Author Topic: What frequency standard is sufficient?  (Read 4544 times)

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Offline Housedad

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What frequency standard is sufficient?
« on: October 08, 2018, 01:15:33 am »
Tcxo, Ocxo, rubidium, GPS disciplined and all combinations thereof.

I can really see the use in having a frequency standard on the bench.  The problem for me is what level and cost is right for me?    I know it is whatever you are working on will dictate it, but I'd rather just buy what I need for future uses now. 

I'm studying now for ham, and am looking to start repairing various things for resale.  Older radios, test equipment, the odd repairable tv,   God knows what.  Calibrating test equipment is a good reason, too.

What is a good balance type standard that is good for the medium beginner and will still be useful a ways down the road and not break the bank?

At least I'm still older than my test equipment
 

Offline Vgkid

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Re: What frequency standard is sufficient?
« Reply #1 on: October 08, 2018, 01:25:12 am »
What is your budget? Do you have access to the sky(not living in a heavily crowded city) gps has many options...
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Offline tsman

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Re: What frequency standard is sufficient?
« Reply #2 on: October 08, 2018, 01:35:20 am »
What test equipment do you already have or plan to get? A frequency standard depends on what you're trying to do and what other equipment you have. If you're just starting out then getting good quality test equipment is far more important than a high precision frequency standard. You can always buy a frequency standard later and hook it up to the external reference inputs anyway. You may find that the built-in oscillators are good enough for what you need to do.
 

Offline retrolefty

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Re: What frequency standard is sufficient?
« Reply #3 on: October 08, 2018, 01:38:39 am »
A used 10MHZ OCXO can be easily found on E-bay and is a good start for a beginners test bench More complex/accurate can follow as the need is justified.
 

Offline radioactive

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Re: What frequency standard is sufficient?
« Reply #4 on: October 08, 2018, 01:52:00 am »
I ran across this open-source GPS disciplined oscillator product the other day... https://www.tindie.com/products/nsayer/gps-disciplined-xcxo/

I am not affiliated with this product in any way, but I think it looks interesting for the price.  Worth a look for the budget conscious in my opinion.
 

Offline Housedad

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Re: What frequency standard is sufficient?
« Reply #5 on: October 08, 2018, 01:55:04 am »
I have plenty of sky.  I live just  outside the edge of the suburbs.  I'd like to keep it well below $200 US.
I have a few pieces of test gear and want to aquire more.  Repairing used equipment if needed to keep the costs down.
Equipment:

DS 1054Z Oscilloscope  (hacked 100MHz)
Instek 100 mhz analog oscilloscope
2 bench DMM's Instek
Rigol dg1022 function generator
(2)Gdp 3032S power supplies
BK precision DMM handheld
De5000 LCR meter on it's way.

Currently trying to repair a Wavetek 2410 Rf generator.

And most of the standard bench repair/test/play stuff and tools.

https://www.eevblog.com/forum/chat/whats-your-work-benchlab-look-like-post-some-pictures-of-your-lab/msg1179703/#msg1179703

At least I'm still older than my test equipment
 

Offline xrunner

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Re: What frequency standard is sufficient?
« Reply #6 on: October 08, 2018, 02:23:23 am »
I have plenty of sky.  I live just  outside the edge of the suburbs.  I'd like to keep it well below $200 US.
I have a few pieces of test gear and want to aquire more.  Repairing used equipment if needed to keep the costs down.

I have a BG7TBL GPSDO version and it works great for $133 -

BG7TBL
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Offline tkamiya

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Re: What frequency standard is sufficient?
« Reply #7 on: October 08, 2018, 02:38:27 am »
Well, at that price range, you can expect to get OCXO GPDO.  Remember; though, you need that, antenna, and distribution amplifier at minimum.  So if you are looking to keep the cost down, all of that needs to be looked at.  Silly thing as coax and connectors can add to quite a bit. 

At this point, I don't think you need one as there is nothing you will slave to a master oscillator. 

