Re: [Discuss-gnuradio] External clock source Info?

[Sylvain Munaut]

Hi,


> It seems that 52MHz /64MHz precision clock references are like hen’s
> teeth, so I’m working on a design.


Have you seen http://code.google.com/p/clock-tamer/ ?

It can provide precision 52 / 64 MHz (and a lot of others) and can also
integrate a GPS receiver to lock itself to the 1 pps.

They're out of stock right now but should have new one in september IIRC.


Cheers,

   Sylvain

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Re: [Discuss-gnuradio] External clock source Info?

[Per Zetterberg]

Gregory Maxwell wrote:

On Tue, Aug 17, 2010 at 3:10 PM, Mark J. Blair  wrote:
  

On Aug 17, 2010, at 2:24 PM, William Pretty Security Inc wrote:


It seems that 52MHz /64MHz precision clock references are like hen’s teeth, so 
I’m working on a design.
What I need to know is what sort of level is the USRP1 looking for ? Is it 3.3V 
CMOS ?

Once I get the design working, I’ll make them available at a reasonable price J
  

I think you will get bonus points if your design can accept an external 10MHz 
reference provided by a GPS-disciplined oscillator. If there was some 
convenient way to adjust out the crystal aging for use when the external 
reference isn't available, that would be even better. I've been thinking of 
designing something along these lines, but I won't complain if you do the hard 
work and I can just buy one from you. ;)




A while back I decided I wanted a better clock in my USRP1s and went
hunting. I didn't have a lot of luck, and the few places I sent
inquiries to didn't get back to me.  My attention was eventually
pulled off onto other projects... but you can count me in as someone
who would like to partake in the results of this work.

I'd also like to echo the 10MHz comment.  GPSDOs Clocks with excellent
long term stability show up at fairly low prices on ebay all the time
(excess from cell site deployments, I assume).  I have a couple of
them.   What they don't usually have is the very low phase noise that
I'd want for a clock which is eventually going to multiplied up to GHz
levels.   So a USRP1 clock board which was primarily a 10->64MHz
up-converter with very low phase noise would be exactly what I would
want.

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I enjoy hearing the discussion on clocks as I have also wanted  very low 
phase-noise. For the USRP2 I hope that a good 10MHz reference will do 
the trick.  A while ago I did some tests which show good results when 
the transmitter and receiver are locked to the same 10MHz (see below). 
From this I assume that the same good results would occur with 
excellent 10MHz references. I have ordered T1220-T17-3.3-SM-10.0MHz 
from  http://www.greenrayindustries.com/tcxo.html. I haven't tested with 
that one yet.



== My experiments:

I have been experimenting with transmitting a tone between two USRP2s 
equipped with basic daughterboards. The daughterboards are tuned to 
20MHz, and the transmitted signal is generated in base-band as a cisoid 
with 5MHz frequency. The decimation and interpolation rate is 25MHz in 
both TX and RX.


In the figure
http://www.s3.kth.se/~perz/usrp/phase_noise1.jpg 
 non of the USRP2s 
are locked to external reference.

In the figure
http://www.s3.kth.se/~perz/usrp/phase_noise2.jpg 
 the two USRPs are 
locked to the 10MHz ref output of a marconi2024 generator.

In the figure
http://www.s3.kth.se/~perz/usrp/phase_noise3.jpg 
 the two USRP2 are 
locked to the 10MHz ref of two _different_ marconi2024.


What I gather from this is that the reference of my marconi2024 is 
better than that built in the usrp2 (first and third figure). Though, it 
can be much better (second figure). However, then I would need an 
extremely clean reference. My marconi2024 is specified to have 
phase-noise better than -124dB/Hz at 20kHz. The results seem to suggest 
that's not true. However, the generator is 15 years old. Is it likely 
that it has aged ?




BR/
Per

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Re: [Discuss-gnuradio] External clock source Info?

[Mark J. Blair]

On Aug 17, 2010, at 2:24 PM, Gregory Maxwell wrote:
> I'd also like to echo the 10MHz comment.  GPSDOs Clocks with excellent
> long term stability show up at fairly low prices on ebay all the time
> (excess from cell site deployments, I assume).  I have a couple of
> them.   What they don't usually have is the very low phase noise that
> I'd want for a clock which is eventually going to multiplied up to GHz
> levels.   So a USRP1 clock board which was primarily a 10->64MHz
> up-converter with very low phase noise would be exactly what I would
> want.


