Author Topic: Economical option for precision frequency reference?  (Read 155306 times)

0 Members and 1 Guest are viewing this topic.

Offline motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #150 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

  • Super Contributor
  • ***
  • Posts: 2260
  • Country: ca
Re: Economical option for precision frequency reference?
« Reply #151 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 motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #152 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...
 

Online G0HZU

  • Super Contributor
  • ***
  • Posts: 3012
  • Country: gb
Re: Economical option for precision frequency reference?
« Reply #153 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 motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #154 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

  • Super Contributor
  • ***
  • Posts: 2710
  • Country: us
Re: Economical option for precision frequency reference?
« Reply #155 on: August 01, 2015, 11:08:14 pm »
@ GOHZU , do you happen to know the model of your epson  oscillator?
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline usagi

  • Frequent Contributor
  • **
  • Posts: 390
  • Country: us
Re: Economical option for precision frequency reference?
« Reply #156 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 »
 

Online G0HZU

  • Super Contributor
  • ***
  • Posts: 3012
  • Country: gb
Re: Economical option for precision frequency reference?
« Reply #157 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

  • Super Contributor
  • ***
  • Posts: 2710
  • Country: us
Re: Economical option for precision frequency reference?
« Reply #158 on: August 02, 2015, 02:58:14 pm »
The one in your anritsu counter,specs will suffice.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Online G0HZU

  • Super Contributor
  • ***
  • Posts: 3012
  • Country: gb
Re: Economical option for precision frequency reference?
« Reply #159 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 motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #160 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

  • Frequent Contributor
  • **
  • Posts: 390
  • Country: us
Re: Economical option for precision frequency reference?
« Reply #161 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 motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #162 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 motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #163 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 motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #164 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

  • Super Contributor
  • ***
  • Posts: 2260
  • Country: ca
Re: Economical option for precision frequency reference?
« Reply #165 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

  • Frequent Contributor
  • **
  • Posts: 390
  • Country: us
Re: Economical option for precision frequency reference?
« Reply #166 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.

Offline motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #167 on: August 03, 2015, 04:48:54 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

Thanks, that is a very helpful explanation.

One interesting thing just happened. I was sitting here watching one of robrenz' videos, and all of a sudden the HUP value reset to its starting point (4.32E-4), and the EFC started going nuts. Checking the logs, it looks like it rebooted. After the reboot, it started another survey. I've turned off the survey on boot feature for now. I'm a bit worried as to why it rebooted. It's plugged into a UPS, and I wasn't moving it or doing anything that should have disturbed the (albeit iffy) power connection.

 

Offline motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #168 on: August 03, 2015, 06:10:23 am »
I discovered why it rebooted. The UPS is faulty. It's a small Eaton that I bought used, so no surprises there.
 

Offline motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #169 on: August 04, 2015, 04:23:22 am »
So I think that UPS had been acting up for some time, perhaps very brief cutouts that were enough to disrupt the power supply to the GPSDO. Since removing that from the circuit, the EFC corrections have really settled down, and HUD was as low as 1.1us today  - currently at 1.7us.  The EFC statistics in Z38XX say the EFC slope is 5.025E+01 per day resembling a relative OCXO drift of 5.024E-11 (Hz) per day.
 

Offline TSL

  • Regular Contributor
  • *
  • Posts: 243
  • Country: au
Re: Economical option for precision frequency reference?
« Reply #170 on: August 04, 2015, 04:49:58 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.

I did a tear down and test of the BG7TBL unit here..

https://drive.google.com/a/skybase.net/file/d/0B9Oysj7clpT7YTBWSjlfNTI1aTQ/view

While I don't have the resolution of KE5FX's test rig ( he has a Cesium standard!) my Thunderbolt based test setup did not show that error.

regards

Tim
VK2XAX :: QF56if :: BMARC :: WIA :: AMSATVK
 

Offline motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #171 on: August 04, 2015, 05:21:15 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.

I did a tear down and test of the BG7TBL unit here..

https://drive.google.com/a/skybase.net/file/d/0B9Oysj7clpT7YTBWSjlfNTI1aTQ/view

While I don't have the resolution of KE5FX's test rig ( he has a Cesium standard!) my Thunderbolt based test setup did not show that error.

regards

Tim

Very nice. From which eBay seller did you purchase this unit?
 

Offline TSL

  • Regular Contributor
  • *
  • Posts: 243
  • Country: au
Re: Economical option for precision frequency reference?
« Reply #172 on: August 04, 2015, 05:38:18 am »

Very nice. From which eBay seller did you purchase this unit?

Thanks - I bought from fly-xy, he has a bunch of interesting RF stuff. URL for the GPSDO I bought here...

http://www.ebay.com/itm/10MHZ-OUTPUT-SINE-WAVE-GPS-DISCiPLINED-CLOCK-GPSDO-GPS-Antenna-Power-supply/121530825744


And here is a newer model with Huawei internals ...

http://www.ebay.com/itm/HW-HUAWEI-GPS-DISCIPLINED-CLOCK-GPSDO-OSCILLATOR-Antenna-power-supply-/111716558224

and yet another model with Trimble internals...

http://www.ebay.com/itm/Trimble-GPS-Receiver-GPSDO-10MHz-1PPS-GPS-Disciplined-Clock-Antenna-power/131564496688

regards

Tim
VK2XAX :: QF56if :: BMARC :: WIA :: AMSATVK
 


Offline motocoderTopic starter

  • Supporter
  • ****
  • Posts: 769
  • Country: us
  • Electrical Engineer
Re: Economical option for precision frequency reference?
« Reply #174 on: August 04, 2015, 06:07:48 am »
« Last Edit: August 04, 2015, 06:09:24 am by motocoder »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf