Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Dan Halperin]

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On Jul 31, 2008, at 12:15 PM, Wireless Monster wrote:

All,
I've been trying to set both Tx and Rx to the same frequency and  
sending a sinusoid to try to measure the frequency offset, but it  
does not seem to work... it looks like as both Tx and Rx are driven  
by the same clock, the frequency the offset gets compensated on the  
Rx signal so it can not be measured... (???)


Anyone has any clue on how to measure it dynamically at the code  
startup?



You have to measure frequency offset between different crystals --  
e.g., 2 USRPs. There's no way to calculate an absolute. Maybe if you  
have the RFX version where each DB has (and uses) its own crystal you  
could do this between two DBs on the USRP, but this would still only  
be their relative offsets.


- -Dan
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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Wireless Monster]

Hi Chris,
Can you briefly explain in what exactly consists the Software PLL?

All,
I've been trying to set both Tx and Rx to the same frequency and sending a
sinusoid to try to measure the frequency offset, but it does not seem to
work... it looks like as both Tx and Rx are driven by the same clock, the
frequency the offset gets compensated on the Rx signal so it can not be
measured... (???)

Anyone has any clue on how to measure it dynamically at the code startup?

Thanks in advance!
[WM]



On Wed, Jul 23, 2008 at 6:57 PM, Chris Stankevitz <[EMAIL PROTECTED]>wrote:

> Wireless Monster wrote:
>
>> Hi All,
>>
>> I am having lots of issues with the USRP  64MHz (20ppm) on board
>> oscillator which does not allow me to get exact and constant RF frequencies
>> out of the RFX900 board. I can not really fix that in SW so I was thinking
>> about replacing the 64MHz crystal with a more precise one. Has anybody a
>> suggestion of which part to use?
>>
>
> I have replaced the crystal with the 20ppm crystal, but I was unable to get
> an "exact and constant" frequency.  I ultimately added a software PLL to
> track the clock errors.  Before I upgraded my software PLL, I used a signal
> generator as an external clock source which worked very well.
>
> Chris
>
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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Matt Ettus]

The one used on USRPs shipping now is 20 ppm with this part number --
   EC2620ETTS-64.000M

From Ecliptek.  They are pin compatible.

Matt

Carles Fernandez wrote:
By the way, could someone tell me what is the part number of the 
VCTCXO shipped with USRP Rev 4.5? In the bill of materials of the 
trunk I see "CTX286LVCT-ND", which seems to correspond to 
CB3LV-3C-64M in the manufacturer's nomenclature.


The datasheet 
(http://www.ctscorp.com/components/Datasheets/008-0256-0_E.pdf) 
specifies +/- 50 ppm, but I have read somewhere that actually it is 
+/- 20 ppm.


Which is the right one?

Thanks,
Carles.




>>> <[EMAIL PROTECTED]
> wrote:
>>> > Hi All,
>>> >
>>> > I am having lots of issues with the USRP  64MHz (20ppm) on board
>>> > oscillator
>>> > which does not allow me to get exact and constant RF
frequencies out of
>>> > the
>>> > RFX900 board. I can not really fix that in SW so I was
thinking about
>>> > replacing the 64MHz crystal with a more precise one. Has
anybody a
>>> > suggestion of which part to use?
>>>




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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Carles Fernandez]

By the way, could someone tell me what is the part number of the VCTCXO
shipped with USRP Rev 4.5? In the bill of materials of the trunk I see
"CTX286LVCT-ND", which seems to correspond to CB3LV-3C-64M in the
manufacturer's nomenclature.

The datasheet (http://www.ctscorp.com/components/Datasheets/008-0256-0_E.pdf)
specifies +/- 50 ppm, but I have read somewhere that actually it is +/- 20
ppm.

Which is the right one?

Thanks,
Carles.




> >>> <[EMAIL PROTECTED]> wrote:
> >>> > Hi All,
> >>> >
> >>> > I am having lots of issues with the USRP  64MHz (20ppm) on board
> >>> > oscillator
> >>> > which does not allow me to get exact and constant RF frequencies out
> of
> >>> > the
> >>> > RFX900 board. I can not really fix that in SW so I was thinking about
> >>> > replacing the 64MHz crystal with a more precise one. Has anybody a
> >>> > suggestion of which part to use?
> >>>
>
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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Wireless Monster]

Thanks everyone for your answer!

I understand that some frequency offset will always be there on the Rx
but I would like to minimize it.

However, the main problem on my system is on the Tx path as the
receiver (which I can not modify) is not having any frequency
correction algorithm, so I must Tx as close as possible to the
"expected" frequency.

Rgds,


On Wed, Jul 23, 2008 at 7:34 PM, Konstantin Tarasov
<[EMAIL PROTECTED]> wrote:
> You still have to remove frequency offset later on in your processing
> however.
>
> Konstantin
>
> On Wed, Jul 23, 2008 at 4:26 PM, Konstantin Tarasov
> <[EMAIL PROTECTED]> wrote:
>>
>> May be you can try FOX924, you can find this oscillator on
>> www.digikey.com. It has 2.5ppm freq. stability, has a 3.3V CMOS output and
>> the pinout is almost the same except one pin which is NC on this chip (# 1).
>> The package is a little smaller but I am sure you can work it in there. You
>> would also have to change settings on the PLL chip since highest frequencey
>> of the FOX924 is 27MHz. It is only 9 bucks (if you get one of) and Digikey
>> is a very fast supplier.
>>
>> Konstantin
>>
>> On Wed, Jul 23, 2008 at 4:06 PM, Brian Padalino <[EMAIL PROTECTED]>
>> wrote:
>>>
>>> On Wed, Jul 23, 2008 at 5:30 PM, Wireless Monster
>>> <[EMAIL PROTECTED]> wrote:
>>> > Hi All,
>>> >
>>> > I am having lots of issues with the USRP  64MHz (20ppm) on board
>>> > oscillator
>>> > which does not allow me to get exact and constant RF frequencies out of
>>> > the
>>> > RFX900 board. I can not really fix that in SW so I was thinking about
>>> > replacing the 64MHz crystal with a more precise one. Has anybody a
>>> > suggestion of which part to use?
>>>
>>> These guys seem to make a whole slew of TCXO's:
>>>
>>>http://www.rakon.com/
>>>
>>> Maybe they'll be interested in supplying you with some more precise
>>> crystals?
>>>
>>> Brian
>>>
>>>
>>> ___
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>>
>
>


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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[kaleem ahmad]

Hi,

Can some one explain in a little bit detail how this software PLL can be
implemented and how this synchronization can be acheived. I am using RFX2400
and facing same problem.

First I tried to solve the problem by calculating the frequency shift at
receiver end by using usrp_fft.py and then by adding/subtracting that
frequency from RX USRP frequency. But it dont solve the problem.

and secondally I am confused why we dont discuss TX clock which is 128MHz
(true...?), I mean what is the role of that.

Thanks


Johnathan Corgan-2 wrote:
> 
> On Wed, Jul 23, 2008 at 3:57 PM, Chris Stankevitz <[EMAIL PROTECTED]>
> wrote:
> 
>>> I am having lots of issues with the USRP  64MHz (20ppm) on board
>>> oscillator which does not allow me to get exact and constant RF
>>> frequencies
>>> out of the RFX900 board. I can not really fix that in SW so I was
>>> thinking
>>> about replacing the 64MHz crystal with a more precise one. Has anybody a
>>> suggestion of which part to use?
>>
>> I have replaced the crystal with the 20ppm crystal, but I was unable to
>> get
>> an "exact and constant" frequency.  I ultimately added a software PLL to
>> track the clock errors.  Before I upgraded my software PLL, I used a
>> signal
>> generator as an external clock source which worked very well.
> 
> Just to reiterate, it's not physically possible to get "exact and
> constant" frequencies in a receiver.  This is not a bug.  All
> realistic radio receivers will have to deal with frequency and phase
> offsets.  Sometimes you can reduce them to the point where you can
> ignore them.  Other times, such as in mobile wireless or satellite
> communications, Doppler effects will make any existing frequency
> stability issues worse.
> 
> Decades of research have gone into algorithms one can use on a
> receiver to recover the exact carrier phase and frequency of a
> received modulated waveform, resulting in many tried and true
> engineering solutions.  Some these, such as software PLLs as Chris
> mentions, are available in GNU Radio for your use.
> 
> I believe (ISTR) that there are 5 PPM versions of the crystal on the
> USRP that fit the same solder pattern.  But that still leaves you with
> many KHz of potential offset and drift at 900 MHz, so unless your
> chosen modulation can withstand that, you'll still need to solve the
> receiver synchronization problem.
> 
> -- 
> Johnathan Corgan
> Corgan Enterprises LLC
> http://corganenterprises.com/
> 
> 
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> 