I have two Nortel GPSTM, which is a GPS trained oven crystal oscillator.  A telecom surplus.  It's enough for me.  I have frequency counters and RF generators slaved.  You can get these within your budget.  (but you will break your bank when you add distribution amplifier)  I use it to establish standard on frequency counters and maintain consistency in my lab. 

Realistically, with it, all of my frequency counters will all zero and display the same number.  And RF generator will agree with frequency counter.  Without it though, last digit or two will flicker and drift as my air conditioner cycles.  BUT, with those drifts and flickers, they are far more accurate than ham radio equipment demands.  Ham gears have TCXO as high stability options.  Unless I'm using two RF generators at the same time, I think of them as sanity check devices.
 

Offline Housedad

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Re: What frequency standard is sufficient?
« Reply #8 on: October 08, 2018, 02:52:35 am »
, I think of them as sanity check devices.


That's what I'm thinking about the first uses too.  My counter and the wavetek RF generator I recently bought are used and I have no assurance if they are even close to spec.  The wavetek has a ocxo so I probably could use one at least as accurate to check it against. 
At least I'm still older than my test equipment
 

Offline CatalinaWOW

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Re: What frequency standard is sufficient?
« Reply #9 on: October 08, 2018, 03:27:44 am »
There are sanity checks you can do with almost no equipment.  Zero beat with a commercial station.  Zero beat with WWV.  Run the oscillator into a long counter (or better a medium counter that generates interrupts for a computerized counter) and then compare the accumulated time to a clock like your cell phone.

The frequency reference is a great convenience and can speed up a sanity check, but it isn't necessary.
 

Offline ZL1CVD

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Re: What frequency standard is sufficient?
« Reply #10 on: October 08, 2018, 04:33:24 am »
I picked up a Symmetricom 089-03861-02 GPSDO module for around $80US a while back. Added a $15 Chinese GPS antenna which came with 5m lead. You can also get some magnetic mount antennas for under $10. Set it up and left it running for a week. Came back and checked it was all good then calibrated my R&S CRTU analyser to it. Packed away the GPSDO  for six to 12 months or so before I do it all again. Most quality brand RF analyzers have a good OCXO inside so once set, your good for at least a year. But yep, its important to have a good time base. I looked at rubidium but you get as good from any GPSDO designed for cellular timing. Plus for us outside the US, rubidium will probably = customs nightmare.
 

Offline tautech

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Re: What frequency standard is sufficient?
« Reply #11 on: October 08, 2018, 04:36:31 am »
I'd be tempted by one of member Leo Bodnar's offerings......no pissing around, just plug it together and load some settings:
http://www.leobodnar.com/shop/index.php?main_page=index&cPath=107
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Online nctnico

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Re: What frequency standard is sufficient?
« Reply #12 on: October 08, 2018, 05:52:42 am »
I'd get a GPDSO and be done with it.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline MrW0lf

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Re: What frequency standard is sufficient?
« Reply #13 on: October 08, 2018, 08:57:31 am »
I have Bodnar's GPSDO. It can serve as sort of 2ch signal gen also, quite useful.
 

Offline tggzzz

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Re: What frequency standard is sufficient?
« Reply #14 on: October 08, 2018, 12:52:37 pm »
Tcxo, Ocxo, rubidium, GPS disciplined and all combinations thereof.

I can really see the use in having a frequency standard on the bench.  The problem for me is what level and cost is right for me?    I know it is whatever you are working on will dictate it, but I'd rather just buy what I need for future uses now. 

I'm studying now for ham, and am looking to start repairing various things for resale.  Older radios, test equipment, the odd repairable tv,   God knows what.  Calibrating test equipment is a good reason, too.

What is a good balance type standard that is good for the medium beginner and will still be useful a ways down the road and not break the bank?

To simply test the equipment you mention, almost anything is good enough. CatalinaWOW's reply is spot on.

There's no point in making any statements/guesses until you know what you want to measure, and to what accuracy, resolution etc.

If you are interested in RF stuff, then sooner or later you will run into phase noise, which is often more important than absolute accuracy and resolution.

Once you start getting into the frequency/timenut domain, you will want to find out about Allen deviations.

