Author Topic: What frequency standard is sufficient?  (Read 4504 times)

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Offline edpalmer42

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Re: What frequency standard is sufficient?
« Reply #50 on: October 11, 2018, 06:15:38 am »
On the subject of 50/75 ohm termination - a lot of instruments aren't either for the ref in anyway. I think my counters are more like a few kohms so you can locally make them either 50 ohm or 75 ohm with suitable additional resistors.

That isn't usually necessary.  AFAIK, the intent is that you run a single daisy-chain from unit to unit with BNC T-connectors and cables.  Terminate the end of the line with a terminator or a device that happens to have a 50 ohm RefIn connection.  The high impedance of the other RefIn connections won't load down the line.  If you find that a particular device is sensitive to this situation you might have to arrange a dedicated connection for it.

Ed
 
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Offline cdev

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Re: What frequency standard is sufficient?
« Reply #51 on: October 11, 2018, 09:55:27 am »
Several things.

The Skytraq GPSs use a Leon3 "Sparc" architecture core running on a chip design that was originally designed for the European space program - to be a highly reliable CPU platform for space vehicles. Also I am pretty sure the clock speed is significantly higher than 48 MHz.

The manual for the Leon 3 is available for download on the net somewhere as well as a toolchain being available.

It seems to me that the same or similar code base is burned onto significantly different hardware for various different applications. I'm not familiar with exactly what factors are involved but it can be found on the Navspark web site. Both platforms are quite nice in their different ways it seems.

Both Skytraq and Ublox sell dev boards and the developer typically uses their configuration applications (windows only but both work under wine) which open up significantly more in the way of interactions with the chip and its internals.

Ublox is particularly good in this respect in some ways, their U-center (Windows) application has the ability to configure what seems like hundreds of different settings, for a great many different applications, via multiple UARTs and USB/UART too. Also all Neo 8 series UBLOX GPSs have at least one, and sometimes two timepulse outputs which are changeable. However to do that you need to send commands to the module by some means. Somebody would have to implement a control interface in a microprocessor that spoke to the chips. The protocol is available, so its likely not very difficult. If you want to experiement with it you need a Ublox timing GPS, though which start at around $70 for a current Neo M8T unit, or you can find an older one for much less. For timing its not at all necessary to have multiple GNSS systems available (what buying an 8 series gets you) so its fine to use a Neo 6 timing unit which still has the two programmable time pulses. They sell for as little as $20-25. One can talk to your module via the UART interface using a USB-UART converter if you need to do it from a separate PC, or over SPI or i2c.

Skytraq approaches the maker community's needs slightly differently because their chip actually is a CPU so it can do double duty as such and run a limited amount of additional code, toggle GPIOs, etc. This makes it possible to use a single chip to do things which otherwise require a GPS plus additional hardware, usually another board. You can also talk to them over UART, SPI or i2c.

Because you can run a limited amount of code on the Leon3 32 bit CPU, you can with skytraq, use that programmability to implement anything that will fit in the space you get, a GPS logger, time stamper, or a clock that displays the location heading, time, etc. with almost no additional parts other than the display and the antenna, battery, SD card, etc.

As far as implementing a GPSDO oven control loop and logic in that additional space, I don't know. It seems that some of the needed info would be available. It depends on how complicated your design is.

------

Also, as far as distributing a signal. Seems to me like having an RF transformer for isolation of the input on any amplifier would likely be useful in reducing noise which might throw readings off. One might even be able to get away with no amplifier using video transformers.. (baluns) as splitters. Especially if your GPSDO has a healthy output level.  Use a trifilar winding to split a signal two ways, quadrifilar winding to spilit it three ways and so on. This would probably work best with very thin insulated wire and small balun cores.
« Last Edit: October 11, 2018, 10:56:57 am by cdev »
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Offline texaspyro

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Re: What frequency standard is sufficient?
« Reply #52 on: October 11, 2018, 11:21:41 am »
Navspark does not make the GPS code available...

Oh, and Lady Heather can configure both the Ublox and Navspark/Venus chips.
 

Offline cdev

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Re: What frequency standard is sufficient?
« Reply #53 on: October 11, 2018, 02:38:17 pm »
Science and the scientific method could be a life saver. Where we're going most of our young people wont have much if any income/money.

People will need low cost tools. Science has a joy component to it, it could be a gateway drug to prevent people from sinking into despair.

