Re: [time-nuts] Adapting my GPSDO to the FE-5680A
> On Apr 22, 2016, at 1:58 AM, GandalfG8--- via time-nuts > wrote: > > Thanks. I’ve taken your suggestion for the sine-to-square converter. > I believe there are two separate commands for tuning the 5680 - one is “ > temporary” and one writes through to the EEPROM. I’ll be using the latter, > of course. > > --- > I'm surprised nobody else seems to have commented on this but I'm pretty > sure that using the write to EEPROM option on an automatically and > regularly repeated basis could be somewhat akin to applying the kiss of > death. I mis-typed. I meant to say that I’d be using the *former* - the temporary command. No writes to EEPROM. I do have code in all of my current GPSDO code that skips the DAC/serial write if the value isn’t changing. I did that for the DAC because there’s a change glitch that’s good to avoid if it’s not absolutely necessary. Anyway, I’ve ordered the first set of boards. In addition to Atilla’s suggestion for the oscillator output conditioning (the DC-block + self-biased inverter), I’ve also broken out the oscillator’s PPS pin to a connector, in case folks might want to experimentally measure it against the GPS PPS or whatever other use one might desire. The physical form factor is designed to be the same width as the FE-5680A, with the connector spaced so that it looks like an extension of it. If you put, oh, 1/3” feet on the four corners of the board, it ought to sit flat adjacent to the oscillator and, well, I’m hoping it looks rather handsome. :) The connectors are the same as the crystal based GPSDOs - a mini-DIN-4 diagnostic connector with the same pinout, an SMA GPS antenna connector, 2.1mm power jack - but this one takes 16-24 VDC @ 25-30W (easily obtainable from a surplus laptop power supply), and two .1” JST output jacks that get separately buffered CMOS square wave outputs (NB3N55 1 + 33Ω series resistor). The boards should get back early May and I’ll post reports to the Hackaday.io project. My results may not be very useful, as I don’t have a significantly better reference for comparison. My control results just sort of look like I’m really measuring the Thunderbolt against the 5680A rather than vice-versa. I suppose I could attempt to buy a second 5680A and square the two off against each other, but buying 5680As is fraught with danger, as there are significant feature and pinout differences that are not always properly called out in eBay auction descriptions. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
Thanks. I’ve taken your suggestion for the sine-to-square converter. I believe there are two separate commands for tuning the 5680 - one is “ temporary” and one writes through to the EEPROM. I’ll be using the latter, of course. --- I'm surprised nobody else seems to have commented on this but I'm pretty sure that using the write to EEPROM option on an automatically and regularly repeated basis could be somewhat akin to applying the kiss of death. Obviously the threat will depend upon how frequently it's decided the incremental change needs applying, presumably quite a bit less frequently for a rubidium module than a crystal oscillator, but in the longer term could it not approach the limits for the device? I would have thought the "temporary" option is to be preferred, and certainly this is what I've always used when manually adjusting FE5680As, until such time as I'm sure the applied value is close enough to be stored permanently. Other than needing to start over in the case of power removal I can't see any obvious disadvantages to this, and if the software can be made to report the current increment on request then presumably that could be noted every few months or so and perhaps an updated value permanently programmed, or even as infrequently as once a year say shouldn't really be too much of an issue. Nigel GM8PZR ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
