Re: [time-nuts] Lars GPSDO
> We have agreed that I will do a board and make Gerber file available. Any comments or recommendations please off list. Perhaps put the DAC and the OCVCXO on a daughter board using .1" headers as an interconnect (like an Arduino shield) so that different DAC/Osc designs can be swapped in and out. Wayne On Sun, Apr 7, 2019 at 12:01 PM ew via time-nuts wrote: > A few more tests on the Lars GPSDO to better understand. The data is > reduced to 2 and 6 hours to reduce data volume. overwhelms our PC's but > tells us what Jim and we are looking for. We have agreed that I will do a > board and make Gerber file available. Most likely two Lars with pulse width > DAC and Jim's with a DAC. > Any comments or recommendations please off list.As part of our related > tests we found little change between Furuno GN87 and GT87 when used in a > GPSDO. > Bert___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Hi One thing to watch out for on any OCXO, but more on ones with crazy large tuning ranges: As the OCXO changes temperature, it’s current draw changes. It’s amazing just how much voltage drop you get in some seemingly well laid out boards when this occurs. The gotcha is that a “bias it up” with a couple resistors approach may or may not help this. It depends on just where the resistors are on the board. On some specs, they go to the extreme of specifying a point on the ground lead (after soldering in a board) that is the “official” ground for the oscillator. Good luck finding that on a data sheet out in the wild … If you have a 0-5V tuning range and a ~2:1 sensitivity variation over that range and a 0.25 ppm EFC: 0.5 ppm / 5 = 0.1 ppm / V. A 10 mv shift (which is not unusual on a 5V OCXO) gets you 1x10^-9 of frequency change. If that’s over 0-50C it’s contributing 2x10^-11 / C. Small changes in air flow quickly change the oven current so the loop may be having fun …. Bob > On Apr 5, 2019, at 10:45 PM, Jim Harman wrote: > > Thanks for your ideas, Ed. In my actual implementation I am using > reasonably low tempco 1% fixed resistors. In the photo they are the blue > resistors between the DAC and the OCXO. > > I have noticed a significant temperature sensitivity in the system, > especially if I run it with the cover off.so the ambient temperature can > change quickly. Moving air has a short term effect on the OCXO and to a > lesser extent the GPS. Sustained ambient temperature changes cause an > offset in the DAC value when the loop is closed, so the loop is doing its > job to compensate for them. Putting the system in a small picnic cooler > with several water bottles makes any temperature changes slower than the > loop time constant, which should minimize temperature effects on the > overall system. > > I calculate the open loop temperature sensitivity at about 7e-11 / deg C. > The C-MAC oscillator spec for what it's worth is "-10 C to +70 C / ref.at > +30 C < +/- 3.0 e-9". If the variation was linear, which I realize it is > probably not, that would be 7.5e-11 / deg C so I am in the same ballpark. > > On Fri, Apr 5, 2019 at 8:21 PM ed breya wrote: > >> I'd recommend that once you get things figured out and tuned up to >> nominal running conditions, you should narrow the offset pot range, and >> use good low-TC resistors to make up most of the network R, with the pot >> having as small an effect as practical. BTW I don't see the pot in the >> pictures, but I do in the schematic. >> >> You may want to consider lowering the entire network resistance by >> scaling everything down, say ten times lower or more. This would reduce >> noise, and lessen effects from the varicap bias (leakage) current in the >> OCXO. Also, I have seen a number of OCXOs with an internal termination >> resistor (like 50-100 k) on the tuning line - that has spoiled a lot of >> fun for me, having to worry about the characteristics of that resistor, >> and including it in the deal. With an unknown or unspecified OCXO, it's >> good to check for any unwanted extra parts. >> >> It may help the stability to put some insulation around the tuning >> resistor network and maybe the DAC too, especially if the waste heat >> from the OCXO is significant. >> >> Ed >> >> ___ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > > > -- > > --Jim Harman > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Thanks for your ideas, Ed. In my actual implementation I am using reasonably low tempco 1% fixed resistors. In the photo they are the blue resistors between the DAC and the OCXO. I have noticed a significant temperature sensitivity in the system, especially if I run it with the cover off.so the ambient temperature can change quickly. Moving air has a short term effect on the OCXO and to a lesser extent the GPS. Sustained ambient temperature changes cause an offset in the DAC value when the loop is