# Flipping problem

I am sending a known sequence of samples from one USRP to another using
the
Basix RX/TX d’boards setting the frequency to 24e6.

When I rx the sequence, the correlation of it keeps flipping, but not in
a
way that suggests residual carrier. It seems as if I am experiencing an
instantaneous flip.

My gain on both the RX and TX is set to 500. The signal amplitude I rx
is
~1200. When I run the GRC model, the Rx says I have no residual carrier,
invert is set to false, the dxc frequency is set to -24e6 and the
baseband
to zero. On the TX side, I have these same values with the exception of
the
dxc freq, it is not set to 24e6. Why the flip?? What do all these
things
mean???

Is 24e6 a bad freq to use with the basic TX/Rx?

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isaacgerg wrote:

to zero. On the TX side, I have these same values with the exception of the
dxc freq, it is not set to 24e6. Why the flip?? What do all these things
mean???

Is 24e6 a bad freq to use with the basic TX/Rx?

I believe what you are seeing here is that the two USRPs are on two
different frequencies. This is not a fault of the USRP. No two
oscillators will be on exactly the same frequency, and so in any

Matt

If this is true, why is it that the residual carrier is reported as
zero?
Are you suggesting that this value is not correct?

Isaac

Matt E. wrote:

Is 24e6 a bad freq to use with the basic TX/Rx?

[email protected]

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On Tue, Jul 29, 2008 at 8:42 AM, isaacgerg [email protected]
wrote:

If this is true, why is it that the residual carrier is reported as zero?
Are you suggesting that this value is not correct?

I think all Matt is saying is that the two receivers are working off
of different local oscillators which cause a frequency offset causing
your correlation phasor to spin around.

Chances are if you plot phase of the correlation over time along with
the magnitude over time, you will see the magnitude doesn’t get
smaller and the phase should smoothly rotate around at the rate which

Does that make sense?

Brian

On Tue, Jul 29, 2008 at 5:42 AM, isaacgerg [email protected]
wrote:

If this is true, why is it that the residual carrier is reported as zero?
Are you suggesting that this value is not correct?

What GRC is reporting as “residual carrier” is not what you seem to
think it is. It refers to how close the software was able to tune the
USRP to your requested frequency. Usually this is in the millihertz
range, or zero, which is what you are seeing.

However, the tuning calculation assumes the USRP clock is exactly 64
MHz, which it never is. Each USRP clock will be slightly different in
frequency, depending on manufacturing tolerances, temperature, age,
and other factors. That is why it is rated at 20 ppm (parts per
million), which means the actual frequency can be anywhere from -20
ppm to +20 ppm away from 64 MHz, or ±1280 Hz. At 24 MHz center
frequency, the USRP/BasicRX can actually be tuned to ±480 Hz around
that frequency. And it’s not even fixed; it will still drift within
this range, primarily due to thermal transients. This difference from
exactly 24 MHz is not what the “residual carrier” term is reporting,
and there is no way for the software to “know” what this offset is.

So even if two USRPs are tuned to what they each think is 24 MHz, and
their tune functions report a “residual carrier” of 0, one will be
receiving at a slightly different frequency that the other is
transmitting, perhaps even a a kilohertz away (2*480 Hz). As many
have mentioned, this will cause your received data to rotate in phase
at the difference in frequency, resulting in “flipped bits”. (The
effect is dependent on which type of modulation you are using,
however.)

Thus, your USRP is working, and 24 MHz is an okay frequency to tune
the BasicRx. You just haven’t implemented any receiver

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

I agree with this analysis unless Isaac tells us he has a carrier and
baud
synchronizer in his system. If he is looking at raw output from the
USRP,
indeed the two oscillators (as they would in any real system) shift
frequency over time and drift in and out of any phase relationship.

Bob

ARRL SDR Working Group Chair
Member: ARRL, AMSAT, AMSAT-DL, TAPR, Packrats,
NJQRP, QRP ARCI, QCWA, FRC.
“Trample the slow … Hurdle the dead”