On Wed, 20 Apr 2016 15:31:04 -0700 Nick Sayer via time-nuts wrote: > I believe there are two separate commands for tuning the 5680 - one is > “temporary” and one writes through to the EEPROM. I’ll be using the latter, > of course. Ah.. ok. Didn't know there are two. > http://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html > > I do agree that the short term stability of the 5680A isn’t as good as an > OCXO, but at tau ~2s or so, the tables are turned. I’m getting a good measure > of my undisciplined 5680A as we speak to get a good control, but it’s > difficult, as I’m testing it against a Thunderbolt, and I think I’m seeing > its “hump” (http://www.leapsecond.com/pages/gpsdo/) from 10s to 300s coming > through. Comparing an Rb to a GPSDO at taus below 1k-10k seconds is probably being dominated by the GPSDO noise anyways. To actually see the stability improvements of using an Rb as oscillator for an GPSDO instead of an OCXO you will need something more stable. Either use an ensemble of good rubidiums, or you need a Cs beam standard. Using multiple GSPDO with different antennas is not going to help as the errors will be correlated due to short baseline. > In any event, I’m getting awfully close to the limits of my 53220a. There are people working on solving this problem :-) > I may go down the road of trying to make a mash-up as you suggest, > but I’m going to start by seeing if I can give myself a choice between > whether I want short term stability (OCXO) or medium term (Rubidium) > for my reference. Hehe.. it looks like you are going down the time-nuts road ;-) Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
On Wed, 20 Apr 2016 22:44:49 + (UTC) Bruce Griffiths wrote: > Sampling at 4x(10MHz + 1kHz) followed by a digital image reject mixer > would work better.The front end analog filter only needs to reject > unwanted nyquist regions, thus it can have a wider bandwidth and low phase > shift tempco. Its PN noise contribution can also be small. Digital filters > can be used to reject the undesired spurs. Yes, the problem with this is that Nick used a ATtiny as uC, which is not powerfull enough to munch 10Msps of data. Going to something more powerfull like a Cortex-M3 or so, should allow to do that, though An alternative would be to use a small FPGA like a Lattice ICE4 or an Altera MAX and use that to do the sampling, and the PLL. Then using the PLL/NCO tune word as output once in a while would be very easy to do and would use very little resources on the uC. But if you have never done anything with an FPGA that's quite a step up in design complexity (though once you know how to do it, it's not more difficult than to use a uC). BTW: if one would go the FPGA way, i would recommend using a more powerfull ADC that can do 40Msps (still quite cheap, especially if only an 8bit variant is used) and clock everything from a 38.8MHz crystal. Either using a PLL to lock the 38.8MHz oscillator to the 10MHz OCXO, or using a dual ADC and sample both reference and OCXO at the same time. Attila Kinali -- Reading can seriously damage your ignorance. -- unknown ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
Hi Most of these telco Rb’s start out around 2x10^-11 at 1 second tau. Their ADEV then drops as square root of tau. At 100 seconds they hit 2x10^-12 and at 10,000 seconds they (might) hit 2x10^-13. A *good* OCXO will do 2x10^-12 at 100 seconds. An excellent part will hold sub 1x10^-12 out to 1,000 seconds. Very roughly speaking the Rb will beat out the OCXO somewhere past 100 seconds and somewhere before 1,000 seconds. Bob > On Apr 20, 2016, at 6:31 PM, Nick Sayer via time-nuts > wrote: > > Thanks. I’ve taken your suggestion for the sine-to-square converter. > > I believe there are two separate commands for tuning the 5680 - one is > “temporary” and one writes through to the EEPROM. I’ll be using the latter, > of course. > > http://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html > > I do agree that the short term stability of the 5680A isn’t as good as an > OCXO, but at tau ~2s or so, the tables are turned. I’m getting a good measure > of my undisciplined 5680A as we speak to get a good control, but it’s > difficult, as I’m testing it against a Thunderbolt, and I think I’m seeing > its “hump” (http://www.leapsecond.com/pages/gpsdo/) from 10s to 300s coming > through. In any event, I’m getting awfully close to the limits of my 53220a. > I may go down the road of trying to make a mash-up as you suggest, but I’m > going to start by seeing if I can give myself a choice between whether I want > short term stability (OCXO) or medium term (Rubidium) for my reference. > > >> On Apr 20, 2016, at 1:57 PM, Attila Kinali wrote: >> >> On Wed, 20 Apr 2016 07:17:58 -0700 >> Nick Sayer via time-nuts wrote: >> >>> I spent some time yesterday mashing together my FE-5680A "breakout" board >>> with my GPSDO to make a GPS discipline board for it. Before I send the >>> board >>> off to OSHPark, I'd like to open the design to criticism (and I mean that >>> in >>> its neutral sense) here first. >> >> Looking at your schematics, I would replace the input squarer (IC4) >> by something different than a schmitt-trigger with an input bias >> voltage. For one, schmitt-triggers are more noisy than normal buffers >> for an other, the bias voltage will result in a slightly skewed duty >> cycle. If you want to use a gate, then the canoncical way would be to >> use an inverter with an input capacitor like you did, but let it self-bias >> itself by using a 100k-1M resistor from its output to its input. >> Important: don't use a buffer, as this will only work with an inverter. >> But I'd rather use a different squaring circuit, if you want to use >> the input directly for the output. There are many discussions on >> squaring circuits in the archives. Probably the most simple, yet sofisticated >> is the one you can find in the TADD-2. >> >> >> But: >> As you can see on http://www.ke5fx.com/rb.htm the phase noise of the >> FE 5680's is horrible at best, hence I wouldn't use it as source for >> anything directly. Additionally, the tuning word you write through >> the RS-232 is stored in an EEPROM inside the FE-5860 (unless i mix >> it up with another Rb). Writing this tuning word often will wear out >> the EEPROM pretty quickly. Hence you should not do this too often. >> >> What I would do instead is, use your current GPSDO design, with OCXO >> and all, but add something with which you can measure the phase/frequency >> of an external 10MHz reference. One way would be to use a digital DMTD[1,2]. >> Another would be to sample the reference using an ADC and build DMTD in >> the digital domain. For this you wouldn't need a high sampling rate, a >> couple of kHz should be enough, as long as the analog bandwidth of the >> ADC is high enough (>10MHz, better >20MHz). What you need is some PLL >> though, as you need to create a frequency that is not an integer divisible >> of 10MHz, as the ADC clock is used to downmix the reference frequency. >> >> Eg: >> If you can generate a 10001Hz ADC sampling clock from the OCXO, >> you will get a 1kHz beat frequency. You can "lock" to this using a >> digital PLL combined with an NCO (numerically controlled oscillator). >> Then use the steering word for the NCO as an input for the control >> loop of the OCXO, toghether with the corrections calculated from the >> GPS PPS. >> >> The advantage of this is, that you get the low phase noise and good >> short term stability of the OCXO, but can use the Rb to get the nice >> mid-term stability (somewhere from 1 to 10s up) while getting the >> accuracy of a GPSDO, whithout ever the need of writing to the tuning >> word of the Rb. That keeps your Rb more stable (the internal conditions >> of the Rb do not change) and allows you to compensate for quite large >> frequency offsets for Rb refernces that are working outside the spec, >> but are otherwise fine. >> >> One thing that you have to take care of is spurs, though. Because >> the ADC does some heavy down-mixing, or rather sub-sampling, this >> approach is quite sensitive to spurs
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
Sampling at 4x(10MHz + 1kHz) followed by a digital image reject mixer would work better.The front end analog filter only needs to reject unwanted nyquist regions, thus it can have a wider bandwidth and low phase shift tempco. Its PN noise contribution can also be small. Digital filters can be used to reject the undesired spurs. Bruce On Thursday, 21 April 2016 10:10 AM, Bruce Griffiths wrote: There's the small matter of the high phase shift tempco of the narrow 10MHz bandpass filters Q~10,000 or so.These would need to either track very closely or be in an oven with very high temperature stability.The PN contribution of these analog filters may also be an issue. Bruce On Thursday, 21 April 2016 9:03 AM, Attila Kinali wrote: On Wed, 20 Apr 2016 07:17:58 -0700 Nick Sayer via time-nuts wrote: > I spent some time yesterday mashing together my FE-5680A "breakout" board > with my GPSDO to make a GPS discipline board for it. Before I send the board > off to OSHPark, I'd like to open the design to criticism (and I mean that in > its neutral sense) here first. Looking at your schematics, I would replace the input squarer (IC4) by something different than a schmitt-trigger with an input bias voltage. For one, schmitt-triggers are more noisy than normal buffers for an other, the bias voltage will result in a slightly skewed duty cycle. If you want to use a gate, then the canoncical way would be to use an inverter with an input capacitor like you did, but let it self-bias itself by using