external 10M ref for scope

[metebalci]

I have a 6 series infiniivision scope. It has I think 15ppm internal clock. I would like to improve freq. counter and jitter measurements. What kind of freq. ref/oscillator is the best fit ? I am a bit confused about differences among alternatives, what should be the selection criteria ?

[mendip_discovery]

Before the Rubidium Clock fanatics wade in. Just get a GPS Based Reference GPSDO like this, it will have a much better ppm than what you currently have and isn't going to break the bank. At least it gives you a frequency source to play with.

[metebalci]

OK, and if I need to use it far from a window etc. I guess it will not work ? so in this case is an OCXO enough before going to rubidium ?

[tggzzz]

So you think better frequency and jitter is the solution. But what is your problem.

Are you sure a scope is the most appropriate tool?

[metebalci]

No but I dont want to buy a freq. counter or another device yet, also I may need to sync some devices including the scope. Basically I think what I am asking is what would be a good choice as the first freq. ref or master clock.

[mendip_discovery]

How far is the window? You can mount the antenna outdoors if you like. The GPS is what gets the ppm lower.

I recently had a fun couple of hours with a customer doing some tests on his rather expensive scope. Lots of messing about with settings and even then it wouldn't always give brilliant results, but they were within spec he just hand looked at the spec much. They were testing some USB stuff and had failed some kit for being out of spec. I think the idea of doing all the testing using 1 bit of kit appealed to them.

[metebalci]

How far is the window? You can mount the antenna outdoors if you like. The GPS is what gets the ppm lower.

I have 3-4m to the window.

By the way I was mistaken about the scope's accuracy, time base accuracy on the datasheet is 1.6ppm + aging (1 year 0.5ppm, 10 years 2ppm).

Diverging a bit from my original question but also on the datasheet it says for the jitter analysis: "Jitter measurement floor: 600 fs rms at 6 GHz sine wave (typical)". This is at least partially related to the time base accuracy right ? so does it improve with a better external ref or not ? also does it change with the bandwidth of the scope ?

If the jitter measurement floor is affected by the external ref in a positive sense, is the important property short term stability ?

[Electro Fan]

Before the Rubidium Clock fanatics wade in. Just get a GPS Based Reference GPSDO like this, it will have a much better ppm than what you currently have and isn't going to break the bank. At least it gives you a frequency source to play with.

Leo Bodnar also makes another version:
http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301

The OP might also want to look at a BG7TBL GPSDO.  BG7TBL sells products through lots of eBay sellers (some are probably better than others).  Search for "BG7TBL10MHz LCD-GNSS DISCIPLINED OSCILLATOR Support GNSS GPS BDS GLONASS GALILEO." This model will support two different GNSS systems such as GPS BDS GLONASS and GALILEO (you pick two).  It also has a LCD readout.  Some users in the metrology forum will probably say you don't need the LCD feature but I find it useful.  For a few $ more you can probably buy from a local distributor but it's possible to have good luck with distributors in other regions.  For about half the price you can find a BG7TBL GPSDO model that won't have the LCD and only do one GNSS system.  Either way, these will provide a good reference clock for other devices on your bench. 

If you want to synchronize multiple devices you can take a look at a distribution amplifier.  Search for "BG7TBL distribution amplifier" on eBay.  I have found that this approach can significantly improve the accuracy of multiple devices such as oscilloscopes, counters, signal generators, and spectrum analyzers, as well as synchronize their clocks.  You just need a small patch antenna (that generally comes with a new GPSDO) - you put that in a window with a reasonably clear view to the sky and you are in business.  You can also connect the GPSDO to a computer and use it with various free software including Lady Heather which will give you some additional information and give you some added control over the GPSDO.  I'd say start with a BG7TBL GPSDO or maybe a Leo Bodnar GPSDO and if you enjoy it go for a distribution amplifier if you have several devices that you would like to drive with the reference signal. 

Not all test equipment is set up to receive a 10 MHz reference (or any reference) so make sure you understand your equipment's capabilities for using an external reference. Some equipment will benefit more than others; depends on how good the internal clock is already performing in each piece of equipment.  Just be careful, as this is a potential slippery slope to becoming a Time Nut and it can add to TEA.