closed, so the loop is doing its job to compensate for them. Putting the system in a small picnic cooler with several water bottles makes any temperature changes slower than the loop time constant, which should minimize temperature effects on the overall system. I calculate the open loop temperature sensitivity at about 7e-11 / deg C. The C-MAC oscillator spec for what it's worth is "-10 C to +70 C / ref.at +30 C < +/- 3.0 e-9". If the variation was linear, which I realize it is probably not, that would be 7.5e-11 / deg C so I am in the same ballpark. On Fri, Apr 5, 2019 at 8:21 PM ed breya wrote: > I'd recommend that once you get things figured out and tuned up to > nominal running conditions, you should narrow the offset pot range, and > use good low-TC resistors to make up most of the network R, with the pot > having as small an effect as practical. BTW I don't see the pot in the > pictures, but I do in the schematic. > > You may want to consider lowering the entire network resistance by > scaling everything down, say ten times lower or more. This would reduce > noise, and lessen effects from the varicap bias (leakage) current in the > OCXO. Also, I have seen a number of OCXOs with an internal termination > resistor (like 50-100 k) on the tuning line - that has spoiled a lot of > fun for me, having to worry about the characteristics of that resistor, > and including it in the deal. With an unknown or unspecified OCXO, it's > good to check for any unwanted extra parts. > > It may help the stability to put some insulation around the tuning > resistor network and maybe the DAC too, especially if the waste heat > from the OCXO is significant. > > Ed > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > -- --Jim Harman ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Hi One thing that may well help in terms of internal bias resistors on an OCXO is that they often are inside the ovenized section of the device. There’s no guarantee of course. If you have an effective thermal gain “in the hundreds”, a common 50 ppm/C resistor set would get down to a fraction of a ppm. Effectively it would be better than almost anything else you might last up outside the device. Again - no guarantees ….. The device needs to hit it’s temperature spec at a range of EFC voltages, whatever is in there does have some constraints on it. So maybe not totally unlikely. Bob > On Apr 5, 2019, at 8:44 PM, ed breya wrote: > > I'd recommend that once you get things figured out and tuned up to nominal > running conditions, you should narrow the offset pot range, and use good > low-TC resistors to make up most of the network R, with the pot having as > small an effect as practical. BTW I don't see the pot in the pictures, but I > do in the schematic. > > You may want to consider lowering the entire network resistance by scaling > everything down, say ten times lower or more. This would reduce noise, and > lessen effects from the varicap bias (leakage) current in the OCXO. Also, I > have seen a number of OCXOs with an internal termination resistor (like > 50-100 k) on the tuning line - that has spoiled a lot of fun for me, having > to worry about the characteristics of that resistor, and including it in the > deal. With an unknown or unspecified OCXO, it's good to check for any > unwanted extra parts. > > It may help the stability to put some insulation around the tuning resistor > network and maybe the DAC too, especially if the waste heat from the OCXO is > significant. > > Ed > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
I'd recommend that once you get things figured out and tuned up to nominal running conditions, you should narrow the offset pot range, and use good low-TC resistors to make up most of the network R, with the pot having as small an effect as practical. BTW I don't see the pot in the pictures, but I do in the schematic. You may want to consider lowering the entire network resistance by scaling everything down, say ten times lower or more. This would reduce noise, and lessen effects from the varicap bias (leakage) current in the OCXO. Also, I have seen a number of OCXOs with an internal termination resistor (like 50-100 k) on the tuning line - that has spoiled a lot of fun for me, having to worry about the characteristics of that resistor, and including it in the deal. With an unknown or unspecified OCXO, it's good to check for any unwanted extra parts. It may help the stability to put some insulation around the tuning resistor network and maybe the DAC too, especially if the waste heat from the OCXO is significant. Ed ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
[time-nuts] Lars GPSDO