a 100k-1M resistor from its output to its input. Important: don't use a buffer, as this will only work with an inverter. But I'd rather use a different squaring circuit, if you want to use the input directly for the output. There are many discussions on squaring circuits in the archives. Probably the most simple, yet sofisticated is the one you can find in the TADD-2. But: As you can see on http://www.ke5fx.com/rb.htm the phase noise of the FE 5680's is horrible at best, hence I wouldn't use it as source for anything directly. Additionally, the tuning word you write through the RS-232 is stored in an EEPROM inside the FE-5860 (unless i mix it up with another Rb). Writing this tuning word often will wear out the EEPROM pretty quickly. Hence you should not do this too often. What I would do instead is, use your current GPSDO design, with OCXO and all, but add something with which you can measure the phase/frequency of an external 10MHz reference. One way would be to use a digital DMTD[1,2].. Another would be to sample the reference using an ADC and build DMTD in the digital domain. For this you wouldn't need a high sampling rate, a couple of kHz should be enough, as long as the analog bandwidth of the ADC is high enough (>10MHz, better >20MHz). What you need is some PLL though, as you need to create a frequency that is not an integer divisible of 10MHz, as the ADC clock is used to downmix the reference frequency. Eg: If you can generate a 10001Hz ADC sampling clock from the OCXO, you will get a 1kHz beat frequency. You can "lock" to this using a digital PLL combined with an NCO (numerically controlled oscillator). Then use the steering word for the NCO as an input for the control loop of the OCXO, toghether with the corrections calculated from the GPS PPS. The advantage of this is, that you get the low phase noise and good short term stability of the OCXO, but can use the Rb to get the nice mid-term stability (somewhere from 1 to 10s up) while getting the accuracy of a GPSDO, whithout ever the need of writing to the tuning word of the Rb. That keeps your Rb more stable (the internal conditions of the Rb do not change) and allows you to compensate for quite large frequency offsets for Rb refernces that are working outside the spec, but are otherwise fine. One thing that you have to take care of is spurs, though. Because the ADC does some heavy down-mixing, or rather sub-sampling, this approach is quite sensitive to spurs. In order to not introduce some weird oscillations in the control loop due to spurs in the reference signal, you should use some narrow 10MHz filter at the input (at most half the sampling frequency wide). One way to achieve that is using a ceramic resonator which are available at 10MHz. Attila Kinali PS: I'm pretty sure I am not the first one with this idea. But I have never seen anyone else mention it, much less implement it. Does anyone know why? [1] "Digital Dual Mixer Time Difference for Sub-Nanosecond Time Synchronization in Ethernet", by Moreira, Alvares, Serrano, Darwezeh and Wlostowski, 2010 [2] "Digital femtosecond time difference circuit for CERN's timing system", by Moreira, Darwazeh, 2011 http://www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf -- Reading can seriously damage your ignorance. -- unknown ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
Thanks. I’ve taken your suggestion for the sine-to-square converter. I believe there are two separate commands for tuning the 5680 - one is “temporary” and one writes through to the EEPROM. I’ll be using the latter, of course. http://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html I do agree that the short term stability of the 5680A isn’t as good as an OCXO, but at tau ~2s or so, the tables are turned. I’m getting a good measure of my undisciplined 5680A as we speak to get a good control, but it’s difficult, as I’m testing it against a Thunderbolt, and I think I’m seeing its “hump” (http://www.leapsecond.com/pages/gpsdo/) from 10s to 300s coming through. In any event, I’m getting awfully close to the limits of my 53220a. I may go down the road of trying to make a mash-up as you suggest, but I’m going to start by seeing if I can give myself a choice between whether I want short term stability (OCXO) or medium term (Rubidium) for my reference. > On Apr 20, 2016, at 1:57 PM, Attila Kinali wrote: > > On Wed, 20 Apr 2016 07:17:58 -0700 > Nick Sayer via time-nuts wrote: > >> I spent some time yesterday mashing together my FE-5680A "breakout" board >> with my GPSDO to