Enabling scientific thinking.

Accurate measurement of time *really* cheaply could be a game changer. There is no point reinventing the wheel if it isn't cheaper.
« Last Edit: October 11, 2018, 03:06:20 pm by cdev »
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Offline texaspyro

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Re: What frequency standard is sufficient?
« Reply #54 on: October 11, 2018, 03:02:28 pm »
What does LH do with the 1PPS? - do you use it?

Nope,  Heather is blissfuly ignorant of PPS signals.   
 

Offline Leo Bodnar

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Re: What frequency standard is sufficient?
« Reply #55 on: October 11, 2018, 10:38:43 pm »
The Leo Bodnar unit doesn't have an oven, right?
No, it does not really need it for its intended use.
Quote
What makes it different than, say a ublox timing GPS, some of which can be found on ebay for $20.
Ublox output has 21 nanoseconds of jitter pk-pk.  GPS clock has few hundred femtoseconds jitter.  This is the biggest difference. I have attached the phase noise plot from mini GPS clock.  Notice the lack of spurs - they have not been removed, they are simply absent in the signal.
I will try to get phase noise plot directly from Ublox, this will be fun.
Quote
They have two internal user programmable square wave outs which can be adjusted typically one is set to 1 pps and the other is set to 10 MHz.
I think with adjustable duty cycle.
One could slap one of them in a box with a Raspberry Pi and a screen or a serial port and have a standalone adjustable DDS.
It has the expected amount of jitter.
Am I wrong about something here?
All four statements are technically correct.  But the Ublox jitter is still 21ns pk-pk at any frequency. Consider that at 10MHz signal period is 100ns and at 100MHz it is 10ns.

Leo

« Last Edit: October 11, 2018, 11:53:25 pm by Leo Bodnar »
 

Offline Leo Bodnar

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Re: What frequency standard is sufficient?
« Reply #56 on: October 12, 2018, 01:53:07 am »
Here is phase noise of direct timepulse output of Ublox M8 module in comparison with mini GPS clock.

I can't add much more to this apart from the fact that phase noise maths usually assumes jitter to be much less than signal period.  With 10MHz 100ns period signal having 21ns quantisation jitter this is probably pushing it but it is what it is.

These two traces will merge somewhere around 0.1Hz on the dark side (in ADEV land.)

Cheers
Leo

Offline tggzzz

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Re: What frequency standard is sufficient?
« Reply #57 on: October 12, 2018, 02:35:14 am »
Here is phase noise of direct timepulse output of Ublox M8 module in comparison with mini GPS clock.

I can't add much more to this apart from the fact that phase noise maths usually assumes jitter to be much less than signal period.  With 10MHz 100ns period signal having 21ns quantisation jitter this is probably pushing it but it is what it is.

Can you configure the Ublox M8 it so the 21ns jitter isn't apparent every cycle, e.g. with an output frequency of 1MHz or 100kHz etc.

That measurement might be more interesting than a 10MHz output.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline Leo Bodnar

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Re: What frequency standard is sufficient?
« Reply #58 on: October 12, 2018, 02:56:02 am »
Can you configure the Ublox M8 it so the 21ns jitter isn't apparent every cycle, e.g. with an output frequency of 1MHz or 100kHz etc.
My kit does not go below 5MHz at the moment.
Not sure what you mean by "apparent every cycle" but you can probably extrapolate correlated phase noise levels with rule-of-thumb 6dB per octave (20dB per decade.)  Spurs will probably stay where they are.
Ublox M8 is a Cortex-M3 core running at 48MHz and using timers and phase accumulation to generate its timepulse output frequencies.  The quantisation steps are side effect of timer timebase being 48MHz.

Offline cdev

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Re: What frequency standard is sufficient?
« Reply #59 on: October 12, 2018, 03:46:00 am »
Leo, how does your frequency generator work, do you have a block diagram anywhere of it?
« Last Edit: October 12, 2018, 04:04:16 am by cdev »
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Offline tggzzz

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Re: What frequency standard is sufficient?
« Reply #60 on: October 12, 2018, 04:35:46 am »
Can you configure the Ublox M8 it so the 21ns jitter isn't apparent every cycle, e.g. with an output frequency of 1MHz or 100kHz etc.
My kit does not go below 5MHz at the moment.
Not sure what you mean by "apparent every cycle" but you can probably extrapolate correlated phase noise levels with rule-of-thumb 6dB per octave (20dB per decade.)  Spurs will probably stay where they are.
Ublox M8 is a Cortex-M3 core running at 48MHz and using timers and phase accumulation to generate its timepulse output frequencies.  The quantisation steps are side effect of timer timebase being 48MHz.

