Calling "tune" on BASIC_RX

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
http://www.sbrac.org

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
:slight_smile:


Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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

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