorials at http://gpstime.com/ . In my past incarnation I ran NASA's Geodetic VLBI program and was responsible for H-masers as time and frequency standards. While I am on here making comments on this thread, I note that Alberto, I2PHD is using a circuit similar to the one I developed for locking an xtal to the 10 kHz output from the Connexant/Navman Jupiter-T receiver. A couple of notes on what I found: My initial effort also used 74HC390 dividers as a ripple counter to get from 10 MHz - 10 kHz. But I found that the propagation delay thru these dividers varied strongly with temperature, amounting to a couple of hundred nsec in a day. I fixed this problem by using a simple, but elegant circuit developed by Tom van Baak (see http://www.leapsecond.com/tools/ppsdiv.zip) which uses a PIC with its clock input driven by the 10 MHz signal and a finite state machine that executes a fixed number of instructions to generate lower frequencies. Not only is it a very stable synchronous divider, but also it need only a couple of $$ worth of parts. I did a lot of work to optimize loop time constants to try to achieve performance at the couple of nsec levels. Most of the time, the Jupiter-T steered the oscillator very well, but about once per hour, the 10 kpps (and 1pps) output sawtooth goes thru a zero-beat, with a fixed bias error spanning intervals of 10s of seconds. You can see some of these sawtooth hanging bridges that really screw up the locking in my tutorials on gpstime.com. And you can see the fix that Rick (W2GPS) is using in his latest CNS clock using the M12+ in the latest of the gpstime.com tutorials. 73, Tom
Re: [Flexradio] Frequency stability and calibration
Tom Really neat stuff. Would love to hear you give the 2005 PPT presentation. What did happen on 09/07/02? Looks to my naked eye, that raw data from GPS is plenty accurate for our purposes as you pose in one of the slides. The flying cursor clock and nixies on gpstime.com is the neatest thing Ive seen in a long time! -pun Thanks for the links. I dont understand all of it but is enjoyable to try to figure it all out! Bob mentioned using pic slaved to the osc, the other night on Teamspeak. REALLY clever idea.! Eric From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Tom Clark, W3IWI Sent: Thursday, November 24, 2005 1:30 AM To: Jim Lux Cc: [EMAIL PROTECTED]; FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Frequency stability and calibration Jim Lux wrote: There ARE actually sources with better close in phase noise than a quartz crystal, just in case you see one at a hamfest or surplus place (or, you're wealthy enough). A hydrogen maser, for instance (that's what we use at work, JPL, when we're concerned about such things.. but then we have an infrastructure to distribute the maser signal around, and a budget for the support staff). Actually, all H-Maser I know rely on a really high quality xtal for their short-term stability (and hence intrinsic phase noise); by high quality, I mean BVA xtal units costing in the $5k range. The transition from the BVA xtal to the maser is typically done at times ~30-100 seconds or so (see the AVARs in my tutorials I mention later, or http://www.leapsecond.com/museum/manyadev.gif to see that the BVA performance is better than the Maser up to ~30 seconds. The goal is to hand off from one oscillator to the next when their Allen deviation is equal). BTW -- we actually have a couple of amateurs that have both passive active H-Masers in their basements. One is Tom vanBaak (no call) whose efforts can be viewed at http://leapsecond.com/ and another is Jim Jaeger (K8RQ) (see http://www.clockvault.com/ if you can stand the music!). TvB offered a review paper on amateur timekeeping at the 2003 PTTI meeting, which can be fetched at http://tycho.usno.navy.mil/ptti/ptti2003/paper35.pdf. Also be sure to note TvB's Most Accurate WristWatch when you log onto leapsecond.com. I've said it before, and I'll repeat it now -- you are better off thinking in the frequency (and phase noise) domain when you are considering oscillators on time scales shorter than tens of seconds, and in the time domain for minutes and longer. If you are interested in these topics, you might want to fetch one of my Timing for VLBI tutorials at http://gpstime.com/ . In my past incarnation I ran NASA's Geodetic VLBI program and was responsible for H-masers as time and frequency standards. While I am on here making comments on this thread, I note that Alberto, I2PHD is using a circuit similar to the one I developed for locking an xtal to the 10 kHz output from the Connexant/Navman Jupiter-T receiver. A couple of notes on what I found: My initial effort also used 74HC390 dividers as a ripple counter to get from 10 MHz - 10 kHz. But I found that the propagation delay thru these dividers varied strongly with temperature, amounting to a couple of hundred nsec in a day. I fixed this problem by using a simple, but elegant circuit developed by Tom van Baak (see http://www.leapsecond.com/tools/ppsdiv.zip) which uses a PIC with its clock input driven by the 10 MHz signal and a finite state machine that executes a fixed number of instructions to generate lower frequencies. Not only is it a very stable synchronous divider, but also it need only a couple of $$ worth of parts. I did a lot of work to optimize loop time constants to try to achieve performance at the couple of nsec levels. Most of the time, the Jupiter-T steered the oscillator very well, but about once per hour, the 10 kpps (and 1pps) output sawtooth goes thru a zero-beat, with a fixed bias error spanning intervals of 10s of seconds. You can see some of these sawtooth hanging bridges that really screw up the locking in my tutorials on gpstime.com. And you can see the fix that Rick (W2GPS) is using in his latest CNS clock using the M12+ in the latest of the gpstime.com tutorials. 73, Tom