Apologies for my loose wording.

Perhaps measure ublox output at 8MHz (48/6) or 6MHz (48/8) - not 10MHz with its non-integral divisor.
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 #61 on: October 12, 2018, 04:47:33 am »
Leo, I'm just curious if you enable the Ublox setting that incorporates the SBAS satellite data, (thats its name in Europe, in US we call it WAAS) into the timing solution?
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Offline Leo Bodnar

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Re: What frequency standard is sufficient?
« Reply #62 on: October 12, 2018, 05:34:11 am »
Leo, I'm just curious if you enable the Ublox setting that incorporates the SBAS satellite data, (thats its name in Europe, in US we call it WAAS) into the timing solution?
It will not affect quantisation jitter at all, only underlying receiver local time corrections.

Correction: Ublox do not recommend enabling SBAS for timing applications.
Quote
14.2 Recommendations
For best time pulse performance it is recommended to disable the SBAS subsystem.
 
European SBAS system is called EGNOS. US SBAS is WAAS.  We are running out of acronyms!

GPS clock is, basically, a GPS receiver with clock cleaner.  It merges long term stability from GPS and low phase noise from local TCXO.  There are a few articles around with its block diagrams.
Leo
« Last Edit: October 12, 2018, 05:50:14 pm by Leo Bodnar »
 

Offline cdev

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Re: What frequency standard is sufficient?
« Reply #63 on: October 12, 2018, 07:14:30 am »
The improvements they have been making with GPS are scary. Useful in disasters, for sure, but also quite scary.
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Online hamster_nz

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Re: What frequency standard is sufficient?
« Reply #64 on: October 12, 2018, 09:38:56 am »
Leo, I'm just curious if you enable the Ublox setting that incorporates the SBAS satellite data, (thats its name in Europe, in US we call it WAAS) into the timing solution?
It will not affect quantisation jitter at all, only underlying receiver local time corrections.

Ublox does recommend enabling SBAS for timing applications.
Quote
14.2 Recommendations
For best time pulse performance it is recommended to disable the SBAS subsystem.
 
European SBAS system is called EGNOS. US SBAS is WAAS.  We are running out of acronyms!

GPS clock is, basically, a GPS receiver with clock cleaner.  It merges long term stability from GPS and low phase noise from local TCXO.  There are a few articles around with its block diagrams.
Leo

Just found this about a SBAS trial in Aus (& NZ),

http://www.ga.gov.au/scientific-topics/positioning-navigation/positioning-for-the-future/satellite-based-augmentation-system

$160M sounds a little bit expensive...

In the "Can I use it" FAQ:
Quote
...while it will depend on your device, many GPS users in Australia will immediately be able to track the SBAS signal from their device and see improved positioning accuracy. The SBAS test-bed will not be certified for safety-of-life use. However, should the SBAS be implemented, it will be certified for safety-of-life use.

and
Quote
The services will continue to be transmitted until 31 January 2019.

I might start a new thread.....
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Offline Leo Bodnar

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Re: What frequency standard is sufficient?
« Reply #65 on: October 12, 2018, 05:47:53 pm »
Uh-oh, just noticed that yesterday I wrote the opposite of what I meant... Ublox do not recommend enabling SBAS for timing applications.
Quote
14.2 Recommendations
For best time pulse performance it is recommended to disable the SBAS subsystem.
 

Offline NivagSwerdna

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Re: What frequency standard is sufficient?
« Reply #66 on: October 12, 2018, 07:28:40 pm »
Just found this about a SBAS trial in Aus (& NZ)
I'm so used to having SA turned off and SBAS augmentation in the UK... I guess you just get used to it.  How do you find the small camouflage geocache in the ivy otherwise!  :)
 
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Online blackdog

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Re: What frequency standard is sufficient?
« Reply #67 on: October 13, 2018, 12:43:07 am »
Hi Leo,  :)

Here a happy user of your LeoNTP device.
I also lookt at your GPS lockt frequency standard, but is it to expensive to use a better TCXO in your device so the phase noise wil be lower?
If i have to pay 30 to 50 Euro's more fo a better phase noise i will buy it :-)

