Author Topic: Economical option for precision frequency reference?  (Read 117776 times)

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Offline evb149

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Re: Economical option for precision frequency reference?
« Reply #150 on: July 31, 2015, 05:09:24 pm »
I haven't looked at the details fully but that photograph looks like there was quite a lot of phase variation relative to the entire 10MHz cycle, which surprised me.  I would have expected the jitter to be visible only on one zoomed in edge of the waveforms and to account for a small fraction of a cycle's phase over a relatively short (hour) interval with there being a bit of phase noise with an underlying phase drift progression due to the frequency discord.

I also wonder about whether the clock outputs wouldn't get cleaner wrt. jitter during GPS LOS periods since there is no further input for the corrections.  Of course I suspect the devices create a mathematical model of a given OCXO's 'inherent' frequency error and temperature sensitivity and may try to keep correcting for the predicted error according to the trained model data (and perhaps also temperature measurement though probably not if it is assumed to simply be stable in the oven's feedback).  But eventually I suppose the 'remembered' feedback due to the frequency error modem might 'age out' and then one would be left with basically the OCXO's intrinsic output given the temperature / power supply / noise related fluctuations.
Or I suppose if you cold started a unit and could get it to output the 15MHz or 10MHz or whatever without having the GPS related corrections enabled due to insufficient data to model the OCXO error at which point you'd be left with less control system jitter, though it may simply not output the high frequency at such a time.  I recall seeing something about a fault described in the RFTG's SW report descriptions as 'flywheel failed, duration > 8 hr' so I wasn't sure what it'd do wrt. HF output after 8hr or 24hr in flywheel.

I suppose it might also be possible to trick the non-GPS RFTG unit into giving 'pure' OCXO output by doing something like feeding its own 15MHz output divided by 15M back into its PPS reference input as if that was coming from the GPS disciplining unit, I suppose one might need to do some phase adjustment to get it to accept that there was a 0 static phase error in that process.

 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #151 on: August 01, 2015, 12:30:17 am »
I haven't looked at the details fully but that photograph looks like there was quite a lot of phase variation relative to the entire 10MHz cycle, which surprised me.  I would have expected the jitter to be visible only on one zoomed in edge of the waveforms and to account for a small fraction of a cycle's phase over a relatively short (hour) interval with there being a bit of phase noise with an underlying phase drift progression due to the frequency discord.

I also wonder about whether the clock outputs wouldn't get cleaner wrt. jitter during GPS LOS periods since there is no further input for the corrections.  Of course I suspect the devices create a mathematical model of a given OCXO's 'inherent' frequency error and temperature sensitivity and may try to keep correcting for the predicted error according to the trained model data (and perhaps also temperature measurement though probably not if it is assumed to simply be stable in the oven's feedback).  But eventually I suppose the 'remembered' feedback due to the frequency error modem might 'age out' and then one would be left with basically the OCXO's intrinsic output given the temperature / power supply / noise related fluctuations.
Or I suppose if you cold started a unit and could get it to output the 15MHz or 10MHz or whatever without having the GPS related corrections enabled due to insufficient data to model the OCXO error at which point you'd be left with less control system jitter, though it may simply not output the high frequency at such a time.  I recall seeing something about a fault described in the RFTG's SW report descriptions as 'flywheel failed, duration > 8 hr' so I wasn't sure what it'd do wrt. HF output after 8hr or 24hr in flywheel.

I suppose it might also be possible to trick the non-GPS RFTG unit into giving 'pure' OCXO output by doing something like feeding its own 15MHz output divided by 15M back into its PPS reference input as if that was coming from the GPS disciplining unit, I suppose one might need to do some phase adjustment to get it to accept that there was a 0 static phase error in that process.

The microprocessor in the unit is making EFC corrections periodically to keep the thing in sync with the GPS signal. I can see in the reported PPS stats that the PPS value is varying between about +/- 40ns of the GPS time, and for others here, whose units arent' quite as well behaved, it's more like +/-90ns. The o-scope capture posted earlier was I assume taken by turning on infinite persistence on the scope and letting it run for 3 hours. So I would entirely expect this sort of display as the EFC value gets updated many times over that 3 hour span.
 