On a real simple GPSDO that I built, I drove the PWM into a 2N700x fet, pulled the FET output up with the OCXO VREF / resistor, and added a RC filter. Seemed to work rather well. The OCXO VREF was around 7V. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Hi The real question is “how worried are you about phase noise?”. Next past that is the bandwidth of the EFC input on the OCXO. Once you have those *and* the LF noise spectrum of the DAC …. you can work it all out :) Since the EFC “does FM” and the noise you worry about is PM ( = phase noise) there is already a 1/F ( = low pass filter) simply because of the physics involved. There is no reason to go insane about this. At the same time, it *could* be an issue. One classic example of all this is the TBolt. You have an OCXO with a “many ppm” sort of EFC. That’s a lot of sensitivity. Cutting it back to a “< 0.1 ppm” sort of range with a couple of resistors would have a big impact on how things work. Bob > On Apr 5, 2019, at 6:04 PM, Jim Harman wrote: > > I see the AD5680 uses a dithering approach to get the last 2 bits, so the > noise may be a problem. I suppose I could suppress it with a LPF on the > DAC output. Also I have no way to measure the effect of any DAC noise on > the oscillator frequency, so for the time being I am blissfully ignorant. > > I suppose I could AC couple a high gain amplifier to the DAC output and > measure the noise, or tune an FM receiver to the 9th harmonic of the 10 MHz > and listen for noise. > > I have successfully hand soldered a 6 pin SOT-23 but the AD5680 is an 8 pin > chip and thus even more challenging, and breakout boards for them are hard > to find and/or expensive. I see Digi-Key has a service where you buy the > chip and they will deliver it soldered to a breakout board for a few $ > extra. I may give that a try > > On Fri, Apr 5, 2019 at 5:21 PM Bob kb8tq wrote: > >> Hi >> >> If you do decide to play with the AD5680, be careful of the “tone” on the >> output >> at about 180 Hz. You might also want to check the noise inside 100 Hz >> since the >> spec sheet is of no help there. If the 180 Hz is the third harmonic of >> something much >> larger at 60 Hz …. yikes ….. Since there don’t seem to be noise specs on >> the MCP4725, >> who knows if things are getting better or worse. I’d bet better ….. >> >> That *is* a weird little package …. >> >> Bob >> >> -- > > --Jim Harman > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
I see the AD5680 uses a dithering approach to get the last 2 bits, so the noise may be a problem. I suppose I could suppress it with a LPF on the DAC output. Also I have no way to measure the effect of any DAC noise on the oscillator frequency, so for the time being I am blissfully ignorant. I suppose I could AC couple a high gain amplifier to the DAC output and measure the noise, or tune an FM receiver to the 9th harmonic of the 10 MHz and listen for noise. I have successfully hand soldered a 6 pin SOT-23 but the AD5680 is an 8 pin chip and thus even more challenging, and breakout boards for them are hard to find and/or expensive. I see Digi-Key has a service where you buy the chip and they will deliver it soldered to a breakout board for a few $ extra. I may give that a try On Fri, Apr 5, 2019 at 5:21 PM Bob kb8tq wrote: > Hi > > If you do decide to play with the AD5680, be careful of the “tone” on the > output > at about 180 Hz. You might also want to check the noise inside 100 Hz > since the > spec sheet is of no help there. If the 180 Hz is the third harmonic of > something much > larger at 60 Hz …. yikes ….. Since there don’t seem to be noise specs on > the MCP4725, > who knows if things are getting better or worse. I’d bet better ….. > > That *is* a weird little package …. > > Bob > > -- --Jim Harman ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Jim Your hitting the nail on the head with my questions. Though I have quite a few GPSDOs, Berts results have my attention. I can also tell that the solution you have should also be relatively low power consumption. Lets see what his 24 hour test does. That said changing to the DAC was a good idea, but that also changed the software. I agree on the SOT 23 technology. I have a microscope and can handle it just barely. There are smaller technologies like msop8s and hand soldering just doesn't work. BGA's heck n. Figuring out the offsets for a ocxo tends to be a challenge. Careful measurements and behavior characterization since everything I ever pickup is random flea market stuff. Regards Paul. On Fri, Apr 5, 2019 at 4:00 PM Jim Harman wrote: > Paul, > > Yes, I switched from Lars's PWM based DAC to the MCP4725, which is > available on an inexpensive breakout board. I wanted to avoid the original > design's sensitivity to the 5V supply voltage. My OCXO has a 4V reference > output and I use that to power the DAC. I use a resistor network between > the DAC and the VFC pin of the OCXO to shift the DC level and reduce the > control range, effectively increasing the 12 bit resolution of the