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Re: [Discuss-gnuradio] interpolation and output level

From: Javs
Subject: Re: [Discuss-gnuradio] interpolation and output level
Date: Fri, 18 Mar 2005 14:05:24 -0800 (PST)

Hello ppl,
I understand that the interpolation factor changes the amplitudes of the signals. The default usrp_siggen.py works fine and I can admire 16mV and 100 K on the scope. But if I use the default intrp of 64 in the example usrp_siggen.py and try to change the amplitude levels using -a option, the signals still come down. Any guesses why ?
Also, When I first boot my USRP and try an example, it fails for the first time. For instance, I tried tx_check.py and i had a runtime error. However, it works fine the second time. Is this because, it takes its time to load firmware into the FX2 and the FPGA or am I missing something here ?
Any help is appreciated :-)
>This is also why most systems like this only allow power of 2 ratios.

Quoting Matt Ettus <address@hidden>:

> Quoting Chuck Swiger <address@hidden>:
> > Hi - Just curious whats the mechanism behind this: using a usrp xmit
> > interpolation of 64 we get max signal level, and it drops off a lot if
> > changed, say to
> > 40 or 80. With interp=64, sample rate of 2000000, sig ampl of 6000, center
> > at 14000e3 and freq of 72e3 my output is 3v peak (external amps).
> > Change it to interp=80, sample rate of 1600000 it drops way down to .6v
> peak
> > (-14db). Try 50, 3200000 and get .5v peak out. Even stranger using
> interp=60
> > gets 2.7v peak but 68 it's down to .6v.
> This happens both with the interpolator and decimator.  It is due to a
> simplification I made to save space in the FPGA.  I will be fixing it in the
> near future.
> Basically, if you interp/decimate by a power of 2 you get full amplitude
> signals.  If you use a non-power of 2, you will get lower amplitudes.  Using
> a
> value just greater than a power of 2 will have the worst effect.  A value
> just
> less than a power of 2 should be less noticeable.  This is why 68 is so much
> lower than 60 in your example above.
> Matt

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