Re: [Flexradio] Frequency stability and calibration
At 05:29 PM 11/22/2005, [EMAIL PROTECTED] wrote: Hello Flexers, I did a freq cal four days ago and have checked it everyday since. Here is what I see on my SDR, -1Hz to +2Hz. Also a comment about the Rubidium, Cesium and Crystal oscillators. Oscillators are characterized by short term(phase noise) and long term(drift) freq stability. The phase noise it what concerns us most when it comes to receiving. We want it low enough so that it doesn't swamp weak signals. Ideally it should be 10db below the noise floor of the receiver so as to not reduce sensitivity and hence dynamic range. It is the 'Q' of the resonant component(s) that determine the phase noise. The quartz crystal is still the leader when it comes phase noise. Rubidium and Cesium are superior in time keeping (long term stability), but have awful phase noise. These are the 'real' atomic clocks. The navy is the largest user of these. No GPS under water! Regarding GPS, its timing is based on the Cesium clocks. There ARE actually sources with better close in phase noise than a quartz crystal, just in case you see one at a hamfest or surplus place (or, you're wealthy enough). A hydrogen maser, for instance (that's what we use at work, JPL, when we're concerned about such things.. but then we have an infrastructure to distribute the maser signal around, and a budget for the support staff). A more portable high Q resonator is a sapphire resonator (which is used in some high performance phase noise test sets). Another one is a superconducting cavity resonator (which isn't as impractical as it might seem, with high temperature superconductors(liquid nitrogen temperature). As far as long term stable sources go, there's also things like Mercury Ion traps, which I believe can provide Cs quality long term, but also are high Q so they're good phase noise. However, as John says.. there's lots of really good quality 10 MHz crystal oscillators out there fairly cheap.
[Flexradio] Frequency stability and calibration
Hello Flexers, I did a freq cal four days ago and have checked it everyday since. Here is what I see on my SDR, -1Hz to +2Hz. I'm using the 1 Watt SDR and I leave it and the PC powered on at all times. I cal by setting the mode to AM and the filter to 25Hz using the Spectrum display this yields five displayed data points or a resolution of ~13Hz. Since the display uses a 'connect the dots approach' you get a linear interpolation. Adjust the clock offset while centering the peak and getting equal amplitude at the one above(+13Hz) display point and on the one below(-13Hz) data point. Caution! When I first did this, I cycled through the various WWV freq's and it appeared 20MHz was off by 6Hz. What I discovered is that the Filter BW has to be the same for each frequency. It appears that when switching BW to 50Hz and maybe 500Hz it shifts the center freq by -6.3Hz. Maybe there's a quick fix for this? Comments? You can easily observe this by looking at a CW signal and changing the BWs. Also a comment about the Rubidium, Cesium and Crystal oscillators. Oscillators are characterized by short term(phase noise) and long term(drift) freq stability. The phase noise it what concerns us most when it comes to receiving. We want it low enough so that it doesn't swamp weak signals. Ideally it should be 10db below the noise floor of the receiver so as to not reduce sensitivity and hence dynamic range. It is the 'Q' of the resonant component(s) that determine the phase noise. The quartz crystal is still the leader when it comes phase noise. Rubidium and Cesium are superior in time keeping (long term stability), but have awful phase noise. These are the 'real' atomic clocks. The navy is the largest user of these. No GPS under water! Regarding GPS, its timing is based on the Cesium clocks. Back to the SDR. Since the crystal has the best phase noise we want to keep it. Long term drift, measured from hours to years, is dominated by temperature. Additionally, aging can cause 20Hz of drift per year at 10 MHz. It's caused by a change in physical properties. Trapped gasses escaping, contaminants, etc. Temperature varies physical properties such as thickness and area of the quartz, which in turn varies the frequency. A 5ppm oscillator can drift 50Hz at 10MHz with a 10 degree F change in temperature! RTXO's (room temp xtal oscillators) range from 2-10ppm. The largest temperature change occurs at turn on, therefore you should wait an hour before performing a freq cal. That's why I leave mine on all the time and have only seen a 2 Hz maximum change so far. I'm sure those using high duty cycle modes with 100W SDRs will notice higher shifts. Don't keep chasing your tail. During transmission the SDR will warm up and the freq will drift in one direction and when you go back to receive the unit will cool and return to it's original frequency. So don't keep recalibrating. The sound card clock is hopefully derived from a crystal. 