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Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements

From: Gayathri Ramasubramanian
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Fri, 15 Aug 2014 17:08:53 -0700


Thank you for your note.

I am trying to get the power of received signal using the blocks you had suggested previously.
I have attached a flow graph snapshot ('uhd_fft_DK.grc.png").

I am using the UHD:USRP Source to receive the signal which in turn sends the signal in the foll path
"complex to Mag^2" block --> "integrate and decimate" block -->  "10 log +k"  block---> "signal probe" block.

However there are errors mainly referring to the in-compatible I/O sizes of source (complex to Mag ^2 block) and sink (Integrate and Decimate block).

The I/O size of "Complex_Mag^2" block is said to be 4096, while the I/p size of i"ntegrate and decimate" block is 4. ==> error as not compatible.
Should I change the Vector length? I am not quite sure?
The error propagates to other blocks if I change anything.

Kindly help solve this.

I have also attached another flowgraph (flowgraph screenshot.png) which works but it doesn't have the "integrate and Decimate" block or the Log10+K block but it works and receives the signal (developed by someone else previously in our lab). Now since the "Log10+K" block is missing, does it mean the output is given in mwatts instead of dBm? 
I cant seem to use this version on as the "probe_signal_vector" block does not seem to be supported in GRC which is the version in my system.

I tried using simple probe_signal block instead but again faced the I/o size incompatibility error?  Is there a work around for this "probe signal vector" block?

Also could you explain briefly how to approach and solve there I/o size errors.

Look forward to your response


On Mon, Aug 11, 2014 at 6:30 AM, madengr <address@hidden> wrote:
You need to be careful with the WBX as input power above -20 dBm could damage
the 2nd stage RX amplifier.  Per the data sheets, the absolute maximum input
power of the 2nd stage amp is below the saturated output power of the 1st
stage amp.

1a) Yes, only valid for linear range.
1b) The device is always going to generate IMD, so it really depends on how
much error you can tolerate.  The IMD3 grow at 3x the rate of the main
tones, so for a slow gain compression, the error added to the IMD levels
won't be much compared with how fast they grow.  On my plots, -30 dBc is
where the single tone power (visually) starts to deviate from linear.

2) Negative of the difference.  The IMD are always going to be negative dBc,
where the "c" means referenced to the carrier.

3) Optional, and tied to the daughterboard, specifically analog imparments
in the I/Q sections.  I don't know the specifics; maybe ask on the USRP
list.  I think each correction file is serialized by the daughterboard,
which make sense if you are running several USRP from one host.


Gayathri Ramasubramanian wrote
> Thanks for doing the TX side one tone and two tone tests for USRPN210. It
> helps me get a reference.
> 1) When I did the test for finding out the calibration factor, I found it
> was a constant value ntil a certain range and after that it dipped (e.g
> ard
> -8 dBm input power @ 400 Mhz). I image this is point near the saturation
> /compression region of the device. This was observed on 5 USRPN210+WBX
> devices that I tested.
> a) So I understand that this calibration factor is valid only in the
> linear
> range. Is it correct?
> b) if so, I should not use the calibration factor to turn the IMD
> products'
> value on FFT to dBm using this factor as they occur only when the device
> is
> operating in the non linear range.
> Please let me know if my understanding is right.
> 2) You told me to plot the IMD plot with difference of power between
> fundamental tone and the IMD product power @ harmonc freq. i.e  [power @
> F1
> - power @ (2F1-F2)] . But this is coming as a +ve value for me while your
> plot shows the Y axis on a negative scale. Would this also be might due to
> the attenuator you are using? or should I consider the negative of the
> difference?
> 2) Also you have told to run the UHD calibration routines should be run
> for
> the N210+WBX:
>    - uhd_cal_rx_iq_balance
>    - uhd_cal_tx_dc_offset
>    - uhd_cal_tx_iq_balance
> Are these mandatory or optional? Do they affect the daughter board or
> mother board of the device? The devices I use are for common use in a lab
> and hence it is preferable not to change any of the configurations for
> specific use for long term. Hence my question.
> As per the information given at
> http://files.ettus.com/manual/page_calibration.html#calibration_ it seems
> that it more tending towards the WBX board operation. Also the factors get
> stored in the machine they are connected to. So in case we want to run the
> USRPN210 device with old configuration, it would be ok to just delete the
> files or run it from some other machine? Is this correct?
> Can any random number be specified for a serial number if not detected
> from
> the device or any specific format is to be used ( i.e min number of digits
> & alphabets etc)
> Kindly clarify the above points.
> Thanks again
> Regards
> Gayathri
> __________
> Discuss-gnuradio mailing list

> Discuss-gnuradio@

> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

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