Re: [Discuss-gnuradio] Multiplexing modulators

[Nick Foster]

On Tue, 2011-06-21 at 15:54 -0400, John Ackermann N8UR wrote:
> Thanks, all, for the replies.
> 
> For my simple test case, I'm trying just two modulators with a frequency 
> translating FIR following each, one set to 0 Hz and the other to +25kHz.
> 
> I set the NBFM modulator quadrature outputs to 128ksps, then add them 
> together and feed to the USRP at 256ksps.  (At this point, I'm not 
> worrying about gain setting; I know I need to address that but wanted to 
> get the basic flow correct first.)
> 
> When I run the program I get this error:
> 
> UHD Warning:
>      The hardware does not support the requested TX sample rate:
>      Target sample rate: 0.256000 MSps
>      Actual sample rate: 0.255102 MSps
> 
> Since I'm summing two 128ksps streams, I thought I'd get a 256ksps 
> stream at the output of the adder.  Where are these 898 samples per 
> second disappearing?  And how do I correct that?  (The single channel 
> version of the program, with the NBFM quadrature rate set directly to 
> 256ksps, worked fine.)

Couple of things. First of all, when you add two signals together, you
don't add the sample rates. Two 256ksps streams added together still
result in a single 256ksps stream. Second, 256ksps is not a factor of
100Msps, so the USRP2 picks the closest available valid rate, which is
100e6/392=255.102ksps. You might have better luck using a sample rate
which is natively supported. Try 250ksps.

--n

> 
> Thanks,
> 
> John
> ----
> Marcus D. Leech said the following on 06/21/2011 12:33 PM:
> > On 21/06/2011 12:05 PM, Alexandru Csete wrote
> >>
> >>
> >>
> >> Sorry, I meant multiply with a complex sine or cosine wave of
> >> frequency f, where f is the offset.
> >>
> >> Marcus suggested using filters, which will also work but it's a bit
> >> overkill for a simple translation, isn't it?
> >>
> >> Alex
> > Yes, it's overkill, but I thought it might be the simplest "form" to get
> > working first.
> >
> >
> >
> >
> >
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