Offline evb149

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Re: Economical option for precision frequency reference?
« Reply #152 on: August 01, 2015, 03:12:53 am »
Interesting.  That is quite a lot of jitter for some of the units (40-90ns).  Particularly out of a 100ns period 10MHz cycle anything more than 1ns of jitter is relatively speaking a lot (3.6 degrees / ns @ 10MHz).

BTW I looked at the data from KO4BB's site but I still haven't seen anything like an output specification for the 15MHz or PPS signals, is there a published value for what the frequency accuracy and jitter and such are supposed to be when the unit is operating normally?

As I understand it the PPS signals for timing oriented equipment are actually supposed to be synchronous with the transition of the GPS referenced second's transition time, generally down to nanoseconds levels of precision.  I'm not sure that the RFTG units are oriented toward "timekeeping" so much as an accurate long term stable frequency reference at 1Hz and 15MHz though I suppose they probably also keep the clock edge of the PPS relatively aligned to the actual transition of the second though it sounds like that is only to the level of ~40ns to ~90ns which really sounds like it is about a half cycle of the 5MHz crystal clock.
Though it is odd if it can flywheel for 24hrs with microseconds of drift accuracy and still get large fractions (90ns) of that as short term corrections.
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #153 on: August 01, 2015, 06:48:29 am »
Interesting.  That is quite a lot of jitter for some of the units (40-90ns).  Particularly out of a 100ns period 10MHz cycle anything more than 1ns of jitter is relatively speaking a lot (3.6 degrees / ns @ 10MHz).

BTW I looked at the data from KO4BB's site but I still haven't seen anything like an output specification for the 15MHz or PPS signals, is there a published value for what the frequency accuracy and jitter and such are supposed to be when the unit is operating normally?

As I understand it the PPS signals for timing oriented equipment are actually supposed to be synchronous with the transition of the GPS referenced second's transition time, generally down to nanoseconds levels of precision.  I'm not sure that the RFTG units are oriented toward "timekeeping" so much as an accurate long term stable frequency reference at 1Hz and 15MHz though I suppose they probably also keep the clock edge of the PPS relatively aligned to the actual transition of the second though it sounds like that is only to the level of ~40ns to ~90ns which really sounds like it is about a half cycle of the 5MHz crystal clock.
Though it is odd if it can flywheel for 24hrs with microseconds of drift accuracy and still get large fractions (90ns) of that as short term corrections.

It's not 40 - 90ns of jitter on a waveform at 10 MHz (100ns). It's 40-90ns delta over the GPS reference over a 1 second interval. The hardware has no mechanism to measure jitter over a individual periods of the 10Mhz clock. What it does it count the number of 10 MHz (probably 5 Mhz in this case - but let's use 10MHz for simplicity of discussion) OCXO cycles that occur from one GPS PPS rising edge to the next GPS PPS rising edge. If the count is 9,999,996, it calculates that it's -40ns relative to GPS, and it uses this value as an error signal into the PID loop that is controlling the EFC voltage. Similarly, if the count is 10,000,009, it calculates that it's 90ns relative to the GPS, feeds that into the PID loop, and adjusts the EFC accordingly.

Regarding the stability over the course of the day, I suspect it estimates this based on some calculation involving the rate of change of EFC and the standard deviation or variance of frequency when in closed-loop mode (GPS tracking active). The theory being that if it goes into an open-loop mode (no GPS), it will leave the EFC voltage at a constant value, and the drift in frequency will be a function of how much it was having to change EFC previously and how much the frequency was varying even when EFC was tightly controlled.
« Last Edit: August 01, 2015, 07:45:53 am by motocoder »
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #154 on: August 01, 2015, 02:44:48 pm »
I have uncovered a couple of interesting data points on my Lucent unit.

1) I noticed that the PPS fluctuations seemed to increase every time the A/C turned on here. So I put a small 12V fan behind the unit blowing air through at a very low CFM rate. Notably, this seemed to cause the fluctuations to get worse. I observed this over many hours, and as soon as I removed the fan, it returned to the previous pattern.
2) After removing the fan, I put a  piece of cardboard around the A/C vent so that the air would not circulate past the Lucent unit as much. This settled things way down, and the PPS value seems to mostly be +/- 10ns with occasional excursions to +/- 20ns and a much rarer spike to +/-30 ns.

During #2, the HUD value started to go down for awhile, but then started increasing again. It appears strongly correlated with the rate of change of the EFC value, which was fairly steady for a short time (while HUD was decreasing), but then started to rise again.