make a GPS discipline board for it. Before I send the board >> off to OSHPark, I'd like to open the design to criticism (and I mean that in >> its neutral sense) here first. > > Looking at your schematics, I would replace the input squarer (IC4) > by something different than a schmitt-trigger with an input bias > voltage. For one, schmitt-triggers are more noisy than normal buffers > for an other, the bias voltage will result in a slightly skewed duty > cycle. If you want to use a gate, then the canoncical way would be to > use an inverter with an input capacitor like you did, but let it self-bias > itself by using a 100k-1M resistor from its output to its input. > Important: don't use a buffer, as this will only work with an inverter. > But I'd rather use a different squaring circuit, if you want to use > the input directly for the output. There are many discussions on > squaring circuits in the archives. Probably the most simple, yet sofisticated > is the one you can find in the TADD-2. > > > But: > As you can see on http://www.ke5fx.com/rb.htm the phase noise of the > FE 5680's is horrible at best, hence I wouldn't use it as source for > anything directly. Additionally, the tuning word you write through > the RS-232 is stored in an EEPROM inside the FE-5860 (unless i mix > it up with another Rb). Writing this tuning word often will wear out > the EEPROM pretty quickly. Hence you should not do this too often. > > What I would do instead is, use your current GPSDO design, with OCXO > and all, but add something with which you can measure the phase/frequency > of an external 10MHz reference. One way would be to use a digital DMTD[1,2]. > Another would be to sample the reference using an ADC and build DMTD in > the digital domain. For this you wouldn't need a high sampling rate, a > couple of kHz should be enough, as long as the analog bandwidth of the > ADC is high enough (>10MHz, better >20MHz). What you need is some PLL > though, as you need to create a frequency that is not an integer divisible > of 10MHz, as the ADC clock is used to downmix the reference frequency. > > Eg: > If you can generate a 10001Hz ADC sampling clock from the OCXO, > you will get a 1kHz beat frequency. You can "lock" to this using a > digital PLL combined with an NCO (numerically controlled oscillator). > Then use the steering word for the NCO as an input for the control > loop of the OCXO, toghether with the corrections calculated from the > GPS PPS. > > The advantage of this is, that you get the low phase noise and good > short term stability of the OCXO, but can use the Rb to get the nice > mid-term stability (somewhere from 1 to 10s up) while getting the > accuracy of a GPSDO, whithout ever the need of writing to the tuning > word of the Rb. That keeps your Rb more stable (the internal conditions > of the Rb do not change) and allows you to compensate for quite large > frequency offsets for Rb refernces that are working outside the spec, > but are otherwise fine. > > One thing that you have to take care of is spurs, though. Because > the ADC does some heavy down-mixing, or rather sub-sampling, this > approach is quite sensitive to spurs. In order to not introduce some > weird oscillations in the control loop due to spurs in the reference > signal, you should use some narrow 10MHz filter at the input (at most > half the sampling frequency wide). One way to achieve that is using a > ceramic resonator which are available at 10MHz. > > Attila Kinali > > PS: I'm pretty sure I am not the first one with this idea. But I have never > seen anyone else mention it, much less implement it. Does anyone know why? > > > [1] "Digital Dual Mixer Time Difference for Sub-Nanosecond Time > Synchronization in Ethernet", by Moreira, Alvare
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
There's the small matter of the high phase shift tempco of the narrow 10MHz bandpass filters Q~10,000 or so.These would need to either track very closely or be in an oven with very high temperature stability.The PN contribution of these analog filters may also be an issue. Bruce On Thursday, 21 April 2016 9:03 AM, Attila Kinali wrote: On Wed, 20 Apr 2016 07:17:58 -0700 Nick Sayer via time-nuts wrote: > I spent some time yesterday mashing together my FE-5680A "breakout" board > with my GPSDO to make a GPS discipline board for it. Before I send the board > off to OSHPark, I'd like to open the design to criticism (and I mean that in > its neutral sense) here first. Looking at your schematics, I would replace the