[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
hardware.

> 
> 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.

-Josh




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