So in conclusion, the unit is sensitive to the environmental air temperature and possibly the EMI generated by that fan. The HUD value seems to be strongly correlated with the rate of change of EFC. The more constant EFC is when the unit is locked to GPS, the lower the HUD value, regardless of how much "jitter" there is of PPS.
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #155 on: August 01, 2015, 03:17:07 pm »
I have uncovered a couple of interesting data points on my Lucent unit.

1) I noticed that the PPS fluctuations seemed to increase every time the A/C turned on here. So I put a small 12V fan behind the unit blowing air through at a very low CFM rate. Notably, this seemed to cause the fluctuations to get worse. I observed this over many hours, and as soon as I removed the fan, it returned to the previous pattern.
2) After removing the fan, I put a  piece of cardboard around the A/C vent so that the air would not circulate past the Lucent unit as much. This settled things way down, and the PPS value seems to mostly be +/- 10ns with occasional excursions to +/- 20ns and a much rarer spike to +/-30 ns.

During #2, the HUD value started to go down for awhile, but then started increasing again. It appears strongly correlated with the rate of change of the EFC value, which was fairly steady for a short time (while HUD was decreasing), but then started to rise again.

So in conclusion, the unit is sensitive to the environmental air temperature and possibly the EMI generated by that fan. The HUD value seems to be strongly correlated with the rate of change of EFC. The more constant EFC is when the unit is locked to GPS, the lower the HUD value, regardless of how much "jitter" there is of PPS.

I noticed that mine reacted strongly when a stray sunbeam fell on it for a few minutes.  I'm surprised that a double-oven OCXO would be that sensitive to external thermal influences.  The high number of holes in the case suggests that when these units were in service there was a fan tray or something similar that flooded the entire bay with a continuous stream of air to maintain a constant temperature.  Closing off those holes would have to be done cautiously to make sure that nothing was going to overheat, but reducing drafts on the oscillator would likely improve performance.

By the way, don't add extra insulation around the OCXO!  The internal heat must be allowed to escape.  If you prevent that, the oven controller won't be able to control the temperature properly.  Worst case, the internal temperature starts to rise and the unit cooks itself.
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #156 on: August 01, 2015, 05:01:55 pm »
I noticed that mine reacted strongly when a stray sunbeam fell on it for a few minutes.  I'm surprised that a double-oven OCXO would be that sensitive to external thermal influences.  The high number of holes in the case suggests that when these units were in service there was a fan tray or something similar that flooded the entire bay with a continuous stream of air to maintain a constant temperature.  Closing off those holes would have to be done cautiously to make sure that nothing was going to overheat, but reducing drafts on the oscillator would likely improve performance.

Yes, my theory on this is that we're talking about such minute changes in frequency, that even a tiny outside temperature change can have an effect. I wish I had some other unit to compare against - not sure if this is normal. Do you think the stray sunbeam was impacting the outside temperature of the oven, or is there perhaps some chip-scale IC inside these units?

BTW, I found an interesting paper from Milliren on crystal oscillator aging: http://www.mti-milliren.com/MTIPapers/Ext_Aging_Perf_Results.pdf

By the way, don't add extra insulation around the OCXO!  The internal heat must be allowed to escape.  If you prevent that, the oven controller won't be able to control the temperature properly.  Worst case, the internal temperature starts to rise and the unit cooks itself.

Noted. Thanks
« Last Edit: August 01, 2015, 05:09:29 pm by motocoder »
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #157 on: August 01, 2015, 05:23:05 pm »
Yes, my theory on this is that we're talking about such minute changes in frequency, that even a tiny outside temperature change can have an effect. I wish I had some other unit to compare against - not sure if this is normal. Do you think the stray sunbeam was impacting the outside temperature of the oven, or is there perhaps some chip-scale IC inside these units?

You mean like the RPi a few months back?  I think these units are too old for that.  If I remember correctly, it took the unit several minutes to recover after I blocked the sunbeam so I think it was a thermal effect.

I have a Z3801A which is equipped with an HP double-oven oscillator.  The oscillator is surprisingly crude for an HP unit.  They basically took an HP 10811 oscillator (really good reputation, by the way), made a few internal changes to increase the electrical tuning range, and then wrapped another heater and insulation around it.  There are autopsy photos online.  Once you dig out the oscillator, it even has the original label on it.  In spite of how crude it is, it works quite well.  The entire unit is in a solid steel box with a reasonable number of slots for cooling.  Thermal changes don't really bother it - even at these tiny levels.