Specifically with respect to phase noise, this might be interesting:
https://twitter.com/leobodnar/status/1075529001836400646

Overview on Phase Noise and Jitter
Phase noise and jitter are two related quantities associated with a noisy oscillator. Phase noise is a frequency-domain view of the noise spectrum around the oscillator signal, while jitter is a timedomain measure of the timing accuracy of the oscillator period. This paper discusses the relationship between phase noise and jitter in free-running oscillators.  The reader is referred to three other papers for a more detailed look at the theory: Demir [1], Hajimiri [2] and Herzel [3]. The phase noise and jitter theory discussed here is mainly taken from these three papers. The jitter in a closed-loop phase-locked loop is not discussed here; see [4, 9, 10].
https://www.keysight.com/us/en/assets/7018-01984/technical-overviews/5990-3108.pdf

[Echo88]

Now im looking at 12 Digit frequency counters, rubidium standards and allan deviation plots. Slippery slope is too slippery, help. 
I like the linked twitter-picture of the bottles and the plush teddy behind the laptop.

[metebalci]

I will try one of the clocks mentioned here or something similar, thank you all.

[Electro Fan]

Now im looking at 12 Digit frequency counters, rubidium standards and allan deviation plots. Slippery slope is too slippery, help. 
I like the linked twitter-picture of the bottles and the plush teddy behind the laptop.

Into the rabbit hole:
https://www.phidgets.com/docs/Allan_Deviation_Primer

[MegaVolt]

Specifically with respect to phase noise, this might be interesting:
https://twitter.com/leobodnar/status/1075529001836400646

It seems to be a very bad generator in BG7TBL. It is necessary to really try so that the generator for one frequency was worse than the integrated PLL .... very surprised.

[jonpaul]

Hello,

We use the excellent Leo Bodnar GPS, has SMA connector and long antenna cable. Seems to sync easily far from windows.

Cost L UK 99.

Can generate any output 1 kHz...2 GHz. Set to 10.0000 MHz and highest level.

Jon

[mendip_discovery]

I have yet to setup my GPS ref but I am wondering what would the measurment uncertainty of the GPS systems used. As it's the source of my timing I would need to know, I did find some stuff on it but that was back in the early 2000s and I think things have moved on a bit since then.

[jonpaul]

Rebonjour, the scopes have trigger jitter and limited frequency and time resiolution.

Even with an NBP perfect ref clock, the scopes intrinsic jitter and other limitations will predominate.

Thus a 0.01% xtal, TCXO, GPS, Rb or Cs will not produce a noticeable difference in measurements.

Refer to your scope mfg spec sheet for jitter, triggering, etc.

Bon Chance

Jon

[metebalci]

I got a Leo Bodnar GPS (with two outputs) and trying to install it to my not very near to window location. First thing is it seems that it is a bit picky because the latest GPS module (not clock just module) I have can find satellites and lock pretty quick but the GPS clock cannot lock with the same antenna at the same location (which is a bit far from the window so limited sky view). So I either need a long antenna cable or a long clock cable ? Does it matter, which one is better ? By long I mean 20m would be great but I may live with less if I use it just to check and adjust a non-GPS OCXO time to time.

[metebalci]

…and to what extent the long clock cable/RG58 would affect the clock signal quality ?

[jonpaul]

Rebonjour cher  metebalci!

1/ We used the Leo Bodnar GPS and 40 pS pulser for years, fine products well engineered and supported.
The GPS is run at the highest output and 10.0000 MHz, into a number of 2 and 4 way BNC 50 Ohm splitters to form a house master clock distribution with no amplifier. All HP, Yokogawa and TEK seem to accept the REF at 200..300 mV.

2/ GPS USA L1 1575.42 MHz  GPS L2 Band: 1227.6. Our  Antenna at ~120 W long 37 lat north facing window, wood structure easily acquires many sats in a few seconds. Your location, structure, floor will affect results as the GPS freq is Leo provided PPS antenna and cable with SMA. Many accessory extender and GPS booster antennas are available.

