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[Discuss-gnuradio] Receive Sensitivity and Transmit Leakage?

From: Delgado, Christopher
Subject: [Discuss-gnuradio] Receive Sensitivity and Transmit Leakage?
Date: Tue, 2 Aug 2011 11:10:58 -0400

Have some questions regarding received signal levels and what seems like LO leakage on transmit side.
We are using N210 with WBX (UHD driver). This issue seems to be pretty independent of carrier frequency.

1) Receive sensitivity - We're cabling in a CDMA2000 carrier (1.2288 MHz bandwidth) from a femtocell
base station we have. The transmit power is about -80 dBm. We've done a simple test with GRC to
receive this signal, write to a file and do PN correlations in Matlab to verify the integrity of the
data. With the signal level of -80 dBm and the rx_gain in the GRC gui set to maximum (38 dBm) the
I/Q data seems to be about 3 bit values (about +/- 4 or 5). These values seem a bit low
for this signal level. On other SDR receivers we use (not usrp) with max gain, signals over the air from base stations
are about -90-95 dBm and toggle about 6 bits on ADC.
In short, I'm wondering if this is expected behavior or if there is another analog gain setting to change on receive side.

2) Transmit leakage
We've set up a simple program in GRC to take the received IQ from the same base station and retransmit them on a different frequency.
The receive is in the 800MHz band, and we've tried transmit on both 500MHz and 1GHz. Looking at the output on a spectrum
analyzer as well as just plotting IQ in Matlab, there is some strong carrier leakage, in fact the carrier is dominant over the
signal for all practical ranges of signal level at the input. With the input at -30 dBm and the rx_gain set to max 38 dBm
the signal begins to finally overtake the carrier however this signal level is obviously not practical. Furthermore, the gain parameter in GRC's UHD: USRP Sink block seems to do nothing based on observing live spectrum of the transmit signal.
The DC offset of the receive data seems to be negligible - we're writing it to a file before it goes out.
Wondering what could be causing such strong LO leakage.

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