Kind regards,
Bram
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Offline Leo Bodnar

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Re: What frequency standard is sufficient?
« Reply #68 on: October 13, 2018, 07:21:31 am »
Bram,
TCXO I use is as good as they come. Current phase noise is at the limit of what you can get with AT-cut crystal that most (all?) TCXOs use.
Lower phase noise needs SC-cut crystal which inevitably means OCXO.  What sort of improvement are you looking for?  Or, at least, at what distance to carrier area?  Are low spurs important?
Yes, you can get PN improvement but OCXOs that are suitable frequency, output, consumed power and availability are pushing €100 already just on their own.
If I know a bit more of what you want I might be able to come up with a solution.
Leo
I also lookt at your GPS lockt frequency standard, but is it to expensive to use a better TCXO in your device so the phase noise wil be lower?
If i have to pay 30 to 50 Euro's more fo a better phase noise i will buy it :-)

Offline Leo Bodnar

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Re: What frequency standard is sufficient?
« Reply #69 on: October 14, 2018, 12:17:46 am »
Perhaps measure ublox output at 8MHz (48/6) or 6MHz (48/8) - not 10MHz with its non-integral divisor.
Here is 8MHz output result.

It does sound counter-intuitive but integer ratio does not matter.  You get same amount of jitter, it is just spread over different time period and its noise density distribution has different shape. RMS of sawtooth does not depend on its period, just amplitude.

It is important to note that 48MHz core clock is coming from local Ublox 26MHz TCXO or XO - it's free running and usually has about 0.5-2ppm error. 8MHz timepulse is coming from GPS timebase. These frequencies are completely asynchronous.

I had to use John's brilliant KE5FX Noise toolkit and E4406A to get 8MHz plot since HP3048 refuses to lock to such a noisy signal.  I have superimposed 8MHz noise trace onto original image.  There is so much going on and you can argue what's noise and what's spur but all unwanted energy is still there.

Leo
 
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Offline Johnny B Good

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Re: What frequency standard is sufficient?
« Reply #70 on: June 21, 2019, 11:36:28 am »
 I know this response is some eight months late now but I thought I'd explain my vote of thanks anyway.

 Having seen just how much I have written below (TL:DR), I'll summarise by saying that you've shown rather neatly in graphical form just why you can't just simply take the 10MHz output from a NEO-M8N directly for use as a reference other than as quick 'n' dirty substitute for the 10MHz WWV broadcasts which, outside of the USA, are virtually impossible to receive due to the non-existent propagation conditions that prevail during this current period of Sunspot minima.

 The 60KHz and 198KHz frequency standard broadcasts that can be received here in the UK, are inconveniently too low for direct use as calibration references, needing to be multiplied up which is a project of difficulty on a par with that of constructing a GPSDO in itself. Those graphs demonstrate just why you need the expense and complication of (typically) an OCXO to turn a GPS receiver into a stable frequency reference that can be used by high grade test and communications equipment where stability and low phase noise is of paramount importance.

 The following is effectively my 'back story' in how I came to appreciate those graphs you made of the NEO-M8N's performance as a jittery 10MHz reference source. Whilst this is probably a story you've heard many times over, others more fresh to the subject, might be able to get something useful out of it.

 I didn't join the eevblog community until just over six months ago when a posting in sed mentioned the ultra low cost of Chinese test gear, notably Feeltech's ridiculously priced FY6600 (circa $100 at that time - I paid just £75.66 for my, now very much modified, 60MHz version after reading all 69 pages of the FY6600 thread twice over as a result of my purchasing a Siglent SDS1202X-E the week before, also prompted by what I'd read in that thread).

 As they say, "One thing leads to another..." and here I am with a revived interest in hobby level electronics, only this time with ambitions of building stuff with three or more magnitudes of calibration accuracy than I could ever have hoped to achieve three decades ago.