3/ Even without GPS lock it recalls the last GPS and give very fine accuracy and phase noise.

4/ Strongly suggest to Contact to Leo Bodnar directly with your questions. Leo is super for customer support!
https://www.leobodnar.com/shop/index.php?main_page=contact_us

5/ Simple test: Setup the scope, meter, counter etc for the most critical signal and test eg jitter in ns range,
Start with internal Xtal ref take data.
Replace INT with EXT ref, use Leo Bodnar or HP8640B set to 10.0000 MHz.

Any difference  in the  measurements? Unless you are using a fancy Lecroy/HP.R&S with >10 gHz FS costing 10K or more, I doubt if you see any difference.

Best Regards,


Jon

[metebalci]

Rebonjour cher  metebalci!

1/ We used the Leo Bodnar GPS and 40 pS pulser for years, fine products well engineered and supported.
The GPS is run at the highest output and 10.0000 MHz, into a number of 2 and 4 way BNC 50 Ohm splitters to form a house master clock distribution with no amplifier. All HP, Yokogawa and TEK seem to accept the REF at 200..300 mV.

2/ GPS USA L1 1575.42 MHz  GPS L2 Band: 1227.6. Our  Antenna at ~120 W long 37 lat north facing window, wood structure easily acquires many sats in a few seconds. Your location, structure, floor will affect results as the GPS freq is Leo provided PPS antenna and cable with SMA. Many accessory extender and GPS booster antennas are available.

3/ Even without GPS lock it recalls the last GPS and give very fine accuracy and phase noise.

4/ Strongly suggest to Contact to Leo Bodnar directly with your questions. Leo is super for customer support!
https://www.leobodnar.com/shop/index.php?main_page=contact_us

5/ Simple test: Setup the scope, meter, counter etc for the most critical signal and test eg jitter in ns range,
Start with internal Xtal ref take data.
Replace INT with EXT ref, use Leo Bodnar or HP8640B set to 10.0000 MHz.

Any difference  in the  measurements? Unless you are using a fancy Lecroy/HP.R&S with >10 gHz FS costing 10K or more, I doubt if you see any difference.

Best Regards,


Jon

Hi Jon,

I actually got in touch with their support but there is not much they can do, it is because of my location like you said (also I think the building is a steel frame). The latest GPS module I mentioned is a latest ublox so it would not be strange if it is more sensitive I guess. I found a 20m RG58 at home and tried both, it looks to me antenna extension works better which is also more convenient. I looked at jitter in both cases with an ext. OCXO ref but I dont know if it would be a good test.

Good to know it is OK to use splitters I may need that.

Mete

[2N3055]

Rebonjour cher  metebalci!

1/ We used the Leo Bodnar GPS and 40 pS pulser for years, fine products well engineered and supported.
The GPS is run at the highest output and 10.0000 MHz, into a number of 2 and 4 way BNC 50 Ohm splitters to form a house master clock distribution with no amplifier. All HP, Yokogawa and TEK seem to accept the REF at 200..300 mV.

2/ GPS USA L1 1575.42 MHz  GPS L2 Band: 1227.6. Our  Antenna at ~120 W long 37 lat north facing window, wood structure easily acquires many sats in a few seconds. Your location, structure, floor will affect results as the GPS freq is Leo provided PPS antenna and cable with SMA. Many accessory extender and GPS booster antennas are available.

3/ Even without GPS lock it recalls the last GPS and give very fine accuracy and phase noise.

4/ Strongly suggest to Contact to Leo Bodnar directly with your questions. Leo is super for customer support!
https://www.leobodnar.com/shop/index.php?main_page=contact_us

5/ Simple test: Setup the scope, meter, counter etc for the most critical signal and test eg jitter in ns range,
Start with internal Xtal ref take data.
Replace INT with EXT ref, use Leo Bodnar or HP8640B set to 10.0000 MHz.