5ppm at 48KHz yields 0.24Hz. Not much concern. Other Options: TXCO The temperature compensated oscillator uses components external to the crystal to offset drift. These parts (caps, inductors, thermistors) have opposite temperature coefficients. The TCXO typically gives a 5X performance increase over the RTXO or 1 ppm. 10Hz drift at 10MHz for a 10 degree change. Oven Controlled Crystal Oscillator. In this case the oscillator is placed in a temperature controlled box. The temperature is maintained at the crystal turnover point. This is where the freq change vs. temp is smallest. The best performance is about 20 degrees F above the highest ambient. Typically 150 deg F. The oven should be under power at all times, if not the crystal temp will rise from room temp to 150F at turn on. During warm up the freq could drift nearly 1KHz at 10MHz. After warm up you can expect 5ppb. Or 0.05 Hz at 10 MHz for a 100F change! Another idea is to take a 10MHz crystal and mount it onto a large piece of metal and place it in an insulated box. The metal will slow the temp variations as will the insulation. If you mount it external to the SDR the PA temp variations won't effect freq stability. As mentioned in other posts you can derive the reference from GPS. Don't overlook your friendly cell phone system. CDMA cell phones have very critical timing requirements and are synchronized to Cesium sources. I try to keep it simple. I'm always battling RF in this, RF in that. I don't plan to get the reference through another receiver. Sources of 10MHz oscillators are abundant, 99 percent of the RF test equipment manufactured in the last 30 years use 10MHz reference oscillators. Many are ovenized, almost all are in MW equipment. Surplus telecom equipment is another resource. I have picked up three ovenized oscillators on EBay for less than ten dollars each. Who said amateurs can't make a better radio? Well I guess this was a little more than a comment on
Re: [Flexradio] Frequency stability and calibration
[EMAIL PROTECTED] said the following on 11/22/2005 08:29 PM: Also a comment about the Rubidium, Cesium and Crystal oscillators. Oscillators are characterized by short term(phase noise) and long term(drift) freq stability. The phase noise it what concerns us most when it comes to receiving. We want it low enough so that it doesn't swamp weak signals. Ideally it should be 10db below the noise floor of the receiver so as to not reduce sensitivity and hence dynamic range. It is the 'Q' of the resonant component(s) that determine the phase noise. The quartz crystal is still the leader when it comes phase noise. Rubidium and Cesium are superior in time keeping (long term stability), but have awful phase noise. These are the 'real' atomic clocks. The navy is the largest user of these. No GPS under water! Regarding GPS, its timing is based on the Cesium clocks. Being slightly insane, I gathered up the published stability and phase noise specs for a bunch of oscillators and synthesizers and put them in a single table at http://www.febo.com/time-freq/hardware/specs.html. The HP 5065A Rb, for example, has phase noise of -93dBc at 1Hz, and -140 at 1kHz. The 5061A Cs with normal tube is only -82dBc at 1Hz but is also -140 at 1kHz. The 10811A crystal ranges (depending on model) from -95 to -103dBc at 1Hz, and -145 to -162 at 1kHz. The small Rbs like the Efratom bricks that are available on eBay, on the other hand, are pretty horrible as they use FM modulation as part of the loop detection. Of course, the problem is that in the SDR-1000 we are multiplying the 10MHz external reference to 200 MHz, and that multiplication is going to multiply the phase noise as well. The question is whether a low jitter can oscillator at 200 MHz is better than a really good crystal (or even Rb) at 10MHz, multiplied by 20. I hope to do some measurements over Christmas to answer that question. 73, John
Re: [Flexradio] Frequency stability and calibration