 For the past couple of months, my timing accuracy mods to the AWG and my fledgling GPSDO project have taken on the character of an "Arms Race" since they complement each other's development. Initially, I'd upgraded the cheap 'n' nasty SMD XO chip that Feeltech had cursed their FY6600 with to a 0.1ppm 50MHz TCXO oscillator board which has in turn since been usurped by a CQE 10MHz OCXO with a 3N502 clock multiplier which is now, for the time being, running around 30ppt adrift from another CQE 10MHz OCXO currently being disciplined by a fake M8N module. I have another five of these CQE OCXOs to hand, thanks to a chance purchase of a 5 volt 13MHz CQE OCXO at a hamfest over seven weeks ago (my searching for manufacturer's data had led me to the chance discovery of a very cheap source for the 10MHz units, hence the "Lifetime's Supply").

 One of the earliest conclusions about the nature of the 10MHz output from the M8N module was that it didn't really make any real difference overall whether an even integer[1] divider ratio was chosen over any non-integer ratio simply because the effects of the sawtooth adjustments against the 48MHz TCXO couldn't be escaped, only masked at best. Your test results very nicely and formally confirmed what I'd already managed to conclude simply by gathering the evidence of my ears by monitoring the effects with 25 year old Kenwood HF transceiver.  :) It was so refreshing to see this point about the NEO M8N being made by such a well respected expert as yourself.

 At the end of the day, the random phase shifting due to the satellite signals having to traverse an ionosphere of varying electron density makes such short term variations pale into insignificance, disappearing below the noise level after the necessary hours long averaging to attenuate this final system deficiency has extended to weeks and then ultimately to years.

 I find all mention of these "non-integer division jitter" and "sawtooth adjustments" 'defects' with any GPS receiver capable of offering a 10MHz or other frequency alternative to the PPS output rather irritating when it should be blindingly obvious that these are side effects that have little to zero detriment to the basic navigational function of the GPS system.

Utilising the long term accuracy of the PPS feature embedded within virtually every GPS receiver, simply requires the use of very long integration periods to average or filter out the effects of jitter and sawtooth noise from the long term timing accuracy of the system to translate it into a an extremely accurate low phase noise frequency reference courtesy of a quality design of GPSDO whether a commercial ready made unit or a DIY fabrication.

 I have to say, in the four months since I purchased that first M8N module (a genuine u-blox module BTW, now sadly unable to output anything on its PPS line thanks to rank carelessness on my part in an experiment involving a 12v OCXO), I've learnt an awful lot about the issues involved in utilising a GPS receiver to discipline a high quality OCXO into a trustworthy low phase noise frequency reference. I've no doubt that there's yet more to learn, especially once I've actually committed my basic breadboard layout to a soldered up PCB (most likely a dead bug on groundplane bodge) fitted into a cheap metal enclosure.  :)

 When I first availed myself of the 'luxury' of getting a "10MHz reference" directly out of the M8N module, I did question the need for a 10MHz XO of any sort at all. A few weeks of experimentation later and my question was answered. Now, I realise that unless you're after a quick 'n' dirty alternative to the 10MHz WWV broadcasts, the first priority of any DIY GPSDO project is an electrically tunable XO, preferably an OCXO over a(n electrically tunable) TCXO (but any electrically tunable XO will do for an initial attempt).

 As far as building "Your Very First GPSDO" goes, almost any GPS reciever with an accessible PPS signal can be pressed into such service given enough determination to lock a 10MHz oscillator to a 1Hz reference. However, a word of warning, if you choose one of those cheap £/$4.00 NEO-6Ms, there's a chance you might get one which whittles its PPS down to nothing before immediately reincarnating itself as the full fat pulse you'd initially programmed.  :wtf: The symptoms had been so confusing, it nearly drove me right round the bend... bibble bibble bibble...

 It hadn't helped that I'd had to divide an OCXO with a tuning range of just 1ppm down by a factor of ten thousand. Getting a phase lock at 1KHz becomes a rather glacial process on the order of 15 to 30 minutes, plenty of time for the PPS to disappear up its own backside two or three times and cause disruption to the phase locking process.

 My example with a 1KHz squarewave setting would run at the programmed 50/50 ratio for the first 5 to 7 minutes of a 10 to 14 minute cycle before starting to narrow down, eventually to disappear up its own fundament to then instantly reincarnate as a 50/50 squarewave pulse again. The timing of this cycle seemed to be tied to a bug related to the sawtooth adjustments used to keep the rising edge synced to the nearest 48MHz rising clock edge.

