Re: [Discuss-gnuradio] PLL Problem

[Matt Ettus]

David Scaperoth wrote:
> I have been testing the capabilities of the pll blocks to hopefully be
> used for a coherent receiver.
> 
> I have been using the gr.pll_ref_out(...) to lock a low IF signal.
> 
> What I have discovered is that the pll seems to be overdriving the
> processor (2.4GHz P4) when its used at high sampling rates (200kS/sec or
> above).  Is this something you have encountered??

It was used without problem for wideband FM reception on a Pentium M
laptop.  I don't recall the sample rate, but I think it must have been
640 ks/s.

Are you using a P4 or a P4 celeron?  The celeron versions perform very
poorly because of the tiny caches.

> The destination block also seems to affect the processing burden.  When
> using the audio card sink as a destination block (24kS/sec) the pll
> seems to work fine for a sinusoidal tone, but when I replace the audio
> card sink with null sink (same sampling rate) or the oscope gui, the
> processor goes crazy.   I may be setting this up wrong, but just the
> same I wanted to see if you have encountered this before.

Are the samples coming from a USRP?  If the chain has nothing pacing it,
like a USRP or an audio card, then you will use 100% cpu no matter how
simple your processing.

> I need this to work at high sampling rates like 2MS/sec so that I can
> detect larger bandwidth signals, but right now I'm no sure.

No work has been done with optimizing that PLL block, so I'm sure you
could speed it up quite a bit.

Matt