On 05/29/2012 07:33 AM, Sanat Gulvadi wrote:

Unless your devices are synchronized to an external reference, there is
a near-certainty that there will be an offset in the sampling
frequencies between the two devices -- the master clocks on the two
devices won't be synchronized, which leads both to sample-rate
differences, and actual tuned-frequency differences.

--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

Thanks for the quick response. Could you help me understand this better
please? The different in the physical constraints that affect the clocks on
board the devices that cause the the carrier frequency offset also causes
differences in sampling rates ? So if I am using same
decimation/interpolation factors, setting the sampling frequency on Tx to
100e6/16 and on the Rx side also to 100e6/16 will still result in them
being sampled at different rates ? What would typically such numbers ? I
have the RFX2400 on both devices and carrier frequency set to  2.44GHz.

On Tue, May 29, 2012 at 2:07 PM, Marcus D. Leech mleech@ripnet.com wrote:

--

The master oscillator on the USRP2 is a +/- 5PPM TCXO oscillator.
Crystal oscillators are imperfect, and they get closer to perfect the
more you spend on them. Which is why one generally uses an external
reference to move from 5PPM to 50PPB, using a GPSDO or similar.

So,
you can take the 5PPM estimate and plug it into your calculations.

-Marcus

On 29 May 2012 10:50, Sanat Gulvadi wrote:

the quick response. Could you help me understand this better please? The
different in the physical constraints that affect the clocks on board
the devices that cause the the carrier frequency offset also causes
differences in sampling rates ? So if I am using same
decimation/interpolation factors, setting the sampling frequency on Tx
to 100e6/16 and on the Rx side also to 100e6/16 will still result in
them being sampled at different rates ? What would typically such
numbers ? I have the RFX2400 on both devices and carrier frequency set
to 2.44GHz.

<mleech@ripnet.com [1]> wrote:

Gulvadi wrote:

this question has already been asked or has already been addressed in
the manual. I was unsuccessful in finding the answer. Should I expect an
offset in the sampling frequencies between the 2 devices ? Just to add a
little background :

system using 2 USRP2-REV4s each with an RFX2400 board. My algorithm is
supposed to estimate and correct the different effects that an OFDM
symbol might experience. With simulated transmission within MATLAB
alone, the whole algorithm works fine with the following simulated
channel effects

of noise samples before and after the sample stream.

offset simulated by multiplying the time domain digital samples by an
exponential term

multipath components each multiplied with a small amplitude with the
original stream

correcting the resulting constellation for each symbol after
demodulation etc.

the whole constellation to rotate proportionally with each subsequent
OFDM symbol which I corrected for.

reading, I thought I had accounted for the different effects that can
cause the signal to degrade.

question for the list is that the only thing that occured to me is
whether there is possible chance for there to be a sampling frequency
offset between the 2 devices when I attempt a real transmission. Is
there any to be expected ?

your devices are synchronized to an external reference, there is a
near-certainty that there will be an offset in the sampling

frequencies between the two devices -- the master clocks on the two
devices won't be synchronized, which leads both to sample-rate

differences, and actual tuned-frequency differences.

Marcus Leech

Consortium

Links:

[1]
mailto:mleech@ripnet.com

I want to design the system without any external synchronization. All my
corrections I plan to do within the receiver. I apologize if I seem to be
asking the same question again. But in my case, with no synchronization
with an external reference clock, should I be assuming that 100e6/16 Hz
sampling rate on one USRP2 will not be the same as 100e6/16 Hz sampling
rate on the other ? If this is the case, am I also right in assuming that
this is because 100MHz doesn't really mean the same resulting real-world
clock frequency on different devices, because of these imperfections in the
crystals ?

Regards,
Sanat

On Tue, May 29, 2012 at 5:00 PM, mleech@ripnet.com wrote:

--

Just factor in the +/- 5PPM uncertainty in the clock on the two
devices:

100Mhz/16 = 6.25e6 * +/-5PPM

= +/- 31.25 Hz uncertainty in
sample rate

On 29 May 2012 11:58, Sanat Gulvadi wrote:

design the system without any external synchronization. All my
corrections I plan to do within the receiver. I apologize if I seem to
be asking the same question again. But in my case, with no
synchronization with an external reference clock, should I be assuming
that 100e6/16 Hz sampling rate on one USRP2 will not be the same as
100e6/16 Hz sampling rate on the other ? If this is the case, am I also
right in assuming that this is because 100MHz doesn't really mean the
same resulting real-world clock frequency on different devices, because
of these imperfections in the crystals ?

Tue, May 29, 2012 at 5:00 PM, <mleech@ripnet.com [2]> wrote:

master oscillator on the USRP2 is a +/- 5PPM TCXO oscillator. Crystal
oscillators are imperfect, and they get closer to perfect the more you
spend on them. Which is why one generally uses an external reference to
move from 5PPM to 50PPB, using a GPSDO or similar.

take the 5PPM estimate and plug it into your calculations.

-Marcus

Thanks for the quick response. Could you help me understand this better
please? The different in the physical constraints that affect the clocks
on board the devices that cause the the carrier frequency offset also
causes differences in sampling rates ? So if I am using same
decimation/interpolation factors, setting the sampling frequency on Tx
to 100e6/16 and on the Rx side also to 100e6/16 will still result in
them being sampled at different rates ? What would typically such
numbers ? I have the RFX2400 on both devices and carrier frequency set
to 2.44GHz.

<mleech@ripnet.com [1]> wrote:

Gulvadi wrote:

advance if this question has already been asked or has already been
addressed in the manual. I was unsuccessful in finding the answer.
Should I expect an offset in the sampling frequencies between the 2
devices ? Just to add a little background :

OFDM based communication system using 2 USRP2-REV4s each with an RFX2400
board. My algorithm is supposed to estimate and correct the different
effects that an OFDM symbol might experience. With simulated
transmission within MATLAB alone, the whole algorithm works fine with
the following simulated channel effects

placing a random number of noise samples before and after the sample
stream.

domain digital samples by an exponential term

using function like like awgn()

cyclicprefixlength/4 number of multipath components each multiplied
with a small amplitude with the original stream

placing the FFT window incorrectly and correcting the resulting
constellation for each symbol after demodulation etc.

a linear phase error that was causing the whole constellation to rotate
proportionally with each subsequent OFDM symbol which I corrected
for.

accounted for the different effects that can cause the signal to
degrade.

is that the only thing that occured to me is whether there is possible
chance for there to be a sampling frequency offset between the 2 devices
when I attempt a real transmission. Is there any to be expected ?

synchronized to an external reference, there is a near-certainty that
there will be an offset in the sampling

devices -- the master clocks on the two devices won't be synchronized,
which leads both to sample-rate

tuned-frequency differences.

Principal Investigator

http://www.sbrac.org

Links:

[1]
mailto:mleech@ripnet.com
[2] mailto:mleech@ripnet.com