Re: [Discuss-gnuradio] Calling "tune" on BASIC_RX

[Josh Blum]

The DDC can shift the IF signal in either direction. So yes, it must 
deal with negative frequencies and you should be able to pass the tune 
function a negative frequency in Hz.

Also, you can simulate the action of the ddc in grc with a complex sine 
source and a complex multiply. Watch the fft.

-Josh

On 03/15/2010 04:32 PM, Marcus D. Leech wrote:
> On 03/15/2010 07:22 PM, Josh Blum wrote:
> > On 03/15/2010 04:09 PM, Marcus D. Leech wrote:
> > > Does the "tune" function actually do anything for the BASIC_RX? Does it
> > > just set the DDC
> > >     appropriately?
> >
> > Tune on the basic and lf rx only sets the ddc. In other words, the IF
> > of the dboard is always 0 Hz in this case.
> >
> > Because this tuning relies entirely on the ddc, you are really
> > aliasing in frequencies above ddc_rate/2. Because of this, your RF
> > frontend should need a bandpass filter so only the desired range can
> > be aliased in.
> >
> > The tune method should do the right thing.
> Well, the front-end converts everything to complex baseband, so in this
> case, do I just hand "tune"
>    the offset frequency?  That is, does it "deal" with negative frequencies?
>
> We'll go back to the example.  Complex quadrature direct-conversion
> receiver, giving "I" and "Q"
>    from DC-4MHz to DC+4MHz.    That is, it converts everything to
> baseband.  Do I tune
>    frequencies below DC, using negative values to the tune function?
>
> I know this seems rather rudimentary, but please have patience for me :-)