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Re: [Discuss-gnuradio] time to reconfigure a block in flowgraph

From: Josh Blum
Subject: Re: [Discuss-gnuradio] time to reconfigure a block in flowgraph
Date: Fri, 17 Dec 2010 12:16:32 -0800
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101208 Lightning/1.0b2 Thunderbird/3.1.7

On 12/17/2010 10:28 AM, Steve Mcmahon wrote:
> Hello Martin Braun:
> I'm not sure what you mean. There is no set_center_freq() method in
> gr_sig_source_c.
> When you say the best way to implement frequency hopping is to "use a
> larger bandwidth and do the tuning digitally", you mean to do the
> tuning in software in the Python flowgraph using Gnuradio blocks?

Typically with frequency hopping applications, the closer you tune to
the host the faster, but the more constrained is your bandwidth, and
therefore tuning capabilities:

1) Tune in software: fastest option but tuning is the most bandwidth
constrained (how much can you tune with (25Msps on USRP2)). This is as
easy as changing your filter taps or changing the freq on a sig source.

2) Tune in FPGA DSP: second fastest option. Latency is the RTT/2 +
cordic delay. However tuning is now constrained by the bandwidth of
100Msps w/ USRP2.

3) Tune in the RF daughterboard: the slowest option with the largest
tuning range.

Two notes:
*) In uhd you can tune the FPGA DSP without re-tuning the daughterboard
settings using the tune_request struct. I dont think the capability is
available in the older gnuradio+USRP2 driver.

*) You may be able to modify the tuner code of the WBX to respond
quicker by example: if you know you dont have to switch VCO selection,
the tuning code could be optimized for latency by only tuning part of
the RF chip.


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