Quadrature Sampling with the TVRX

GNU Radio
1

Hi all,

I’m confused about how quadrature sampling is done with different
daughterboards. I noticed the daughterboard FAQ says the TVRX is “not a
quadrature board.”

It says:

“Because we’re using the digital up converter in the AD9862 in ‘Dual
Channel Complex DAC Data Mode’, both D/A outputs are related (I&Q)
and as a result, all TX daughterboards are ‘quadrature’ boards. On the
receive side we’ve got a bit more variation. The TV_RX board (for
example) is not a quadrature board, since it only uses a single A/D.”

I’ve looked at the mux value (0xF0F0F0F0) set by
determine_rx_mux_value() for the TVRX, and sure enough, the DDC Q inputs
are grounded. Still, it is my understanding complex mixing is done
digitally in the FPGA, which should yield perfectly valid quadrature
samples, right?

However, this document implies the TVRX has separate quadrature outputs,
I think. For example, on page 61, the mux value (0x----3210) is set in
accordance with the TVRX having I and Q outputs. Is this a sensible mux
value for two TVRXs?

I guess I am asking what the difference is between quadrature and
non-quadrature boards and if that has any impact on your ability to get
valid quadrature samples out. Is it that “quadrature boards” do the
complex mixing in hardware themselves on the board and send the I and Q
signals to separate ADCs, and the complex mixing isn’t done in the FPGA
in that case?

Thanks!

Thomas

2

On Wed, Feb 10, 2010 at 5:22 PM, Thomas [email protected] wrote:

Hi all,

I’m confused about how quadrature sampling is done with different
daughterboards. I noticed the daughterboard FAQ says the TVRX is “not a
quadrature board.”

[…]

I guess I am asking what the difference is between quadrature and
non-quadrature boards and if that has any impact on your ability to get
valid quadrature samples out. Is it that “quadrature boards” do the complex
mixing in hardware themselves on the board and send the I and Q signals to
separate ADCs, and the complex mixing isn’t done in the FPGA in that case?

The TVRX mixes from whatever frequency you’ve tuned to an IF of 20MHz.
The channel is 6 MHz wide, so your signal content is 17MHz to 23 MHz.
The ADC samples only one channel. The digital downconverter then
mixes the rest of the way to DC and produces quadrature outputs.

So it looks like a quadrature board from a software point of view, but
it’s really not a quadrature board, and you have no quadrature phase
error.

–Andy