[USRP-users] SBX daughtercard bandwidth problem..!!

Josh Blum josh at ettus.com
Wed Feb 15 01:58:54 EST 2012

On 02/14/2012 09:16 PM, salman dinani wrote:
> Hi,
> Thanks for your answers, what I have understood from all that discussion is
> that :
> 1. I cannot set the decimation or interpolation factor below "4" in
> Simulink's  SDRu block. It's a     limitation. Am I right in this
> conclusion??
> For elaborating and explaining my problem in a much better and clear way,
> I have attached Snap shots of a very simple SDRu Transmit project in which
> a sine wave block is connected to SDRu Transmit block (see *Sine_prj.png*).
> The frequency of the sine wave is set to be 10MHZ , the configuration of
> sine block is shown in pic *"sine_block.png"*. The centre frequency is set
> to be "1 GHz" and the interpolation is set to "4", SDRu block setting is
> shown in pic "*SDRu_block.png"*.  The output is taken on a Spectrum
> analyzer and the snap shot of that is shown in *"Spectrum_anylzr.png"*. It
> clearly shows two peaks ,10MHZ apart from centre frequency.
> Sir, the problem starts from here that this all process works well up to 12
> MHZ. But when I increased the sine wave frequency above  12MHZ (that's why
> I was emphasizing on word 12 MHZ in my last email), let say 15MHZ or 20MHZ,
> the output doesn't show peaks at 15 or 20MHZ apart from centre frequency,
> In fact the peaks starts moving back as I increase the frequency of sine
> wave and at 25MHZ, both peaks meet each other on zero on spectrum analyzer.

You may not realize it, but you are describing aliasing.
Please consider learning about some fundamentals:

* Nyquist frequency and aliasing
* Complex baseband representation
* Euler's formula

After hitting the books (or the wikipedia), you can use Simulink or
gnuradio-companion to experiment with the concepts of sampling theorem
entirely in software. Then, when it all makes sense, then you go to the

> I hope sir I have explained my problem better than previous. The DAC in
> USRP N200 is of 400MSPS and despite of interpolation is set to "4", the
> sampling rate becomes 400e6/4= 100MSPS, and there is also complex sampling
> involve. Obviously I won't able to get 100MSPS because of the reasons as
> You have mentioned in your earlier email, but I am not able to get even
> 25MHZ of output. I don't know why it's happening like that. I have checked
> and varied all the parameters but the response remains the same.
>  Please help me :
> 1. what is going wrong? or where I am wrong that I am not able to get even
> 25 MHZ of output?
> 2  Is this because of any kind of limitation in software or hardware?

Everything looks fine. According to the spectrum analyzer screen shot,
your signal is occupying 20 MHz of bandwidth centered at 1 GHz. This is
a successful use of 25 MHz of bandwidth from the PC.


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