Does the “tune” function actually do anything for the BASIC_RX? Does it
just set the DDC
appropriately?
For example, if I have a quadrature HF receiver, centered at let’s say
20MHz, with 4MHz either
side of 20MHz, can I use “tune” and setting the decimation to pick out
a particular chunk.
Let’s say I wanted the 250KHz “chunk” that’s +500KHz from the center
frequency, would I set
the decimation to 256, and ask “tune” for 500KHz?
–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
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.
For example, if I have a quadrature HF receiver, centered at let’s say
20MHz, with 4MHz either
side of 20MHz, can I use “tune” and setting the decimation to pick out
a particular chunk.
yes
Let’s say I wanted the 250KHz “chunk” that’s +500KHz from the center
frequency, would I set
the decimation to 256, and ask “tune” for 500KHz?
Assuming center_freq+500KHz is the center of the “chunk”, that is
center_freq+375khz to center_freq+625khz: you would do
tune(center_freq+500KHz) and the tune method figures out what the ddc
should be set to.
-Josh
On 03/15/2010 07:22 PM, Josh B. wrote:
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
–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
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