Hi Dan,
given you want to sweep within one TX bandwidth (which is the complex
sampling rate), something like the following flow graph fed into a USRP
sink tuned to the right center frequency:
Sweeping FG
If you want your sweep to span more than the sampling rate, things get
more complicated.
Basically, you'd have to retune. However, retuning the local oscillator
used to mix up your baseband signal, no matter how it's done, is "slow"
compared to a single sample interval. You will inevitably get some gaps
or other irregularities.
The USRPs typically have two sampling rates that are relevant to the
user: the sampling rate at which you feed in samples from your PC, and
the master clock rate, which boils down to the rate at which the DAC/ADC
runs at. Imagine the MCR being at least as large as the analog (blue)
bandwidth $b_\text{analog}$ below:
bandwidths in digital tuning
Within the bandwidth defined by the MCR (and analog anti-imaging
filters), you can tune very quickly – but for the B210, in the single
channel case, it's 61.44 MHz and in the dual channel case 56 MHz, so
you're not winning Gigahertzes of "fast tunable range". Bonus is that
this digital offset tuning can be done with timed commands, i.e. you can
send a message to the USRP sink block that tells the USRP to tune
*exactly* at this and that sample time.
Unlike other devices, the B2xx can't currently tune the LO frequency
$f_\text{RF}$ with a timed command.
Best regards,
Marcus
On 29.04.2016 00:16, Dan McKenna wrote:
> Hi There,
>
> I am using a B210 to record band widths from 1 to 20 MHZ, center
> frequencies from 400Mhz to 2 Ghz.
> To test my post detection software I would like to use a Tx channel to
> make a sweep generator as a real time test source
> at sweep rates of around 50 Mhz/second over 1 to 20 mhz range with a
> center frequency of 400 Mhz to 2Ghz.
>
> To be clear I wish to use companion to generate a frequency sweep at a
> rate, defined by start/stop frequency and sweep time
> or frequency, range, rate..... for example Freq 400 Mhz, range 2 mhz,
> rate 50Mhz/second.
>
> Is this possible with the standard gnu radio companion block set?
>
> Thanks
>
> Dan
>
>
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