Re: [Discuss-gnuradio] Sample Rate

[Eric Blossom]

On Mon, Jan 28, 2008 at 02:42:33PM -0600, Richard Jaeger wrote:
> On Jan 28, 2008, at 1:32 PM, Eric Blossom wrote:
>
>> On Mon, Jan 28, 2008 at 09:43:13AM -0600, Richard Jaeger wrote:
>>> I have been playing with a number of the examples - FM NB and WB 
>>> receivers,
>>> AM receiver, etc.
>>> The sample rate appears to default to 64 MSPS, but I have been unable to
>>> find where that default rate is set.
>>> Where is it defined?  Can it be changed from the examples themselves?
>>>
>>> For example, 64MPS is not a proper subsample rate for the FM broadcast
>>> band.
>>> It results in aliasing and should be reduced.
>>>
>>> Dick...
>>
>> It's not settable, but is determined by the master oscillator on the
>> USRP.   If you're using the Basic Rx or LF Rx daughterboards, yes you
>> will need some kind of preselector.  If you're using one of the RFX or
>> TVRX boards, the 64MS/s sample rate won't be a problem.
>>
>> Eric
>
>
> Eric,
>
> Thanks for the quick reply.

No problem.  Be sure to use the "Reply-to-all" command in your mailer
to ensure that both the sender and the list get a copy.

> So, then I guess the ADCs run at the maximum sample rate all the time.
> I thought I saw some other Python examples that set the sample rate to 
> 20MSPS. I may have misinterpreted what I was reading.  I'm new to Python.

There is some very old code kicking around that we used with the
Measurement Computing PCI-DAS 4020/12.  It was a 20 MS/s device.

> Once you've sampled the FM band at 64MSPS, I think it's too late to do 
> anything to solve the aliasing problem.
> The spectrum has already been folded.  One would need to use some form of 
> down conversion before the ADC's.

Yes, that's why I mentioned the TVRX and RFX-* daughterboards.
The Basic Rx and LF Rx are really designed to be used with external
converters, filters, etc.

> Is the oscillator running at 64 MHz, or is it divided down from a higher 
> frequency?    I guess can inject an external oscillator if needed.

It's running at 64 MHz.

> My primary application is going to be low frequency phased antenna
> arrays (Ham radio - 160M), so the high sample rate is ok (but not
> really needed.)  I can just decimate right away.

Good.

I recommend that you take a look at
gnuradio-examples/python/multi-antenna/*.py.
They use two Basic Rx daughterboards and 4 DDCs to generate 4 streams
of coherent samples -- perfect for beam forming, etc.

Eric