 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #158 on: August 01, 2015, 05:43:40 pm »
Yes, my theory on this is that we're talking about such minute changes in frequency, that even a tiny outside temperature change can have an effect. I wish I had some other unit to compare against - not sure if this is normal. Do you think the stray sunbeam was impacting the outside temperature of the oven, or is there perhaps some chip-scale IC inside these units?

You mean like the RPi a few months back?  I think these units are too old for that.  If I remember correctly, it took the unit several minutes to recover after I blocked the sunbeam so I think it was a thermal effect.

I have a Z3801A which is equipped with an HP double-oven oscillator.  The oscillator is surprisingly crude for an HP unit.  They basically took an HP 10811 oscillator (really good reputation, by the way), made a few internal changes to increase the electrical tuning range, and then wrapped another heater and insulation around it.  There are autopsy photos online.  Once you dig out the oscillator, it even has the original label on it.  In spite of how crude it is, it works quite well.  The entire unit is in a solid steel box with a reasonable number of slots for cooling.  Thermal changes don't really bother it - even at these tiny levels.

I wonder why these Lucent units are so recommended on Time-Nuts. There seem to be a lot of compromises to them: 15 Mhz sine instead of 10, "dirty" 10 MHz output only available on redundant unit, low sensitivity 8-channel GPS, funky connectors  (power and others), ... And so far the performance does not seem to match the alternatives out there. Am I missing something here?

BTW - my HUD has now risen to 6.5us. It was as low as 1.9us at one point. Not sure what's causing this? Maybe this is a normal pattern as the crystal breaks in?

 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #159 on: August 01, 2015, 07:12:49 pm »
I wonder why these Lucent units are so recommended on Time-Nuts. There seem to be a lot of compromises to them: 15 Mhz sine instead of 10, "dirty" 10 MHz output only available on redundant unit, low sensitivity 8-channel GPS, funky connectors  (power and others), ... And so far the performance does not seem to match the alternatives out there. Am I missing something here?

BTW - my HUD has now risen to 6.5us. It was as low as 1.9us at one point. Not sure what's causing this? Maybe this is a normal pattern as the crystal breaks in?

If you want, you can go out and buy a brand new, shiny GPSDO with a 50 channel receiver that can hear a whisper from another planet.  You probably don't want to know the price!

In the past, we were spoiled by units like the Trimble Thunderbolt and the other members of the Z38 series.  Their performance is amazing.  The Z3801 has a 6 channel receiver that needs a bit higher level.  So what?  It works great!  The Thunderbolt only has an eight channel receiver.  It doesn't need any more.

I don't think it's fair to say that the Lucent units were 'recommended' on the Time-Nuts mailing list.  It wasn't discussed, but I suspect that people were hoping that these units would have similar performance to the Thunderbolt and other Z38xx units which are now either unavailable or uneconomical.  They have good names behind them, a good pedigree and, of course, they were reasonably priced.  Most of the chatter on Time-Nuts involved the reverse-engineering process and modifications to make the unit more time-nut-friendly.  That's part of the fun!  Until the units ran for an extended period, there was no way to know what its ultimate performance would be.  I suspect that there's some disappointment within the list, but not enough to cause outright bitching and moaning.  After all, these are surplus units rather than new and they were priced accordingly.  They were just designed with different parameters than the Thunderbolt and the other Z38xx units.

The HUP will wander around a lot - particularly while the crystal is breaking in.  On my well-broken-in Z3801, there's an interesting oscillation in the HUP.  It goes from a high of 3 to 4 us down to a low of < 500 ns and back up.  The period of this oscillation is about 7 days!  This oscillation isn't visible on any other parameter in the box.  I haven't attempted to get to the bottom of it.  It seems like HUP values in the 5 us range are not surprising and there's significant unit-to-unit variability - probably determined by the characteristics of the particular OCXO in the unit.