Until you have a positive requirement, my inclination would be to go for a decent secondhand spectrum analyser before a frequency source. After all, they contain a decent frequency source plus a frequency synthesiser to observe any frequency you want! (Plus you can often use the LO output).
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline cdev

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Re: What frequency standard is sufficient?
« Reply #15 on: October 08, 2018, 01:59:08 pm »
I paid $45 on ebay for a telecom surplus TruePosition GPSDO, implemented on a card that plugs into a rack-resident mother board, which I dont have. But the instructions to use it are on the web and also here, (a long and informative thread about them). 

The TruePosition is built far better than anything I could ever build myself. Its likely as accurate as one of the Trimble or Symmetricom units that cost more than twice as much, even used.

It's gradually emerged that it has a lot of configurability, thanks to a number of people's contributions. I wish we had the manufacturers specifications but what we do have is still quite good.

This post is way too long and pieces are redundant.

Basically, your needs are totally dependent on what you are doing, but the availability of this specific unit makes that choice a pretty easy one for most users here who are seeking a lab quality 10 MHz time and frequency standard.

Why not buy a standalone timing GPS?

For some uses, that makes sense too. For example, for an NTP server, I think a timing GPS is a better choice. A plain GPS which gives you 25-40 ns accuracy is much more accurate than you need.

 All of the better GPSs end up giving the exact same results to me, all the time, with NTP once I figured out how to configure the offset correctly.

(Anybody who has more info, please share it here!)

Any good GPS thats successful in a mobile or UAV application likely has a good TCXO, already (or they wouldn't work in the wide range of environmental conditions they encounter, or at best they would work unreliably)

Timing applications in telecom and especially space to earth applications like GPS (where there is a very low signal to noise ratio are much more demanding than other radio applications.

An OCXO smooths out the frequency vs. time. Otherwise the GPSDO's frequency will vary in a sawtooth manner between two extremes, within a range of typically 50 nanoseconds or so. The best ones might do better, maybe half that.

Experts all will tell us that the jitter on any GPSs (and all GPSs have this problem) is too high for some (RF) applications (for the GPS-resident oscillator to act as a frequency standard in the most demanding applications) all by itself without an oscillator with a long time constant- a means of smoothing this signal out.. A quartz or rubidium oscillator - 'free running' will emit a pure sine wave that changes in frequency very slowly, but it may be off a bit unless its aligned with a GPS both periodically (calibrated to cancel out the crystals natural aging) and also on an ongoing basis by the GPS's atomic clock.
 
One feature one should look for in a timing GPS is "Timing Receiver Autonomous Integrity Monitoring" (abbreviated "TRAIM")

Once the GPS is installed in its permanent home and you know its not going to move using TRAIM you can train it so that after that it can maintain its lock for timing purposes with only one satellite.

The commercial GPSDOs that originated in cell phone equipment, although used, are likely the cheapest high quality signal source youre likely to find. Some of the GPSDOs on ebay, taobao, etc. are comparable in quality or better. But I really doubt if most of the homemade ones - with the possible exception of the very best designs - perhaps ones that were in essence crowd sourced, improved by a community of high knowledge users.

- without that I doubt if most attempts are going to be as good as a telecom surplus unit thats not at the end of its useful life due to aging or broken in some way.

Personally I would make sure that I got one with an oven if I planned on serious radio use in the future.  There probably is a lot of variation between ebay sourced GPSDOs with some of the best being made by a Chinese ham I think his call is bg7tbl.  See this thread here.

https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/

But, here is the problem with his unit, if somebody wants to have the ability to tweak the settings, you really can't.

Some of the functionality on most GPSs is accessed by sending commands to the unit via a serial port (well, more often than not on newer units it is a 3.3 or 5 volt- serial port, make sure you know which - lower voltage UART)

As far as I know some of the homemade units don't have any way of sending commands to the units to do much in the way of configuration. Some may have a button to initiate training the unit for a new location, and thats it. Some don't even have that.

OTOH, some of the GPS-only units are very flexible and configurable to emit square waves over a wide range of frequencies, basically you get a DDS. Considering that some Ublox 6T units that have that dual output capability, two configurable square wave outputs can be found for as little as $25, thats an amazing deal.

IF you can live with the jitter. Or get both.
« Last Edit: October 10, 2018, 10:31:42 pm by cdev »
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Offline tggzzz

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Re: What frequency standard is sufficient?
« Reply #16 on: October 08, 2018, 04:16:57 pm »
It totally depends on what you are doing, but I should say that a standalone GPS, even a very good one, as was previously described, may be able to supply a 10 MHz "gps disciplined" reference signal but even its own manufacturer (Sktraq, I think) cautions that the jitter on their GPSs (and all GPSs have this problem) is too high for some (RF) applications by itself.

My limited understanding is that the cheap units have a 48MHz internal clock, and the 1pps (etc) output signal is synchronous with that. Consequences:
  • it is avisable to choose an output frequency that can be derived from 48MHz by simple division: 1MHz/4MHz/etc is good, but 10MHz isn't
  • there will be a drift between the 48MHz clock and the GPS received signal, resulting in an occasional 21ns jump
  • and then there is all the behaviour associated with GPS signals
More expensive units may or may not derive the 48MHz clock from the GPS signal.

So, GPS is very good for long-term absolute accuracy, but is non-ideal for short-term stability. Hence the use of GPS receivers to "discipline" (a.k.a. correct) rubidium and crystal oscillators - which are stable in the short term but can drift in the long term.

Hence the OP needs to figure out what he needs. GPS may be precisely the wrong technology.

In the absence of other information, I suspect the OP should stick to a simple TCXO, and optionally use that as a DDS's clock source if he wants other frequencies.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline TurboTom

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Re: What frequency standard is sufficient?
« Reply #17 on: October 08, 2018, 04:59:37 pm »
I tend to defend a Rb oscillator, simply for the convenience of it. It's portable, independant of unobstructed view to the sky and three minutes after powering up, it provides a fairly accurate reference signal that gets better over the next thirty some minutes. Jitter, accuracy and cleanlieness of the output signal very much depends on the particular oscillator used. I've got very good results with my Efratom LPRO101, much better than the Temex LPFRS. I didn't get my hands dirty on other atomic oscillators so I cannot tell anything about those.
 

Online nctnico

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Re: What frequency standard is sufficient?
« Reply #18 on: October 08, 2018, 05:05:24 pm »
Recently I've seen some measurements of a Rb clock from Ebay compared to a very good oscillator and the Rb clock showed a lot of short term instability. The measurement was repeated using a different Rb clock from Ebay and it showed the same results.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline coppercone2

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Re: What frequency standard is sufficient?
« Reply #19 on: October 08, 2018, 05:19:18 pm »
Was it superior to a TC Crystal though? (like a good one in good agilent equipment)

I had some doubts about using my cheap RB one.
 

Offline CatalinaWOW

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Re: What frequency standard is sufficient?
« Reply #20 on: October 08, 2018, 05:35:14 pm »
Define superior.  As noted in prior posts and threads on the subject, each has areas of superiority.  Which is why they are combined in GPSDO in an attempt to get the best of both worlds.  Rb alone is satisfactory for many uses.  As is OCXO.  Depends on your application and requirements.

When I was a young engineer I used to laugh at the note our internal calibration department placed on counters.  "Only calibrated to 1 part in 10^8".  Since the work we were doing at the time wasn't particularly sensitive to frequency errors, we were more than satisfied with accuracy of 1 part in 10^3, or if we were going to get all anal about it 1 part in 10^4.  The extra four orders of magnitude of accuracy really was a joke to us.  Of course this was back when you still had to tune a radio, not just set the frequency you wanted so by today's standards no one cared about accuracy.
 