Any difference  in the  measurements? Unless you are using a fancy Lecroy/HP.R&S with >10 gHz FS costing 10K or more, I doubt if you see any difference.

Best Regards,


Jon

Hi Jon,

I actually got in touch with their support but there is not much they can do, it is because of my location like you said (also I think the building is a steel frame). The latest GPS module I mentioned is a latest ublox so it would not be strange if it is more sensitive I guess. I found a 20m RG58 at home and tried both, it looks to me antenna extension works better which is also more convenient. I looked at jitter in both cases with an ext. OCXO ref but I dont know if it would be a good test.

Good to know it is OK to use splitters I may need that.

Mete

Fun fact is that many GPS manufacturers recommend RG6 as an antenna cable despite it being 75 ohm, because of much lower loss.

[metebalci]

In case a future reader of this post has the same issue, here is my working solution. I use a 20m RG 58/U cable (specified DC-4GHz) terminated with BNC connectors . I use the GPS antenna (which also has a 3m cable) that comes with Leo Bodnar GPSDO, and SMA-BNC adapters on both ends of the 20m cable, so I extend the antenna cable. GPSDO locks quick and I havent seen a problem yet.

One point I am confused looking at the datasheets of various products: some products particularly say the ref clock input is expected to be a sine wave, some products do not specify it (the waveshape). For oscilloscopes, spectrum analyzers and frequency counters etc., is there a consensus about this ? Do I have to check each equipment individually, or it doesnt matter ?

[Wallace Gasiewicz]

I think you have to check the specs on each unit.
However, many times it does not matter and the receiving unit will correctly use any wave of the correct amplitude and freq and you can use either a sine or square wave
Some units are fussy about the rise time of the frequency ref and only take square wave, or something that at least resembles a square wave.
Sometimes a unit will take a pulse.
No easy answer.

[jonpaul]

Rebonjour a tous:

My experience is with

HP RF gen 8640B, counters, spectrum/network analyzer   4195A and synthesizer 3325A
Yokogawa scope DL7440 and  time analyzer TA720
TEK scopes with REF input (2465B CTT options)

 REF inputs use 50 Ohm inputs  with wave shaping, so not critical for waveshape,  sine, square, pulse ~  100 mV or greater. 


We set our Leo Bodnar  Mini GPS, set for 32 mA (max) RMS output

http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=301

 Measured square wave Trise ~ 650 pS (on 500 MHz Yokogowa DL7440) , 1.8V P-P into 50 Ohms

We distribute as a master clock with 50 Ohm BNC passive (transformer) splitters, we can drive 4...6 instruments easily.

 10 MHz capable 2X, 4X 6X and 8X splitters  are common, e.g.  Mini-Circuits,  with BNC, SMA, N  and other connectors.

With a 4 way splitter Merrimac, we got 930 mV P-P on each output and ~ 3 nS Trise.

Even after a 6way split the outputs are 200 mV or more, so all instruments sync.

https://www.minicircuits.com/WebStore/Splitters.html

Old Splitters are on epay, eg  Blonder-Toung and Merrimack.

Even old CATV cable splitters (F connectors)  work,   at 10 MHz.

Hope this is useful!

Bon Chance,

Jon

[metebalci]

Thanks for the info regarding the splitters, I will probably need one soon.

We set our Leo Bodnar  Mini GPS, set for 32 mA (max) RMS output

http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=301

 Measured square wave Trise ~ 650 pS (on 500 MHz Yokogowa DL7440) , 1.8V P-P into 50 Ohms

Just for the reference, and I have the other GPSDO unit with two outputs, I measure 2.5Vpp (at 32mA setting) and Trise ~1ns. Same for both outputs (I set both to 10MHz).

[metebalci]

Also on the website of GPSDO, it is written:

"Short term signal quality is defined by internal TCXO clock source providing high-quality, low phase noise clock signal with sub-picosecond RMS jitter."

If I measure 1-Period jitter, I see (sigma=) 6ps. Also if I enter the phase noise values given on the website to the calculator, the result is also ~5-6ns (nice example). Not sure what is meant by short term but I think it should be like >= 1MHz.