These units do exactly what they claim.  They provide an absolute frequency and time reference that's traceable to international standards and are within the reach of any hobbyist.  For the majority of users, any imperfections will be irrelevant.  If you're not satisfied with their performance, well, I guess that means you're a Time-Nut, too!   ;)

Ed
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #160 on: August 01, 2015, 07:51:13 pm »
I was just curious where the performance of this unit is going to end up relative to the Thunderbolt. ANd I don't agree that people weren't recommending these units on the time-nuts mailing list. I went through literally every post on this unit in the archives, and there are several occasions where someone there recommends the unit.  I think the performance is fine for my needs - like I said, it's already paid for itself by calibrating my 53131A.

I do have some remaining work to do to get the antenna into a permanent location. Not looking forward to crawling around on the roof...
 

Offline G0HZU

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Re: Economical option for precision frequency reference?
« Reply #161 on: August 01, 2015, 09:50:13 pm »
Rather than venture onto the roof, one (lazy) option would be to just dig it out once every 6 months (or once a year) to calibrate your counter if it now has the high stability OCXO option fitted.

I've got numerous bits of RF test gear that contain a high stability OCXO option and I've just gone round several of my signal generators and the OCXO in each is still within 0.02Hz of 10MHz. This is a much better result than I was expecting because at least one of the generators hasn't been adjusted for a year or so.

I usually find that my various OCXOs stay within 0.1Hz of 10MHz over 6 months although I've only had one of these (used Agilent) generators for about 8 months and the other one for about 18months so this is new data. The other one I tested was a Marconi 2024 and this hasn't been adjusted for ages because it requires a fiddly menu/password access routine. I suspect there may be an element of luck in the fact that it is still within 0.02Hz of 10MHz as I've had this generator several years and it usually drifts a bit more than this over a year or so.

The huge and expensive Toyocom OCXO in my Anritsu counter was also within about 0.01Hz of 10MHz but I think I adjusted it back in January.

If you fit an external antenna on the rooftop will you be leaving the GPSDO running 24/7?  I don't leave my OCXOs running 24/7 and the generators just get used as and when required and they still seem to perform well. Although they are all  >10 years old and the 2024 Marconi must be >18 years old.

« Last Edit: August 01, 2015, 10:12:46 pm by G0HZU »
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #162 on: August 01, 2015, 10:30:27 pm »
Rather than venture onto the roof, one (lazy) option would be to just dig it out once every 6 months (or once a year) to calibrate your counter if it now has the high stability OCXO option fitted.

Unfortunately, the Lucent wasn't even able to acquire a lock on a single satellite when the antenna was indoors next to the window. So that means every time I would go to set it up, I'd have to temporarily mount the antenna outside somewhere, and run the cable through the window. Not a huge big deal, but it's another factor in the decision on whether or not to permanently set up an antenna.

If you fit an external antenna on the rooftop will you be leaving the GPSDO running 24/7?  I don't leave my OCXOs running 24/7 and the generators just get used as and when required and they still seem to perform well. Although they are all  >10 years old and the 2024 Marconi must be >18 years old.

I was thinking of just leaving the Lucent GPSDO running 24/7, and connecting the external reference on the other test gear to that via some sort of distribution amplifier (there are a number of them available for cheap on eBay). I definitely don't want more than one OCXO running, as they generate a fair bit of heat. I'd also like to not have to lave that 53131A on as it runs the fan continuously. I've set up a remote switch for it currently so that I can shut it off when not in use. Having the external reference means I wouldn't have to wait for anything to warm up.

But setting up the antenna is a big task, as I have to set up lightning suppression, grounding, and a wife-approved method to get the coax into the house. I know what to do there, but it's quite a bit of work.
« Last Edit: August 01, 2015, 10:32:02 pm by motocoder »
 

Offline Vgkid

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Re: Economical option for precision frequency reference?
« Reply #163 on: August 01, 2015, 11:08:14 pm »
@ GOHZU , do you happen to know the model of your epson  oscillator?
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Offline usagi

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Re: Economical option for precision frequency reference?
« Reply #164 on: August 02, 2015, 03:00:34 am »
in comparison to my bg7tbl (oscilloquartz ocxo + ublox gps), the lucent is deaf. receiver sensitivity is abysmal and synchronization is extremely slow. like motocoder, i had to run an antenna outside to get it to work. it absolutely would not sync with the antenna inside a window. the 10mhz output is extremely distorted, and if you want to talk to it you have to jump through rs422 hoops. you even have to hack up a db9 power connector. ugh. you also have to source your own gps antenna. each unit sucks about 12W while running, so 24W total if you want to run the setup as designed. also due to its age, the lucent knows nothing about WAAS. the lucent is maybe 8 channels max.