Offline DaJMasta

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Re: What frequency standard is sufficient?
« Reply #21 on: October 08, 2018, 06:04:41 pm »
Outside of very specific applications, an internal high stability reference from almost any piece of timing equipment is more than sufficient for testing and verification.  Once you get into OCXO territory, with single digit ppm accuracy even with some aging, there are very few applications for faster absolute time, and typically, a good OCXO is going to give you some of the lowest phase noise and short term stability, so it's sufficient for any tight timing requirements that don't need an absolute reference.  While a regular internal reference with an output is likely sufficient for most applications (the output is required to synchronize with another piece of gear), a high stability reference gets you another couple digits worth of absolute accuracy and lower phase noise and guilds the lily, so to speak.


That being said, most people looking at an external reference like this don't necessarily require it for measurements they're making, they just want the certainty of a disciplined reference and the stability a high performance reference can offer.  For value concerns, I'd say the peak right now is a GPSDO with an OCXO, either one of the units brewed up by hobbists (like the BG7TBL) or one of the surplus telecom units (like many of the Trimble options).  Both should get you around 10^-9 stability/accuracy after warmed up and with a good GPS fix, which is equivalent to the mid to high tier frequency reference upgrade option on many bits of gear, except with the disciplining to keep the long term drift at basically zero.

Rubidium sources cost more, but good ones can achieve another digit or two worth of accuracy and stability when disciplined.  You have to be a bit careful because some rubidium sources are just directly driving the output and these won't be as stable in the short term or have as low phase noise, the best rubidium references have a Rb source disciplining an OCXO (and which in turn are disciplined to a GPS).  The biggest benefit of Rubidium is long term stability, so by using one to discipline a crystal oscillator you effectively reduce the aging and drift effects of an OCXO by a couple digits, so a rubidium reference disciplined to GPS is not really inherently better performing than a GPSDO OCXO because the GPS signal is providing that same kind of long term stability, but the rubidium reference is a much better performer if the GPS signal is ever lost, since it's long term stability is much better.  A GPS signal disciplining a really well put together rubidium reference which disciplines an OCXO is going to be somewhat more stable than most GPSDOs without rubidium, but it's basically going to be due to the long integration time used to discipline the rubidium source being able to better compensate for small fluctuations in GPS time, so it's a much smaller difference between OCXO vs. Rb when not locked to GPS.

Since GPS is ubiquitous and good OCXOs are fairly cheap, that GPSDO with OCXO offers really good performance for money.  Since it's generally easy to get a signal, the benefits of rubidium are much smaller than it would be without GPS.
 

Offline coppercone2

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Re: What frequency standard is sufficient?
« Reply #22 on: October 08, 2018, 07:18:47 pm »
So it sounds for general lab work, if you have a choice between a OSCO and a Rb stand alone unit, you should just go for the OSCO, because of phase noise concerns.

Over what time period does an frequency specific but not phase sensitive experiment need to be run to benefit from a pure Rb clock (the small ~4x3 inch or less modules) rather then a OSCO?

Are there any other situations where a pure Rb undisciplined clock can benefit?

I am imagining something like, if you are testing a newly made filter to see the effects of burn in/aging, you can hook it up to a pure Rb source to categorize the distortion easier with less post processing and measurement requirements, vs a OSCO, say you wanna monitor it for a month (some kinda active filter etc).. this way you can monitor the distortion drift carefully... and you don't give a hoot about short term phase noise or drift since it will just look like a sineusoid riding some lower frequency, in the frequency domain (say doing extrapolations from a waterfall)...

Am I correct in this?

I like the term OSCO (oven stabilized crystal oscillator) better then something with a X in it, I keep thinking of the kool aid man and the cheetos cheetah.. my sunglasses are not dark enough and my ollies are too lame for that, man
« Last Edit: October 08, 2018, 07:27:47 pm by coppercone2 »
 

Offline usagi

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Re: What frequency standard is sufficient?
« Reply #23 on: October 08, 2018, 07:33:19 pm »
GPSDO OCXO is going to be far beyond the needs of most hobbyists. Few will need anything like a Rb or Cs unit.

Offline MrW0lf

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Re: What frequency standard is sufficient?
« Reply #24 on: October 08, 2018, 08:59:27 pm »
If OCXO one is far beyond needs sounds just right to have one to check if TCXO one is working good... So have to buy 2! I have only TCXO one and already very nervous because cannot check it :-//
 


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