-- 
View this message in context: 
http://www.nabble.com/64MHz-USRP-Oscillator-tp18621090p18632191.html
Sent from the GnuRadio mailing list archive at Nabble.com.



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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Johnathan Corgan]

On Wed, Jul 23, 2008 at 3:57 PM, Chris Stankevitz <[EMAIL PROTECTED]> wrote:

>> I am having lots of issues with the USRP  64MHz (20ppm) on board
>> oscillator which does not allow me to get exact and constant RF frequencies
>> out of the RFX900 board. I can not really fix that in SW so I was thinking
>> about replacing the 64MHz crystal with a more precise one. Has anybody a
>> suggestion of which part to use?
>
> I have replaced the crystal with the 20ppm crystal, but I was unable to get
> an "exact and constant" frequency.  I ultimately added a software PLL to
> track the clock errors.  Before I upgraded my software PLL, I used a signal
> generator as an external clock source which worked very well.

Just to reiterate, it's not physically possible to get "exact and
constant" frequencies in a receiver.  This is not a bug.  All
realistic radio receivers will have to deal with frequency and phase
offsets.  Sometimes you can reduce them to the point where you can
ignore them.  Other times, such as in mobile wireless or satellite
communications, Doppler effects will make any existing frequency
stability issues worse.

Decades of research have gone into algorithms one can use on a
receiver to recover the exact carrier phase and frequency of a
received modulated waveform, resulting in many tried and true
engineering solutions.  Some these, such as software PLLs as Chris
mentions, are available in GNU Radio for your use.

I believe (ISTR) that there are 5 PPM versions of the crystal on the
USRP that fit the same solder pattern.  But that still leaves you with
many KHz of potential offset and drift at 900 MHz, so unless your
chosen modulation can withstand that, you'll still need to solve the
receiver synchronization problem.

-- 
Johnathan Corgan
Corgan Enterprises LLC
http://corganenterprises.com/


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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Brian Padalino]

On Wed, Jul 23, 2008 at 5:30 PM, Wireless Monster
<[EMAIL PROTECTED]> wrote:
> Hi All,
>
> I am having lots of issues with the USRP  64MHz (20ppm) on board oscillator
> which does not allow me to get exact and constant RF frequencies out of the
> RFX900 board. I can not really fix that in SW so I was thinking about
> replacing the 64MHz crystal with a more precise one. Has anybody a
> suggestion of which part to use?

These guys seem to make a whole slew of TCXO's:

http://www.rakon.com/

Maybe they'll be interested in supplying you with some more precise crystals?

Brian


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Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Chris Stankevitz]

Wireless Monster wrote:

Hi All,

I am having lots of issues with the USRP  64MHz (20ppm) on board 
oscillator which does not allow me to get exact and constant RF 
frequencies out of the RFX900 board. I can not really fix that in SW so 
I was thinking about replacing the 64MHz crystal with a more precise 
one. Has anybody a suggestion of which part to use?


I have replaced the crystal with the 20ppm crystal, but I was unable to 
get an "exact and constant" frequency.  I ultimately added a software 
PLL to track the clock errors.  Before I upgraded my software PLL, I 
used a signal generator as an external clock source which worked very well.


Chris


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[Discuss-gnuradio] 64MHz USRP Oscillator

[Wireless Monster]

Hi All,

I am having lots of issues with the USRP  64MHz (20ppm) on board oscillator
which does not allow me to get exact and constant RF frequencies out of the
RFX900 board. I can not really fix that in SW so I was thinking about
replacing the 64MHz crystal with a more precise one. Has anybody a
suggestion of which part to use?

Thanks in advance!
[WM]
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