very NOT recommended.

get a bg7tbl with oscilloquartz (2015-07-08) or trimble (2015-07-17) sub-board. you'll have a much more compact unit that uses much less power, much more sensitive and capable gps receiver, rs232, and true sine output. he even throws in a psu and gps antenna. it has no problem with the antenna inside the same window the lucent refused to work from.
« Last Edit: August 02, 2015, 10:19:14 am by usagi »
 

Offline G0HZU

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Re: Economical option for precision frequency reference?
« Reply #165 on: August 02, 2015, 01:38:31 pm »
@ GOHZU , do you happen to know the model of your epson  oscillator?

Can you clarify which one you mean?

I've got several Agilent instruments here with the 1E5 option OCXO but I've never tried to open up the OCXO module on any of them so I don't know who actually makes this OCXO or what the model number is. Apart from that I don't think any of my other OCXOs are made by Epson.
 

Offline Vgkid

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Re: Economical option for precision frequency reference?
« Reply #166 on: August 02, 2015, 02:58:14 pm »
The one in your anritsu counter,specs will suffice.
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Offline G0HZU

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Re: Economical option for precision frequency reference?
« Reply #167 on: August 02, 2015, 04:00:16 pm »
In my Anritsu counter there is a Toyocom OCXO and I think it's a special version of their old 615 range. These are big old OCXOs 50mm x 50mm x 60mm high and they take a long time to warm up.

I think the yearly ageing rate is <1.5e-8 and over temperature is < 5e-9 over 0 to 50degC. But I really don't like this OCXO because it takes so long to warm up. To get even basic settling takes about 50 minutes. i.e. this is how long it takes to allow the first overshoot and then undershoot and get to within 0.1Hz of 10MHz. There's a similar looking 615 series OCXO in my Advantest analyser but this warms up much faster even though it is the same package etc.

So I really don't recommend the OCXO in my counter even though it performs quite well apart from the slow warmup. I rarely need to use a counter these days but I usually use this counter with an external OCXO with very fast warmup. i.e. my Quintenz or my MFC OCXO that has almost the same spec but can warm up in well under 5 minutes :)
« Last Edit: August 02, 2015, 04:49:21 pm by G0HZU »
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #168 on: August 02, 2015, 04:44:52 pm »
get a bg7tbl with oscilloquartz (2015-07-08) or trimble (2015-07-17) sub-board. you'll have a much more compact unit that uses much less power, much more sensitive and capable gps receiver, rs232, and true sine output. he even throws in a psu and gps antenna. it has no problem with the antenna inside the same window the lucent refused to work from.

Have you seen a write-up on either of these units? The Trimble board actually says "Designed in the US" on the board. I am curious where it comes from, and how it performs relative to the other unit you mention, which I think is just the BG7TBL design with a different OCXO.
 

Offline usagi

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Re: Economical option for precision frequency reference?
« Reply #169 on: August 02, 2015, 05:55:04 pm »
get a bg7tbl with oscilloquartz (2015-07-08) or trimble (2015-07-17) sub-board. you'll have a much more compact unit that uses much less power, much more sensitive and capable gps receiver, rs232, and true sine output. he even throws in a psu and gps antenna. it has no problem with the antenna inside the same window the lucent refused to work from.

Have you seen a write-up on either of these units? The Trimble board actually says "Designed in the US" on the board. I am curious where it comes from, and how it performs relative to the other unit you mention, which I think is just the BG7TBL design with a different OCXO.

the oscilloquartz board is a surplus gpsdo that comes out of some huawei telco equipment. it uses the exact same bg7tbl adapter board as the trimble. it's not just an ocxo swap.

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #170 on: August 02, 2015, 06:14:42 pm »
get a bg7tbl with oscilloquartz (2015-07-08) or trimble (2015-07-17) sub-board. you'll have a much more compact unit that uses much less power, much more sensitive and capable gps receiver, rs232, and true sine output. he even throws in a psu and gps antenna. it has no problem with the antenna inside the same window the lucent refused to work from.

