Re: [Discuss-gnuradio] High pass frequency response with LFRX boards

[Eric Brombaugh]

On 11/04/2010 04:24 PM, Drew Read wrote:
> It seems that there is a very significant roll-off at lower
> frequencies on the receive side which pretty much makes it unusable
> for our baseband signals.
> The aformentioned forum post suggests that it is not caused by the
> LFRX board and so is likely to be something in the fpga...
>
> Just to reiterate the questions from this forum post.
> 1) What on the FPGA is causing this frequency response? (and what can
> we do about it?)
>
> 2) Why does this happen only in the RX path?

You're probably seeing the high-pass behavior of the RX DC offset 
removal. There's a logical block inside the FPGA which integrates the 
ADC DC offset and subtracts it from the incoming signal.

This function isn't needed on the TX side.

Eric