Re: [Discuss-gnuradio] How can we measure the frequency offsets between two USRPs?

[John Andrews]

I figured it out. The USRP input must have an input amplitude between 0 and 32767. When the amplitude was low the signal power was below noise so I couldn't see anything in the FFT. When I raised it to 20000 I was able to see the peak around ( signal frequency +/-  offset ). 

Marcus about your statements about the SAW filter; I will know a little more about it first from the mail archive and then ask more about it. I don't know anything about it yet so before I ask you something more about it I want to do a little homework myself.

Thanks

On Mon, May 16, 2011 at 5:31 PM, Marcus D. Leech <address@hidden> wrote:

Let me give you the details this time. sorry for the confusion.

USRP - The first version

Daughterboard - RFX2400

The signal source has the following settings.

1. Sample Rate - 1Msps

2. Waveform - Sine & Complex

3. Frequency - 100kHz

4. Amplitude - 10

5. Offset - 0

This source is connected to the USRP Sink directly. There is no other block in between and not even a throttle.

USRP sink settings:

1. Interpolation - 128

2. Frequency - 2.5GHz

3. Gain (dB) - 30

4. Side - A (Tx/Rx) The USRP is connected fine and works well with benchmark_tx/rx tests.

On the receiver side I use a GRC flowgraph with USRP sink and FFT block only.

USRP Sink:

1. Decimation - 64

2. Frequency - 2.5GHz

3. Gain (dB) - 10

FFT sink has proper settings too.

I am not using any filters as I hope the SNR is high enough.

Thanks

On Mon, May 16, 2011 at 3:47 PM, Marcus D. Leech <address@hidden> wrote:

On Mon, 2011-05-16 at 15:34 -0500, John Andrews wrote:

I am running it at 2.5GHz.

What magnitude are the samples you're feeding into the USRP sink?

--n

Also, by default the RFX2400 has a TX filter that's centered at 2.441GHz and about 85Mhz wide at the 3dB points.  So, there'll be some attenuation
 at 2.5GHz, roughly 6-8dB or so.

It would be useful to see your .grc flow-graph files.  Are you using UHD, or "classic" (sorry if this has already been asked).

I'll reiterate my earlier comments.  The RFX2400, by default, has a *hardware* SAW filter with a center frequency of 2.414GHz, and a nominal
  bandwidth of 85MHz or so in the TX/RX path, so if you're running it at 2.5GHz, you're somewhere on the skirts of the filter, which would lead
  to attenuated output.  This filter can be bypassed easily by carefully cutting the traces going to FIL1, and populating C204 with a 30-100pF
  SMD capacitor.

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