Have you seen a write-up on either of these units? The Trimble board actually says "Designed in the US" on the board. I am curious where it comes from, and how it performs relative to the other unit you mention, which I think is just the BG7TBL design with a different OCXO.

the oscilloquartz board is a surplus gpsdo that comes out of some huawei telco equipment. it uses the exact same bg7tbl adapter board as the trimble. it's not just an ocxo swap.

Any data or opinions on the relative merits of the two boards? Is one better than another?

What is actually on the bg7tbl adapter board - just connectivity to power and the SMA connectors, or is there actually some logic there (i.e. a microcontroller). What I am mainly interested in is if these units are subject to the frequency problem (not exactly 10MHz) that the other BG7TBL design is.

 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #171 on: August 02, 2015, 08:15:08 pm »
I wrote little program that communicates over the serial port to pull the TI value from the Lucent every second. I was curious how frequently this value gets updated.

My Lucent is mostly reporting +/- 10ns values, with occasional excursions to as high as 50ns. Notably the value never changes at an interval of less than 10 seconds. So I wonder if actually this value is the deviation over a 10 second interval. I wish I  had a way to measure this.
 

Offline motocoder

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Re: Economical option for precision frequency reference?
« Reply #172 on: August 03, 2015, 12:18:55 am »
You know, I just don't believe these TI numbers. Perhaps I don't understand what they represent. I hooked up the Lucent GPSDO 10MHz output to my 53131A, which has its own OCXO that has been calibrated to the Lucent.

Now if the Lucent was really drifting by 10 - 50ns per 1 second interval, unless the timer OCXO was drifting in sync (impossible), I would expect to see variation in the 0.1Hz digit. Instead, it is rock solid all the way out to the 0.0001 digit.

I guess maybe I need to sign up for the time-nuts mailing list and ask someone to explain it (or maybe Ed can explain here).
 

Offline edpalmer42

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Re: Economical option for precision frequency reference?
« Reply #173 on: August 03, 2015, 03:47:34 am »
You know, I just don't believe these TI numbers. Perhaps I don't understand what they represent. I hooked up the Lucent GPSDO 10MHz output to my 53131A, which has its own OCXO that has been calibrated to the Lucent.

Now if the Lucent was really drifting by 10 - 50ns per 1 second interval, unless the timer OCXO was drifting in sync (impossible), I would expect to see variation in the 0.1Hz digit. Instead, it is rock solid all the way out to the 0.0001 digit.

I guess maybe I need to sign up for the time-nuts mailing list and ask someone to explain it (or maybe Ed can explain here).

Believe your counter.  The GPSDO is reporting the TI between its PPS and the calculated PPS based on the received signals.  Problem is, the received signal gets distorted by the ionosphere and maybe by multipath.  This creates jitter in the calculated result.  Over a long period, this jitter averages out to zero.  The OCXO's function is to act as a flywheel to filter out the short-term jitter.  The  PPS is then generated from the OCXO.  The 'gotcha' here is that in the RFTG, the averaging appears to degrade the OCXO's performance - or doesn't completely filter out the short-term jitter.

There are a few things that can be checked to ensure that the jitter is minimized.  Like they say, "Location, location, location".  If the GPSDO doesn't have its location figured correctly, this can be a major cause of jitter.  I don't remember if the RFTG has this feature, but some GPSDOs give you the option to set the length of time for the location survey.  Longer is better!  The survey is just as vulnerable to jitter as the PPS.  A longer survey helps average out the jitter.  Give it a day or two - compare the before and after results. 

Note that measuring the output with a frequency counter isn't a very precise measurement because the counter's gate time results in an averaging function that hides things.  It's much more effective to measure the time interval between the GPSDO and another local reference.  Of course, you have to have another reference!  You also need to be able to capture the readings and log them to a PC.

Ed
 

Offline usagi

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Re: Economical option for precision frequency reference?
« Reply #174 on: August 03, 2015, 04:30:06 am »
What is actually on the bg7tbl adapter board - just connectivity to power and the SMA connectors, or is there actually some logic there (i.e. a microcontroller). What I am mainly interested in is if these units are subject to the frequency problem (not exactly 10MHz) that the other BG7TBL design is.

from what I can tell the adapter board just provides power to the surplus board, and provides rs232 / 1pps / 10mhz / front panel status LEDs. and mounting holes for the spacers to mount the surplus board. the adapter board appears to be a "universal" board for both the huawei and trimble.


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