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Re: [Discuss-gnuradio] Receive Sensitivity and Transmit Leakage?

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Receive Sensitivity and Transmit Leakage?
Date: Tue, 02 Aug 2011 11:23:52 -0700
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On 08/02/2011 08:10 AM, Delgado, Christopher wrote:
> 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.

You didn't state what frequency you are using or what decimation, but
the noise figure should be under 6 dB, and that is the only true measure
of sensitivity.

You can always add more digital gain in the fpga, but I don't think you
need that.

Are you sure you are using the correct antenna input?  Also, the
grand-daughterboard is static sensitive.  In order to check to see if
yours is working, please put a known level sine wave in from a signal
generator, and send us a screenshot of the uhd_fft display along with
the settings you used.

> 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.

I am confused here.  Is this carrier leakage out of the transmit port,
or leakage of TX into the RX?


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