[USRP-users] Oversampling and Automatic Gain Control

Zhihong Luo zhluo at umich.edu
Sat Mar 5 09:26:42 EST 2016


Hi Marcus,

I am using SBX120.

Oh, that's surprising to me.  Does that mean once we set the sample rate,
the bandwidth is automatically half of the sample rate? But if so, what is
the point of the "set-rx-bandwidth" function?

Just google the term non-line-of-sight-only, I don't think that is the
problem, because we connect the transmitter with USRP directly with a
cable. Moreover, if the transmitter sends continuous sinusoidal signal, the
received data look alright.

Thanks a lot :)

Zhihong

On Sat, Mar 5, 2016 at 8:26 PM, Marcus Müller <usrp-users at lists.ettus.com>
wrote:

> 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
>
>
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>
>
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