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Re: [Discuss-gnuradio] How to minimize Spillage of energy in neighboring

From: abhinav narain
Subject: Re: [Discuss-gnuradio] How to minimize Spillage of energy in neighboring frequencies while transmission
Date: Wed, 21 Oct 2015 11:37:10 -0700

Hi Marcus,

On Wed, Oct 21, 2015 at 3:18 AM, Marcus Müller <address@hidden> wrote:
Hi Abhinav,

which USRP are you using?
I'm pretty sure none of our devices supports a 2kS/s sampling rate -- it's just far too low to get directly interpolated to a "usable" DAC/ADC rate. Especially those USRPs that can be equipped with RF frontends (daughterboards) that can operate at near-baseband (400kHz) shouldn't be able to work at such low rates. So: which daughterboard are you using?

I see your point, I am using Basic TX/RX daughterboard.
I didn't notice the shell output but it makes sense now :

Terminal output:
    The hardware does not support the requested TX sample rate:
    Target sample rate: 0.001000 MSps
    Actual sample rate: 0.195312 MSps
-- Tune Request: 0.400000 MHz
--   The RF LO does not support the requested frequency:
--     Requested LO Frequency: 0.400000 MHz
--     RF LO Result: 0.000000 MHz
--   Attempted to use the DSP to reach the requested frequency:
--     Desired DSP Frequency: 0.400000 MHz
--     DSP Result: 0.400000 MHz
--   Successfully tuned to 0.400000 MHz 

Basically, the minimum sampling rate is 196 Ksamples/sec

Hence, UHD should have told you that you can't use that sampling rate, and a higher sampling rate was automatically selected, inherently frequency-"stretching" the signal by the ratio of (actual rate/2kHz).

You should heed Sylvain's advice. Just oversample your signal by using an interpolating FIR filter to something that the USRP can happily work with.

So, here is what I am not clear about- I should use 196 KHz as sampling rate ? 
So, the minimum BW of my signal will 196/2 =98KHz, and that is the best I can do with this equipment, right ?

Best regards,

On 21.10.2015 08:49, abhinav narain wrote:
I am transmitting using the flowgraph: vector src-> FIR interpolator (with RRC filter as below) -> MultiplyConst -> USRP Block

FIR filter - firdes.root_raised_cosine(32, samp_rate, sps, 0.55, int(11*sps*32)), with
center freq= 400kHz 
samp_rate= 1k

I am listening using other USRP with
center freq= 400kHz
samp_rate= 2k 

You can see the spectrogram at the receiver. I want to do narrowband(1 kHz) transmission hence I am keeping the sampling freq at transmitter at 1k.

While I increase the signal amplitude(using MultiplyConst block with slider), I see there is more and more spillage of energy in the neighboring frequencies!

Is there a way to remove that spillage from the transmitter and have a cleaner transmitter ?
If not, how can I atleast mitigate it to the minimum ? is there something I can do with RRC filter parameter or any other way ?

In the figure: 1 - when amplitude is large; 2 - when amplitude is turned down


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