 I tried reprogramming this pulse to the narrowest possible (1μs on a 1ms interval - the only way I could get a 1KHz PPS) and use the PD2 output from the 4046 I was using in the hope the phase/frequency detector could tolerate such insults. Zooming in on the 'scope trace to see the final second of the pulse's disappearing act revealed it literally vanishing altogether, albeit briefly (at a guess, for a whole one millisecond) which not even the PD2 circuit could tolerate. So it was "Goodbye NEO-6N" and "Hello (fake) NEO-M8N" a week or so later.

 It's just possible that this is an effect that only shows as a result of programming a higher frequency pulse. Leaving it set to the default 1 PPS 10ms wide pulse may avoid this issue but I haven't tested for this possibility. It just might prove to be fine at the default setting when I do eventually find the time to test this hypothesis.

[1] I specifically mentioned "even" since odd divisors such as 3 (16MHz output), 5, 7 and 9 (I didn't bother going any further down this particular rabbit hole) result in a 1:2 mark/space ratio which alternates on every sawtooth alignment to a conveniently placed 48MHz clock edge.

 Quite frankly, in view of the sawtooth jitter, there's little point in worrying about using a non-integer divider ratio to get a 10MHz (or 100KHz in my case) unless nice clean 'scope traces are your over-riding objective. It all comes out in the wash anyway, once you've applied some filtering to the PD's output (typically a CR based LPF with a cut off in the region of a few milli Hertz to be followed by a DSP LPF with a cut off frequency of  that's gradually reduced to a micro Hertz or so to filter out the GPS/GLONASS system errors - ionospheric variations etc).

JBG
« Last Edit: June 21, 2019, 11:46:50 am by Johnny B Good »
 

Offline aix

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Re: What frequency standard is sufficient?
« Reply #71 on: June 21, 2019, 02:36:04 pm »
I use an Extron video amp, worked great out of the box. Cost me almost nothing.

+1 to that.  I've done exactly the same.  Lots of very cheap used Extron units on ebay.
 

Offline Johnny B Good

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Re: What frequency standard is sufficient?
« Reply #72 on: June 21, 2019, 08:06:28 pm »
I use an Extron video amp, worked great out of the box. Cost me almost nothing.

+1 to that.  I've done exactly the same.  Lots of very cheap used Extron units on ebay.

 It's worth keeping in mind that if you're terminating the 50 ohm cable feeds with 50 ohm loads, there's no urgent need to modify the 75 ohm matching resistors (replacing with 50 ohms or paralleling them with 150 ohm resistors) as long as it can provide sufficient output voltage (typically in this case, it's unlikely to be a problem). When it comes to the issue of transmission line matching where the signals only go one way, the impedance of the driving source isn't critical, provided the loads at the other end match the cable impedance and there's sufficient headroom for the amps to make up for the extra volt drop in the terminating resistors.

 The transmitting amps can range from zero impedance (constant voltage source) to very high impedance (constant current source) as long they can supply the required voltage or current to the load. In practice, the normal alternative to matching the line driver impedance to the load is to make them constant voltage rather than constant current. However, this exposes the amp outputs to possible fault conditions (short and open circuit faults which can cause overloads and/or instabilities).

 The matching resistors not only protect the amps from these hazards, they also improve stability by acting as snubbing resistors as well as making any fault finding easier, hence their presence in a typical distribution amplifier. Also, there's the fact that any reflections from a shorted or open circuited cable will only be making one trip back to the amp's terminating resistor limiting any deleterious effects such faults might otherwise have on the rest of the distribution network. The only downside to wasting 50% of an amplifier's output in such resistors is the need to find an extra 6dB's worth of headroom. In distribution amps dealing with signals at the milliwatt level, this is not usually regarded by the designer as an expensive luxury.

JBG
 

Offline tkamiya

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Re: What frequency standard is sufficient?
« Reply #73 on: June 24, 2019, 02:42:26 pm »
To do what you said, frequency standards are really not necessary.  Most reputable test equipment already comes with good enough standard.

Having said that, GPSDO is hard to pass by.  It's cheap and it's more than reasonably accurate.  While short term stability may be questionable, certainly, for my use, sufficient.  When you can spend $200 and have a complete set, why not?

Myself, I started there, added a second one, rubidium, rubidium+GPS, and now cesium.  Out of all, GPSDO is the one I use most often.  One thing to think about is, frequency standard is something you keep it on at all times.  Power consumption and warming of the room is a concern.  If I turn all of them on, my A/C cannot keep up.  Rubidium gets quite warm.
 


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