DAC. A > change I am contemplating is to switch to an AD5680 18 bit DAC which looks > ideal for this application, but I have been put off by the tiny SOT23 > package. > > A schematic is attached. Recent changes, not reflected in this schematic, > are a circuit to linearize the diode-R-C integrator and also output buffers > for the 10 MHz and 1 pps. > > > On Fri, Apr 5, 2019 at 12:10 PM paul swed wrote: > > > Jim > > The whole gpsdo seems impressive and also seems to be performing well. > > When I look at the Lars GPSDO what you have built doesn't quite seem to > > match. Such as using a real DAC. Is this one of the variants that can > > exist? > > Regards > > Paul > > WB8TSL > > > > > > > > -- > > --Jim Harman > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Hi If you do decide to play with the AD5680, be careful of the “tone” on the output at about 180 Hz. You might also want to check the noise inside 100 Hz since the spec sheet is of no help there. If the 180 Hz is the third harmonic of something much larger at 60 Hz …. yikes ….. Since there don’t seem to be noise specs on the MCP4725, who knows if things are getting better or worse. I’d bet better ….. That *is* a weird little package …. Bob > On Apr 5, 2019, at 3:27 PM, Jim Harman wrote: > > Paul, > > Yes, I switched from Lars's PWM based DAC to the MCP4725, which is > available on an inexpensive breakout board. I wanted to avoid the original > design's sensitivity to the 5V supply voltage. My OCXO has a 4V reference > output and I use that to power the DAC. I use a resistor network between > the DAC and the VFC pin of the OCXO to shift the DC level and reduce the > control range, effectively increasing the 12 bit resolution of the DAC. A > change I am contemplating is to switch to an AD5680 18 bit DAC which looks > ideal for this application, but I have been put off by the tiny SOT23 > package. > > A schematic is attached. Recent changes, not reflected in this schematic, > are a circuit to linearize the diode-R-C integrator and also output buffers > for the 10 MHz and 1 pps. > > > On Fri, Apr 5, 2019 at 12:10 PM paul swed wrote: > >> Jim >> The whole gpsdo seems impressive and also seems to be performing well. >> When I look at the Lars GPSDO what you have built doesn't quite seem to >> match. Such as using a real DAC. Is this one of the variants that can >> exist? >> Regards >> Paul >> WB8TSL >> >> >> > > -- > > --Jim Harman > v4.pdf>___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Paul, Yes, I switched from Lars's PWM based DAC to the MCP4725, which is available on an inexpensive breakout board. I wanted to avoid the original design's sensitivity to the 5V supply voltage. My OCXO has a 4V reference output and I use that to power the DAC. I use a resistor network between the DAC and the VFC pin of the OCXO to shift the DC level and reduce the control range, effectively increasing the 12 bit resolution of the DAC. A change I am contemplating is to switch to an AD5680 18 bit DAC which looks ideal for this application, but I have been put off by the tiny SOT23 package. A schematic is attached. Recent changes, not reflected in this schematic, are a circuit to linearize the diode-R-C integrator and also output buffers for the 10 MHz and 1 pps. On Fri, Apr 5, 2019 at 12:10 PM paul swed wrote: > Jim > The whole gpsdo seems impressive and also seems to be performing well. > When I look at the Lars GPSDO what you have built doesn't quite seem to > match. Such as using a real DAC. Is this one of the variants that can > exist? > Regards > Paul > WB8TSL > > > -- --Jim Harman Arduino GPSDO Schematic v4.pdf Description: Adobe PDF document ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Jim The whole gpsdo seems impressive and also seems to be performing well. When I look at the Lars GPSDO what you have built doesn't quite seem to match. Such as using a real DAC. Is this one of the variants that can exist? Regards Paul WB8TSL On Thu, Apr 4, 2019 at 8:03 PM Jim Harman wrote: > On Thu, Apr 4, 2019 at 5:00 PM paul swed wrote: > > > Bert > > Have to say I like the construction method. Guessing you shifted it to > the > > middle picture. > > Can you please tell me what the middle box with the -12 counter is? > > Regards > > Paul > > > > > > Yes the GPSDO is built on a "Perma-Proto" board that mimics the layout of > those solderless prototype boards. I wanted everything on 0.1 in. centers > for easy soldering. > > It was originally in the cardboard Samsung cellphone box at the left. Bert > suggested it might not survive shipping to Florida, so I moved it to the > plastic box with a clear cover. In addition to being sturdier, it lets you > see the blinking LEDs. while it is operating. > > Thanks again to Bert for his kind testing assistance. > -- > > --Jim Harman > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