Hi John, Very cool. TNX. You want to calculate the ppm at the freq of the LO. At first glance it seems you would use 200MHz, but since it is divided down to the rx/tx freq the drift also gets divided. It goes the other way also, when used to get to the MW bands you multiply the drift. Regards, k2ox -Original Message- From: John Ackermann N8UR [mailto:[EMAIL PROTECTED] Sent: Tuesday, November 22, 2005 9:09 PM To: [EMAIL PROTECTED] Cc: FlexRadio@flex-radio.biz Subject: Re: [Flexradio] Frequency stability and calibration [EMAIL PROTECTED] said the following on 11/22/2005 08:29 PM: Also a comment about the Rubidium, Cesium and Crystal oscillators. Oscillators are characterized by short term(phase noise) and long term(drift) freq stability. The phase noise it what concerns us most when it comes to receiving. We want it low enough so that it doesn't swamp weak signals. Ideally it should be 10db below the noise floor of the receiver so as to not reduce sensitivity and hence dynamic range. It is the 'Q' of the resonant component(s) that determine the phase noise. The quartz crystal is still the leader when it comes phase noise. Rubidium and Cesium are superior in time keeping (long term stability), but have awful phase noise. These are the 'real' atomic clocks. The navy is the largest user of these. No GPS under water! Regarding GPS, its timing is based on the Cesium clocks. Being slightly insane, I gathered up the published stability and phase noise specs for a bunch of oscillators and synthesizers and put them in a single table at http://www.febo.com/time-freq/hardware/specs.html. The HP 5065A Rb, for example, has phase noise of -93dBc at 1Hz, and -140 at 1kHz. The 5061A Cs with normal tube is only -82dBc at 1Hz but is also -140 at 1kHz. The 10811A crystal ranges (depending on model) from -95 to -103dBc at 1Hz, and -145 to -162 at 1kHz. The small Rbs like the Efratom bricks that are available on eBay, on the other hand, are pretty horrible as they use FM modulation as part of the loop detection. Of course, the problem is that in the SDR-1000 we are multiplying the 10MHz external reference to 200 MHz, and that multiplication is going to multiply the phase noise as well. The question is whether a low jitter can oscillator at 200 MHz is better than a really good crystal (or even Rb) at 10MHz, multiplied by 20. I hope to do some measurements over Christmas to answer that question. 73, John
[Flexradio] Frequency stability and calibration
Can I request that solving the stability issue of the SDR1000 be brought towards the top of the "to-do" pile. Having to recalibrate regularly is becoming a chore. Also it may be my calibrating techniques, but I notice that the calibration settings using spectrum and panadadapter are slightly different, with the spectrum calibration being 6hz lower, ie the clock offset is different, dependent on whether you use spectrum or panadapter. Thanks Ross ZL1WN
Re: [Flexradio] Frequency stability and calibration
At 02:25 PM 11/21/2005, richard allen wrote: Ross, One thing you need to remember is that the fft bin widths will greatly effect sub 20 Hz accuracy that you are looking for. Also, the pixel width in the display, indicating the cursor position, may be many Hz. Of course the sound card may be contributing as was pointed out earlier. Before I had my first unit properly heat sinked and mounted inside a case, I complained about the drift also. Any breezes about the oscillator are gonna cause it to drift. While not as tight as the $300 tcxo in my icom 756, the sdr1000 now does pretty well inside a case with all the latest heatsinks and mods. I use the auto calibrate in 10 MHz WWV. Actually the signal from my 10 MHz gps system is usually louder than WWV. Indeed... I find that blowing on the radio from a few feet away will shift the frequency several 10s of Hz. (Hook I and Q audio outputs to X and Y on an oscilloscope, and tune for zero beat against a good quality reference for a very good demo of this effect). The reference oscillator was selected first for its very low phase jitter, and important consideration for achieving the fantastic receiver specs. The second consideration was probably price. The best solution for your stability if you also care about the receiver specs will probably today be the Reflock II from TAPR along with a 1 pps gps receiver locking a 200 MHz osc. The lower frequency oscillators like that in the z3801 are gonna give you good stability and accuracy but at the expense of more phase noise. There is also a German company that is now producing a good 10 MHz system I believe. An even better solution is to measure the low jitter oscillator in the SDR1000 against your external reference, and fix the frequency in software. That way, you're not multiplying the phase noise of the external source up. ANY oscillator with an external control (needed to discipline it against something else, be it GPS or other standards) is going to have worse phase noise than the same oscillator without the external control, because the noise on the control port manifests itself as phase noise on the output. Regards, Richard W5SXD -Original Message- Jim, W6RMK
Re: [Flexradio] Frequency stability and calibration
