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 :-)
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