I originally did it for Lars but the results are an eye opener and fits nicely in our GPSDO evaluations. A 24 hour test starts at 6 AM all my tests start at the same time. Hope to have a plot by 8 to morrow.Bert In a message dated 4/4/2019 8:03:10 PM Eastern Standard Time, j99har...@gmail.com writes: On Thu, Apr 4, 2019 at 5:00 PM paul swed wrote: > Bert > Have to say I like the construction method. Guessing you shifted it to the > middle picture. > Can you please tell me what the middle box with the -12 counter is? > Regards > Paul > > > Yes the GPSDO is built on a "Perma-Proto" board that mimics the layout of those solderless prototype boards. I wanted everything on 0.1 in. centers for easy soldering. It was originally in the cardboard Samsung cellphone box at the left. Bert suggested it might not survive shipping to Florida, so I moved it to the plastic box with a clear cover. In addition to being sturdier, it lets you see the blinking LEDs. while it is operating. Thanks again to Bert for his kind testing assistance. -- --Jim Harman ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
On Thu, Apr 4, 2019 at 5:00 PM paul swed wrote: > Bert > Have to say I like the construction method. Guessing you shifted it to the > middle picture. > Can you please tell me what the middle box with the -12 counter is? > Regards > Paul > > > Yes the GPSDO is built on a "Perma-Proto" board that mimics the layout of those solderless prototype boards. I wanted everything on 0.1 in. centers for easy soldering. It was originally in the cardboard Samsung cellphone box at the left. Bert suggested it might not survive shipping to Florida, so I moved it to the plastic box with a clear cover. In addition to being sturdier, it lets you see the blinking LEDs. while it is operating. Thanks again to Bert for his kind testing assistance. -- --Jim Harman ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
It is an Adret multiplier like the Tracor 527 but in stead of meters has LED's we use both can read the LED's from 10 feet away but the meter of the Tracor shows short changes, that is how I found out the tbolt problem.Bert In a message dated 4/4/2019 5:00:25 PM Eastern Standard Time, paulsw...@gmail.com writes: Bert Have to say I like the construction method. Guessing you shifted it to the middle picture. Can you please tell me what the middle box with the -12 counter is? Regards Paul On Thu, Apr 4, 2019 at 4:00 PM ew via time-nuts wrote: > Recently Lars GPSDO came up on time nuts. I was working with Lars but we > focused on other subjects. Never got around to test one. So when Jim Harman > mentioned it I contacted him off list and offered to test his unit. He send > me a picture of his unit. Oh boy but a promises is a promises. Encouraged > him to stabilize it for shipping. Got it in the mail today. See attached > picture. Had it up and running in 5 minutes. 5 minutes later 1 E -10. 10 > minutes later 3 E-11. After an hour up and down in the E-12 range like all > the other ones.Will let it stabilize for 48 hours and do a 24 hour plot > like we have done on all our other GPSDO's to better understand frequency > over time do to ionosphere. Mainly do to the delay of ublox F9.Jim can go > in detail about his work with Lars > > Bert___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO
Bert Have to say I like the construction method. Guessing you shifted it to the middle picture. Can you please tell me what the middle box with the -12 counter is? Regards Paul On Thu, Apr 4, 2019 at 4:00 PM ew via time-nuts wrote: > Recently Lars GPSDO came up on time nuts. I was working with Lars but we > focused on other subjects. Never got around to test one. So when Jim Harman > mentioned it I contacted him off list and offered to test his unit. He send > me a picture of his unit. Oh boy but a promises is a promises. Encouraged > him to stabilize it for shipping. Got it in the mail today. See attached > picture. Had it up and running in 5 minutes. 5 minutes later 1 E -10. 10 > minutes later 3 E-11. After an hour up and down in the E-12 range like all > the other ones.Will let it stabilize for 48 hours and do a 24 hour plot > like we have done on all our other GPSDO's to better understand frequency > over time do to ionosphere. Mainly do to the delay of ublox F9.Jim can go > in detail about his work with Lars > > Bert___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO on EEVblog
