# Gr_correlate_access_code_bb question

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Hey all,

Sorry to bring up painful memories :). I did some calculations to work
out frequency drift of the USRPs, and I wanted to know where I’m going
wrong.

The crystals in the RFX 2400 are spec’ed at 50ppm error, which is to say
that the maximum frequency offset between the sender and receiver is
then 2 * 50 ppm * 2.462 GHz (802.11b Channel 11) = 246.2 kHz.

246.2 kHz sampled at 64 MHz is 246.2 kHz * 2pi / 64 MHz which is

At, say, DBPSK 250kbps, oversampled by a factor of 2, that means we
decimate by 128 (500 ksamples per second), to then get at most ~3.094

With DBPSK this could flip all of our bits! Does that mean we in fact do
need to worry about the case where the complement of the access code is
detected? Is my understanding of the meaning of ppm error wrong? Does
the distribution we assume for the frequency offset make this
possibility negligible?

Thanks!

• -Dan
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On 5/1/07, Dan H. [email protected] wrote:

that the maximum frequency offset between the sender and receiver is
then 2 * 50 ppm * 2.462 GHz (802.11b Channel 11) = 246.2 kHz.

I think 50ppm is a total drift of the 64MHz oscillator, so no +/- just
50ppm being the max deviation between two crystals.

Brian

Dan H. wrote:

that the maximum frequency offset between the sender and receiver is
need to worry about the case where the complement of the access code is
detected? Is my understanding of the meaning of ppm error wrong? Does
the distribution we assume for the frequency offset make this
possibility negligible?

No it means you have to estimate the carrier frequency offset and remove
it, even in DBPSK. You do this by using the access code to find the
frequency offset and the differentially detect it.

Thanks!

• -Dan

Bob

AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
“If you’re going to be crazy, you have to get paid for it or
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