input squarer (IC4) by something different than a schmitt-trigger with an input bias voltage. For one, schmitt-triggers are more noisy than normal buffers for an other, the bias voltage will result in a slightly skewed duty cycle. If you want to use a gate, then the canoncical way would be to use an inverter with an input capacitor like you did, but let it self-bias itself by using a 100k-1M resistor from its output to its input. Important: don't use a buffer, as this will only work with an inverter. But I'd rather use a different squaring circuit, if you want to use the input directly for the output. There are many discussions on squaring circuits in the archives. Probably the most simple, yet sofisticated is the one you can find in the TADD-2. But: As you can see on http://www.ke5fx.com/rb.htm the phase noise of the FE 5680's is horrible at best, hence I wouldn't use it as source for anything directly. Additionally, the tuning word you write through the RS-232 is stored in an EEPROM inside the FE-5860 (unless i mix it up with another Rb). Writing this tuning word often will wear out the EEPROM pretty quickly. Hence you should not do this too often. What I would do instead is, use your current GPSDO design, with OCXO and all, but add something with which you can measure the phase/frequency of an external 10MHz reference. One way would be to use a digital DMTD[1,2].. Another would be to sample the reference using an ADC and build DMTD in the digital domain. For this you wouldn't need a high sampling rate, a couple of kHz should be enough, as long as the analog bandwidth of the ADC is high enough (>10MHz, better >20MHz). What you need is some PLL though, as you need to create a frequency that is not an integer divisible of 10MHz, as the ADC clock is used to downmix the reference frequency. Eg: If you can generate a 10001Hz ADC sampling clock from the OCXO, you will get a 1kHz beat frequency. You can "lock" to this using a digital PLL combined with an NCO (numerically controlled oscillator). Then use the steering word for the NCO as an input for the control loop of the OCXO, toghether with the corrections calculated from the GPS PPS. The advantage of this is, that you get the low phase noise and good short term stability of the OCXO, but can use the Rb to get the nice mid-term stability (somewhere from 1 to 10s up) while getting the accuracy of a GPSDO, whithout ever the need of writing to the tuning word of the Rb. That keeps your Rb more stable (the internal conditions of the Rb do not change) and allows you to compensate for quite large frequency offsets for Rb refernces that are working outside the spec, but are otherwise fine. One thing that you have to take care of is spurs, though. Because the ADC does some heavy down-mixing, or rather sub-sampling, this approach is quite sensitive to spurs. In order to not introduce some weird oscillations in the control loop due to spurs in the reference signal, you should use some narrow 10MHz filter at the input (at most half the sampling frequency wide). One way to achieve that is using a ceramic resonator which are available at 10MHz. Attila Kinali PS: I'm pretty sure I am not the first one with this idea. But I have never seen anyone else mention it, much less implement it. Does anyone know why? [1] "Digital Dual Mixer Time Difference for Sub-Nanosecond Time Synchronization in Ethernet", by Moreira, Alvares, Serrano, Darwezeh and Wlostowski, 2010 [2] "Digital femtosecond time difference circuit for CERN's timing system", by Moreira, Darwazeh, 2011 http://www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf -- Reading can seriously damage your ignorance. -- unknown ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Adapting my GPSDO to the FE-5680A
On Wed, 20 Apr 2016 07:17:58 -0700 Nick Sayer via time-nuts wrote: > I spent some time yesterday mashing together my FE-5680A "breakout" board > with my GPSDO to make a GPS discipline board for it. Before I send the board > off to OSHPark, I'd like to open the design to criticism (and I mean that in > its neutral sense) here first. Looking at your schematics, I would replace the input squarer (IC4) by something different than a schmitt-trigger with an input bias voltage. For one, schmitt-triggers are more noisy than normal buffers for an other, the bias voltage will result in a slightly skewed duty cycle. If you want to use a gate, then the canoncical way would be to use an inverter with an input capacitor like you did, but let it self-bias itself by using a 