[USRP-users] Oversampling and Automatic Gain Control

Marcus Müller marcus.mueller at ettus.com
Sat Mar 5 10:10:14 EST 2016


Zhihong,

On 05.03.2016 16:07, Zhihong Luo wrote:
> Marcus,
>
> So, if I understand it right, set_rx_bandwidth has no effects on
> setting the bandwidth of front end for X300, and in order to set the
> bandwidth, I have to apply extra low-pass filter by myself?
Yes.
> If I want to insert a DSP CE module in fpga using RFNoC, I have to
> insert a filter CE between radio and it, maybe the fir_filter?
Exactly!
>
> Ok, I'll reduce the power to see if it is the problem. So -15dBm is
> roughly the maximum to prevent damage and malfunctioning?
Damage. "Malfunctioning", e.g. strong intermodulation due to driving the
RX amplifiers out of their linear range, might happen earlier. This
depends on a frequency and gain; see the performance data measurements
of the SBX board on [1] for details.

Best regards,
Marcus

[1]
http://files.ettus.com/performance_data/sbx/SBX-without-UHD-corrections.pdf
>
> Thanks,
> Zhihong
>
> On Sat, Mar 5, 2016 at 10:55 PM, Marcus Müller
> <marcus.mueller at ettus.com <mailto:marcus.mueller at ettus.com>> wrote:
>
>     Dear Zhihong,
>
>     On 05.03.2016 15:26, Zhihong Luo wrote:
>>     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?
>     As mentioned, there's devices that support adjusting the analog
>     bandwidth of the frontend. So that function is relevant for them!
>>
>>     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.
>     How high is the signal power? Please make sure to stay
>     sufficiently away from -15dBm, or you could damage the SBX!
>     This sounds like your pulse might be so strong that it has bad
>     effects on the receiver amplifiers or mixers. But that's just a
>     guess; could you reduce the pulse power by 10dB, 20dB, 30dB and
>     see if the behaviour stays the same?
>
>     Best regards,
>     Marcus
>
>>
>>     Thanks a lot :)
>>
>>     Zhihong
>>
>>     On Sat, Mar 5, 2016 at 8:26 PM, Marcus Müller
>>     <usrp-users at lists.ettus.com <mailto: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
>>>
>>>
>>>         _______________________________________________
>>>         USRP-users mailing list
>>>         USRP-users at lists.ettus.com <mailto:USRP-users at lists.ettus.com>
>>>         http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>>
>>         _______________________________________________
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>>
>>
>
>

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