[USRP-users] B205mini: 0-value Samples When Switching Center Frequency

mleech at ripnet.com mleech at ripnet.com
Fri Oct 27 13:38:54 EDT 2017


So, I wrote the following script for testing this morning: 

http://www.ccera.ca/files/tuning_test.grc 

It does random frequency hopping with 1MHz offsets, with a base
frequency of 2.4GHz. 

It runs for 60 seconds and then just dumps complex I/Q data to a file. 

I then wrote a quick C program to do run-length analysis, and indeed,
there are long runs of exact zeros after a tuning event, averaging about
4.2ms with the above parameters. 

I made no attempt to use any of the fancy-tuning options available via a
"tune_request_t".  

What I do know is that the AD9361 isn't ideal for frequency-hopping
applications.  A lot of "stuff" needs to happen every time it is tuned. 
My recollection is that tune times used to be in the range of 50-100msec
for this part, but that an optimization was put in place to reduce this
for "short hops". 

On 2017-10-27 01:23, Marcus D. Leech via USRP-users wrote:

> On 10/27/2017 12:44 AM, Gilad Beeri (ApolloShield) wrote: 
> 
>> Thanks Marcus. 
>> 
>> I was told by Ettus in the past that nearby hops (X00's of MHz) should take some tens of microseconds to settle (for the AD936x chip), while I see 3 milliseconds. 
>> Can you reconfirm the former saying, or confirm that you also experience ~3 milliseconds of 0's when you change frequency with a delta of tens of MHz? 
>> 
>> In addition, I would expect that during the settling time, I will get garbage data / noise, and not 0 samples. Do you know the reason I get 0's?
> My *guess* is that this is the AD9361 doing a "mute during tuning" thing.  I don't know whether that's a feature that can be disabled, and whether at
> some point in the code history, it was enabled.   A cursory inspection of the code doesn't reveal anything obvious.
> 
> When you experience your zero-valued samples, is it a sharp cutoff, or is there a ramp into, and out-of the zero-valued-samples state?
> 
> Are you using manual or automatic gain control?
> 
> On Thu, Oct 26, 2017 at 5:27 PM Marcus D. Leech via USRP-users <usrp-users at lists.ettus.com> wrote: 
> 
> On 10/26/2017 06:43 AM, Gilad Beeri (ApolloShield) via USRP-users wrote: 
> Hi, 
> Whenever I switch center frequency with a USRP B205mini (changing to a nearby frequency), I get 3 milliseconds of 0 samples. 
> 
> * What's the reason behind that?
> * Can I make some changes that will reduce that timespan without data?
> 
> The analog hardware takes a finite amount of time to switch frequencies.  During that time, there will inevitably be *some* type of glitch.
> 
> A PLL synthesizer will take some amount of time to change frequency.  Every time you change frequency, you take it out of its converged state, and
> it has to re-converge.
> 
> https://electronics.stackexchange.com/questions/76197/pll-loop-bandwidth-lock-time-and-jitter
> 
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