Charles, Attached is another way to do it. A higher charging voltage increases capacitor voltage linearity over the range of the ADC making curve compensation easier. The bus switch has a very low on resistance to pretty much discharge the cap. The design is pretty old - I'm sure there are better parts available now. I believe there was also a calibration cycle that occurred fairly often which is why U21 looks so busy. Cheers, Bob Martin On 9/8/2018 8:17 PM, Jim Harman wrote: Charles wrote, According to my tests, the B-C junction of a high-quality 2N3904 has about 50pA of leakage at 20vDC reverse voltage. That's good to know, but the problem I had with this circuit was with the forward current at a low forward voltage. With this phase detector, the HC4046 makes a 0 to 1 usec pulse representing the time difference between the 1 pps from the GPS and the 1 MHz derived from the OCXO. This charges the capacitor to a voltage proportional to the width of the pulse, the ADC measures the capacitor voltage, and the cap discharges through the 1 Meg resistor during the rest of the second. When the HC4046 output goes low at the end of the pulse, the voltage on the other side of the 1N5711 diode goes down to about 0.4 V. With a 1N4148 or similar diode instead of the transistor, the forward current after the capacitor is mostly discharged through the 1 meg resistor is enough to prevent the capacitor from discharging all the way and the minimum voltage at the ADC is about 0.3 V. or about 120 counts on the ADC. I was able to get this below 90 counts by using the transistor. It might be possible to reduce the 1 Meg resistor, but then we risk significantly discharging the capacitor in the short time between the end of the pulse and the A/D reading. I would certainly be interested in any suggestions on improving this circuit. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO on EEVblog
Use the phase detector output to drive the tristate control input of a fast CMOS tristate state buffer (eg 74HC126 or faster) which in turn drives the RC network eliminating the diode. Then correct for the exponential charging characteristics using the micro. For best results increase ADC resolution by 1 bit or more and set RC charge time constant to around the maximum expected time interval. Using a 2 stage synchroniser to drive the tristate control input of the buffer to minimise metastability issues may also be useful. Even linear Time to amplitude converter circuits using a current source exhibit nonlinearities. Its easy enough to calibrate the nonlinearity of a simple TAC. Bruce > On 09 September 2018 at 14:17 Jim Harman wrote: > > > Charles wrote, > > > > According to my tests, the B-C junction of a high-quality 2N3904 has > > about 50pA of leakage at 20vDC reverse voltage. > > > That's good to know, but the problem I had with this circuit was with the > forward current at a low forward voltage. With this phase detector, the > HC4046 makes a 0 to 1 usec pulse representing the time difference between > the 1 pps from the GPS and the 1 MHz derived from the OCXO. This charges > the capacitor to a voltage proportional to the width of the pulse, the ADC > measures the capacitor voltage, and the cap discharges through the 1 Meg > resistor during the rest of the second. > > When the HC4046 output goes low at the end of the pulse, the voltage on the > other side of the 1N5711 diode goes down to about 0.4 V. With a 1N4148 or > similar diode instead of the transistor, the forward current after the > capacitor is mostly discharged through the 1 meg resistor is enough to > prevent the capacitor from discharging all the way and the minimum voltage > at the ADC is about 0.3 V. or about 120 counts on the ADC. I was able to > get this below 90 counts by using the transistor. It might be possible to > reduce the 1 Meg resistor, but then we risk significantly discharging the > capacitor in the short time between the end of the pulse and the A/D > reading. > > I would certainly be interested in any suggestions on improving this > circuit. > > > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO on EEVblog
Charles wrote, > > According to my tests, the B-C junction of a high-quality 2N3904 has > about 50pA of leakage at 20vDC reverse voltage. That's good to know, but the problem I had with this circuit was with the forward current at a low forward voltage. With this phase detector, the HC4046 makes a 0 to 1 usec pulse representing the time difference between the 1 pps from the GPS and the 1 MHz derived from the OCXO. This charges the capacitor to a voltage proportional to the width of the pulse, the ADC measures the capacitor voltage, and the cap discharges through the 1 Meg resistor during the rest of the second. When the HC4046 output goes low at the end of the pulse, the voltage on the other side of the 1N5711 diode goes down to about 0.4 V. With a 1N4148 or similar diode instead of the transistor, the forward current after the capacitor is mostly discharged through the 1 meg resistor is enough to prevent the capacitor from discharging all the way and the minimum voltage at the ADC is about 0.3 V. or about 120 counts on the ADC. I was able to get this below 90 counts by using the transistor. It might be possible to reduce the 1 Meg resistor, but then we risk significantly discharging the capacitor in the short time between the end of the pulse and the A/D reading. I would certainly be interested in any suggestions on improving this circuit. > > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO on EEVblog
