Re: [Discuss-gnuradio] [USRP-users] How to correct for the drift in an (

Hi Marcus,

Thanks for your response.

>>What is the magnitude of the frequency drift?
>>What is the magnitude of the gain drift?

Please see description below (and Figures e-1 and e-2).

What are you measuring backscatter *from*?

Sorry, I meant to say from loop-backing the Tx to Rx (without any antennas involved).

Description of the test:

1- The test I did takes the Rx signal from direct loopback of the Tx (LFMCW) chirp.

2- Phase and magnitude offsets of the Rx signal relevant to the Tx chirp are calibrated out from a prerecorded training signal.

3- The calibrated Rx signal is mixed (i.e., dechirped) through complex-conjugate multiplication with the digital Tx chirp.

4- The mixed signal is decimated to 625 Hz.

5- The decimated signal is resolved to individual sweeps.

6- This test was run for around three hours. Hence, in total, I have 10838 sweeps with 490 samples in each sweep.

7- The LFMCW chirp parameters are: upsweep, 0.98 peak, 100 KHz, 0.784 s sweep time.

8- Attached are the following figures:

a- Magnitude of successive sweeps (superimposed).

b- Instantaneous phase of successive sweeps (superimposed).

c- A plot for the mag and phase of a complex-valued sweep divided by itself:

i.e., a_r * exp (j phi_r) = a_s exp (j phi_s)/ a_s exp (j phi_s)

Ideally, this should give magnitude a_r = 1 and phi_r =0.

However, as shown in the attachment, phi_r is not perfectly zero due to the floating-point precision (which is fine!).

d- Similar to 'c' (i.e., relative mag and phase) but for two neighboring sweeps (three superimposed cases are depicted).

e- Similar to 'c' (i.e., relative magnitude and phase) between the last sweep (i.e., sweep 10838) and the 10th sweep. The first 9 sweeps are skipped just to allows the N200 to settle down.

For cases (d) and (e), ideally, one would expect to see the relative mag and phase in the vicinity of (c). However, this is not the case as shown. Also, the tiny drifts in both magnitude and phase are increasing with time as shown in (e).

Back to my original question, what should I do to correct for this?

Thanks in advance.

Best,
Khalid

On Fri, Nov 28, 2014 at 12:08 PM, <address@hidden> wrote:

What is the magnitude of the frequency drift?

What is the magnitude of the gain drift?

What are you measuring backscatter *from*?

On 2014-11-28 10:14, khalid.el-darymli via USRP-users wrote:
Hi,

Given a set of synced (i.e., using external GPS REF/PPS), time-commanded and calibrated (i.e., through compensating for the phase/mag offset between digital Tx chirp prior to transmission and ADC'ed Rx signals) N200 devices with LFTX/LFRX daughterboards, that work with coherent LFMCW chirps, I am still seeing a tiny drift (both in the magnitude and frequency) of the calibrated back-scatter Rx chirp received at time t1 when compared to an Rx chirp received at an earlier time t0.

The more the N200 device runs (e.g., 5 hours), the greater the drift is. Obviously, this drift is pertinent to both the DAC and ADCs and the GPS referenced clocks of the N200 devices.

My questions are:

1- Why I still see such drift although my devices are synced with an external GPS? and how do I correct for it?

2- Can the PLL Carrier Tracking block in GRC be used to track and correct for such a drift? If so, how do I set the max/min freq inputs for this block?

3- Can AGC2 or AGC3 block be useful in this regard? If so, are there any examples to explain how the input parameters of these blocks can be set up?

Thanks.

Best regards,

Khalid
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From:	khalid.el-darymli
Subject:	Re: [Discuss-gnuradio] [USRP-users] How to correct for the drift in an (FMCW) Rx signal?
Date:	Fri, 28 Nov 2014 17:11:25 -0330