USRP Daughterboard bandwidth

GNU Radio
1

Hi guys,
I have two questions about the USRP’s architecture that I have been
unable to answer from looking around.

  1. I understand that when a daughter board is tuned for Rx, the exact
    frequency is not always possible, so the receiver tunes to the nearest
    possible frequency and the ‘requested’ frequency is the  sythesized in
    the DDC. Coming from a conventional RF background, I don’t understand
    why the receivers can only tune to specific frequencies - is it
    something like resolution in the digital control of the VCO’s in the
    receiver? (Just my guess)
  2. What is the raw bandwidth of the output of a daughterboard? The USRP2
    can process a 25MHz wide chunk so I’m guessing its at least as big as
    that.
    Cheers,
    Matt
2

On 06/25/2010 01:31 AM, Matt R. wrote:

something like resolution in the digital control of the VCO’s in the
receiver? (Just my guess)

PLL synthesizers often have a certain minimum resolution, or “step
size”. It varies by specific
daughter-card. That’s a very, very, very conventional “problem” to
run into. In many communications
systems, there’s a standard “raster” that systems are set up on. In
television, for example, channels
are 6MHz wide, so it’s no unusual to find 6MHz or 3MHz as a minimum
“resolution” for the
synthesizer. In amateur radio systems in VHF and above, channels are
often 25KHz wide, so you’ll
find synthesizers with 25KHz or 12.5KHz minimum step-size in the
synthesizer.

The downconverter in the DBS_RX for example is designed for DBS (Direct
Broadcast Satellite)
applications, where 1MHz and wider channel spacings are the norm, so
the resolution of the PLL is
quite “coarse” compared to what you might find in a HF synthesizer for
example.

  1. What is the raw bandwidth of the output of a daughterboard? The
    USRP2 can process a 25MHz wide chunk so I’m guessing its at least as
    big as that.

Cheers,
Matt

Again, varies by daughter-card. The TV_RX, for example, has a 6MHz-wide
IF filter, so you’ll never get
more than 6MHz of useful bandwidth out of it. The DBS_RX has a
programmable baseband filter that
goes anywhere from a coupla of MHz to 30MHz, depending on programming.