Hi Jim, On 09/08/2018 10:02 PM, Jim Harman wrote: > Magnus said, > > It would be interesting to test the linearity of the TIC separately for > > instance. > > I have done some testing of the TIC. It works quite well for the Arduino > Uno with its 1 V full scale ADC setting, but the exponential shape of the > RC charging from the 5 V is quite evident if you use a processor like the > Micro's 32u4 with a 2.56 V sensitivity. This circuit is also sensitive to > any noise on the 5 V supply. The linearity is not too important for this > application as long as it does not affect the loop gain so much that the > loop becomes unstable. Agreed. It is expected, but it would be fun to see how well it would do. > However I have modified the circuit to use a simple 2 ma current source as > shown in the attached schematic, with significantly improved linearity. H/T > to Horowitz and Hill's The Art of Electronics for the idea of using an LED > as the voltage reference for the current source. The diode-connected 2N3904 > has less leakage at a small forward voltage than any small-signal diodes I > tested, so the output voltage with a very narrow input pulse is close to > zero. A more serious current source helps. It comes as no big surprise that this has been a topic of interest to several designers, hence there exists a number of patents on it. Different approaches have been used. The HP5335A for instance uses a three transistor setup, where the current source, a resistor, is buffered behind the transistor pair that also switches it in and out. The HP5335A then acts as pulse extender with x200 and a TTL counter to complete the ADC process. Modern designs use a proper ADC instead but the basic problem remains the same. > I could run some tests on my version by setting the oscillator slightly > off-frequency and capturing the resulting sloped TIC output, and would be > happy to share the results if you are interested. Please do. This is one of the methods and probably the easiest to setup for most. Another approach is to use a programmable delay generator with enough resolution, but having one of those around isn't as common as a RF generator which can be detuned with sufficient resolution. As I spend the evening refactoring the lab-bench, the RF generators gets a more prominent placement as well as one of the delay generators. Hope to get the TICs stacked up nicely for ones. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Lars GPSDO on EEVblog
Magnus said, It would be interesting to test the linearity of the TIC separately for instance. I have done some testing of the TIC. It works quite well for the Arduino Uno with its 1 V full scale ADC setting, but the exponential shape of the RC charging from the 5 V is quite evident if you use a processor like the Micro's 32u4 with a 2.56 V sensitivity. This circuit is also sensitive to any noise on the 5 V supply. The linearity is not too important for this application as long as it does not affect the loop gain so much that the loop becomes unstable. However I have modified the circuit to use a simple 2 ma current source as shown in the attached schematic, with significantly improved linearity. H/T to Horowitz and Hill's The Art of Electronics for the idea of using an LED as the voltage reference for the current source. The diode-connected 2N3904 has less leakage at a small forward voltage than any small-signal diodes I tested, so the output voltage with a very narrow input pulse is close to zero. I could run some tests on my version by setting the oscillator slightly off-frequency and capturing the resulting sloped TIC output, and would be happy to share the results if you are interested. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
[time-nuts] Lars GPSDO on EEVblog
Lady Heather now has some partial support for the Lars GPSDO. It does not directly send any commands to the device (I don't have one yet to implement that), but you can use the !u or !t keyboard commands to do that, Heather treats the Lars GPSDO as a time interval counter. You need to start Heather with /rxi (time interval counter as input device) and /itl (interval counter device is a Lars GPSDO). You can also use the Lars GPSDO as an "extra" input device with the GPS receiver as the main input device (/ei=port /itl). This mode lets Heather calculate true ADEV values while monitoring the GPS receiver. When using the Lars GPSDO, you do want to manually set the DAC gain setting () so that the frequency calculations based on the DAC setting are accurate. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