100k-1M resistor from its output to its input. Important: don't use a buffer, as this will only work with an inverter. But I'd rather use a different squaring circuit, if you want to use the input directly for the output. There are many discussions on squaring circuits in the archives. Probably the most simple, yet sofisticated is the one you can find in the TADD-2. But: As you can see on http://www.ke5fx.com/rb.htm the phase noise of the FE 5680's is horrible at best, hence I wouldn't use it as source for anything directly. Additionally, the tuning word you write through the RS-232 is stored in an EEPROM inside the FE-5860 (unless i mix it up with another Rb). Writing this tuning word often will wear out the EEPROM pretty quickly. Hence you should not do this too often. What I would do instead is, use your current GPSDO design, with OCXO and all, but add something with which you can measure the phase/frequency of an external 10MHz reference. One way would be to use a digital DMTD[1,2]. Another would be to sample the reference using an ADC and build DMTD in the digital domain. For this you wouldn't need a high sampling rate, a couple of kHz should be enough, as long as the analog bandwidth of the ADC is high enough (>10MHz, better >20MHz). What you need is some PLL though, as you need to create a frequency that is not an integer divisible of 10MHz, as the ADC clock is used to downmix the reference frequency. Eg: If you can generate a 10001Hz ADC sampling clock from the OCXO, you will get a 1kHz beat frequency. You can "lock" to this using a digital PLL combined with an NCO (numerically controlled oscillator). Then use the steering word for the NCO as an input for the control loop of the OCXO, toghether with the corrections calculated from the GPS PPS. The advantage of this is, that you get the low phase noise and good short term stability of the OCXO, but can use the Rb to get the nice mid-term stability (somewhere from 1 to 10s up) while getting the accuracy of a GPSDO, whithout ever the need of writing to the tuning word of the Rb. That keeps your Rb more stable (the internal conditions of the Rb do not change) and allows you to compensate for quite large frequency offsets for Rb refernces that are working outside the spec, but are otherwise fine. One thing that you have to take care of is spurs, though. Because the ADC does some heavy down-mixing, or rather sub-sampling, this approach is quite sensitive to spurs. In order to not introduce some weird oscillations in the control loop due to spurs in the reference signal, you should use some narrow 10MHz filter at the input (at most half the sampling frequency wide). One way to achieve that is using a ceramic resonator which are available at 10MHz. Attila Kinali PS: I'm pretty sure I am not the first one with this idea. But I have never seen anyone else mention it, much less implement it. Does anyone know why? [1] "Digital Dual Mixer Time Difference for Sub-Nanosecond Time Synchronization in Ethernet", by Moreira, Alvares, Serrano, Darwezeh and Wlostowski, 2010 [2] "Digital femtosecond time difference circuit for CERN's timing system", by Moreira, Darwazeh, 2011 http://www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf -- Reading can seriously damage your ignorance. -- unknown ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Adapting my GPSDO to the FE-5680A
I spent some time yesterday mashing together my FE-5680A "breakout" board with my GPSDO to make a GPS discipline board for it. Before I send the board off to OSHPark, I'd like to open the design to criticism (and I mean that in its neutral sense) here first. https://cdn.hackaday.io/files/6872294011648/GPS%20Disciplined%20FE5680%20v1_0.pdf or https://hackaday.io/project/6872-gps-disciplined-xcxo And skip down to the latest log entry for discussion. I've seen John M's page that talks about the ADEV of the 5680A - in particular that it's low tau ADEV doesn't (to my eye) appear to be a whole lot better (if at all - I'm currently getting 8E-12 @ 2s) than the OH300. But I've got one, and it's got nothing better to do, and it'd be interesting to see how much benefit is to be had by improving the mid-tau performance over the OH300. I've seen here posts about folks modifying the 5680 as part of a GPSDO. I'm not interested in doing that as my aspirations are not quite so high. I just want to see what happens when the 5680A can be made accurate (well, more, anyway) as well as stable. Sent from my iPhone ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