I completely agree with the external oscillator software fix for offset and I even use it for experiments where I am tuned to 10 Mhz. I can't figure out how to keep the reference tuned and the signal of interest tuned with the single IF! Please fix my ignorance. Bob Jim Lux wrote: At 02:25 PM 11/21/2005, richard allen wrote: Ross, One thing you need to remember is that the fft bin widths will greatly effect sub 20 Hz accuracy that you are looking for. Also, the pixel width in the display, indicating the cursor position, may be many Hz. Of course the sound card may be contributing as was pointed out earlier. Before I had my first unit properly heat sinked and mounted inside a case, I complained about the drift also. Any breezes about the oscillator are gonna cause it to drift. While not as tight as the $300 tcxo in my icom 756, the sdr1000 now does pretty well inside a case with all the latest heatsinks and mods. I use the auto calibrate in 10 MHz WWV. Actually the signal from my 10 MHz gps system is usually louder than WWV. Indeed... I find that blowing on the radio from a few feet away will shift the frequency several 10s of Hz. (Hook I and Q audio outputs to X and Y on an oscilloscope, and tune for zero beat against a good quality reference for a very good demo of this effect). The reference oscillator was selected first for its very low phase jitter, and important consideration for achieving the fantastic receiver specs. The second consideration was probably price. The best solution for your stability if you also care about the receiver specs will probably today be the Reflock II from TAPR along with a 1 pps gps receiver locking a 200 MHz osc. The lower frequency oscillators like that in the z3801 are gonna give you good stability and accuracy but at the expense of more phase noise. There is also a German company that is now producing a good 10 MHz system I believe. An even better solution is to measure the low jitter oscillator in the SDR1000 against your external reference, and fix the frequency in software. That way, you're not multiplying the phase noise of the external source up. ANY oscillator with an external control (needed to discipline it against something else, be it GPS or other standards) is going to have worse phase noise than the same oscillator without the external control, because the noise on the control port manifests itself as phase noise on the output. Regards, Richard W5SXD -Original Message- Jim, W6RMK ___ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz -- Laziness is the number one inspiration for ingenuity. Guilty as charged!
Re: [Flexradio] Frequency stability and calibration
Title: Message All While the spectrum display does have the FFT bin size as the limiting factor, by using the phase display with an unmodulated signal,you can seeto asub-hertz level the drift between the incoming signal and the two SDR oscillators (the 200 MHz oscillator in the SDR chassis and the cheap oscillator on the sound card). An interesting parlor trick is to watch the phase display with a Z3801 at 10 MHz as the RF source. Make the frequency cal perfect - get the dot on the phase display to stop moving. Switch to 20 or 30 MHz. Watch the dot. If it moves, it is the error caused by the sound card crystal. Should be an easy cal to automate - or so I have been told by a South Alabama native. 73 Mike, W3IP - Original Message - From: richard allen To: 'Ross' ; FlexRadio@flex-radio.biz Sent: Monday, November 21, 2005 5:25 PM Subject: Re: [Flexradio] Frequency stability and calibration Ross, One thing you need to remember is that the fft bin widths will greatly effect sub 20 Hz accuracy that you are looking for. Also, the pixel width in the display, indicating the cursor position, may be many Hz. Of course the sound card may be contributing as was pointed out earlier. Before I had my first unit properly heat sinked and mounted inside a case, I complained about the drift also. Any breezes about the oscillator are gonna cause it to drift. While not as tight as the $300 tcxo in my icom 756, the sdr1000 now does pretty well inside a case with all the latest heatsinks and mods. I use the auto calibrate in 10 MHz WWV. Actually the signal from my 10 MHz gps system is usually louder than WWV. The reference oscillator was selected first for its very low phase jitter, and important consideration for achieving the fantastic receiver specs. The second consideration was probably price. The best solution for your stabilityif you also care about the receiver specs will probably today be the Reflock II from TAPR along with a 1 pps gps receiver locking a 200 MHz osc. The lower frequency oscillators like that in the z3801 are gonna give you good stability and accuracy but at the expense of more phase noise. There is also a German company that is now producing a good 10 MHz system I believe. Regards, Richard W5SXD -Original Message-From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of RossSent: Monday, November 21, 2005 12:28 PMTo: FlexRadio@flex-radio.bizSubject: [Flexradio] Frequency stability and calibration Can I request that solving the stability issue of the SDR1000 be brought towards the top of the "to-do" pile. Having to recalibrate regularly is becoming a chore. Also it may be my calibrating techniques, but I notice that the calibration settings using spectrum and panadadapter are slightly different, with the spectrum calibration being 6hz lower, ie the clock offset is different, dependent on whether you use spectrum or panadapter. Thanks Ross ZL1WN ___FlexRadio mailing listFlexRadio@flex-radio.bizhttp://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz
