[USRP-users] Oversampling and Automatic Gain Control

Marcus Müller marcus.mueller at ettus.com
Sat Mar 5 07:26:16 EST 2016


Dear Zhihong Luo,

Which daughterboard are you using?

> The first question is that, since I want to do oversampling with a
> small bandwidth to filter out the noises, I want to make sure that
> this can be done by setting the bandwidth manually, and the sample
> rate won't affect the bandwidth.
That's impossible. The decimators are in the same functional unit as the
low-pass anti-aliasing filter. So, sampling rate sets the filter
bandwidth. That's not really a problem, if you have a high
decimation==high oversampling: The sample rate will then be low enough
to do an extra low-pass filter in software on your PC. For example[1],
you can expect about 80MS/s filtering throughput with a multithreaded
real-tapped FFT 1/10-band low pass filter on complex data with a
somewhat normal PC (i7 CPU, DDR3 RAM).

> Secondly, the magnitude of my received data starts at a high value,
> then drops and remains almost constant at a very low level, which
> makes it difficult to tell whether there are any pulses. Someone told
> me that it might be due to the automatic gain control (AGC). I didn't
> know much about it, does X300 have this function? Can I somehow turn
> it off?
AGC is an analog thing, so it can only be implemented on the
daughterboard. However, the only modular daughterboard that has AGC (and
on that, it's always on) is the TVRX/TVRX2¹. And the TVRX2 is really not
what you should be using with the X310 to detect pulses (I'm not even
sure TVRX works with the X310).

So, no, this is not an AGC. If I had a blind guess, this sounds like you
have some non-line-of-sight-only channel?

Best regards,
Marcus

¹ Non-modular devices like the B2xx or the E3xx can have ADC, but that's
a different story.
[1]
http://stackoverflow.com/questions/32916536/is-there-a-constant-time-algorithm-for-generating-a-bandlimited-sawtooth/32931019#32931019
On 05.03.2016 07:17, Zhihong Luo via USRP-users wrote:
> Hi all,
>
> I am using USRP x300, and I have two pretty basic questions after
> trying to receive  a sparsely distributed pulse signal. For example,
> the signal will be high-frequency sinusoidal for 1 us, then remains
> constant low for 28 us, repeatedly. 
>
> The first question is that, since I want to do oversampling with a
> small bandwidth to filter out the noises, I want to make sure that
> this can be done by setting the bandwidth manually, and the sample
> rate won't affect the bandwidth.
>
> Secondly, the magnitude of my received data starts at a high value,
> then drops and remains almost constant at a very low level, which
> makes it difficult to tell whether there are any pulses. Someone told
> me that it might be due to the automatic gain control (AGC). I didn't
> know much about it, does X300 have this function? Can I somehow turn
> it off?
>
> Thanks in advance for any help :) 
>
> Zhihong Luo
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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