For my bench frequency reference, I selected a surplus Trimble Thunderbolt with 
the newer version of OCXO that's supposed to be nearly as good as the HP 
double-oven OCXOs. I'll power it from an HP bench supply, since the switchers 
that are often supplied with the surplus Thunderbolts are said to add a lot of 
phase noise to the oscillator output. If the statistics from the monitoring 
program that I used during my first test run of the oscillator are to be 
trusted, then it should be able to provide a reference accurate to within tens 
of parts per trillion. I haven't finished installation of my GPSDO yet; I still 
need to finish repairing the (cheap, broken) HP bench supply that I got from 
eBay and assemble a power cable. I may get this done tonight, as the parts I 
needed just arrived today. The outdoor antenna (a surplus Lucent +26dB antenna 
as used at cell sites) is already installed and cabled into my house.

My own idea for a USRP clocking replacement was to use a common and fairly 
inexpensive VCTCXO rated for temperature stability of around +/-2.5ppm 
(available at Digi-Key in frequencies commonly used in cell phones, GPS 
receivers, etc.), drive a PLL+VCO with that to generate 64 MHz (or 52 MHz, or 
any other frequency that the TCXO+PLL+VCO can generate), with an additional PLL 
to lock the VCTCXO to an external 10 MHz input. I'd also include a 
microcontroller which could measure the VCTCXO control voltage while locked to 
an external reference, and then drive that same voltage with a DAC when the 
external reference isn't present. Thus, while attached to the GPSDO the 
internal reference would be slaved to a very good reference, and then when that 
reference isn't available (such as when operating "in the field") the on-board 
VCTCXO would at least be trimmed to compensate for aging. The memorization of 
the nominal VCTCXO tuning voltage might be triggered automatically, or by a 
push button, or by a GPIO controlled by the USRP hardware.

The VCTCXO might be replaced with a voltage-controllable OCXO if desired for 
better holdover stability, but I figured that VCTCXO should be adequate for my 
needs.

Does that architecture sound reasonable? If so, would anybody like to save me 
the trouble of designing and building it myself? ;)



-- 
Mark J. Blair, NF6X 
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.





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Re: [Discuss-gnuradio] External clock source Info?

[Gregory Maxwell]

On Tue, Aug 17, 2010 at 3:10 PM, Mark J. Blair  wrote:
>
> On Aug 17, 2010, at 2:24 PM, William Pretty Security Inc wrote:
>> It seems that 52MHz /64MHz precision clock references are like hen’s teeth, 
>> so I’m working on a design.
>> What I need to know is what sort of level is the USRP1 looking for ? Is it 
>> 3.3V CMOS ?
>>
>> Once I get the design working, I’ll make them available at a reasonable 
>> price J
>
> I think you will get bonus points if your design can accept an external 10MHz 
> reference provided by a GPS-disciplined oscillator. If there was some 
> convenient way to adjust out the crystal aging for use when the external 
> reference isn't available, that would be even better. I've been thinking of 
> designing something along these lines, but I won't complain if you do the 
> hard work and I can just buy one from you. ;)


A while back I decided I wanted a better clock in my USRP1s and went
hunting. I didn't have a lot of luck, and the few places I sent
inquiries to didn't get back to me.  My attention was eventually
pulled off onto other projects... but you can count me in as someone
who would like to partake in the results of this work.

I'd also like to echo the 10MHz comment.  GPSDOs Clocks with excellent
long term stability show up at fairly low prices on ebay all the time
(excess from cell site deployments, I assume).  I have a couple of
them.   What they don't usually have is the very low phase noise that
I'd want for a clock which is eventually going to multiplied up to GHz
levels.   So a USRP1 clock board which was primarily a 10->64MHz
up-converter with very low phase noise would be exactly what I would
want.

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Re: [Discuss-gnuradio] External clock source Info?

[Mark J. Blair]

On Aug 17, 2010, at 2:24 PM, William Pretty Security Inc wrote:
> It seems that 52MHz /64MHz precision clock references are like hen’s teeth, 
> so I’m working on a design.
> What I need to know is what sort of level is the USRP1 looking for ? Is it 
> 3.3V CMOS ?
>  
> Once I get the design working, I’ll make them available at a reasonable price 
> J

I think you will get bonus points if your design can accept an external 10MHz 
reference provided by a GPS-disciplined oscillator. If there was some 
convenient way to adjust out the crystal aging for use when the external 
reference isn't available, that would be even better. I've been thinking of 
designing something along these lines, but I won't complain if you do the hard 
work and I can just buy one from you. ;)


-- 
Mark J. Blair, NF6X 
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.





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[Discuss-gnuradio] External clock source Info?

[William Pretty Security Inc]

Hi All;

 

It seems that 52MHz /64MHz precision clock references are like hen's teeth,
so I'm working on a design.

What I need to know is what sort of level is the USRP1 looking for ? Is it
3.3V CMOS ?

 

Once I get the design working, I'll make them available at a reasonable
price J

 

 

Bill

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