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

From: Eric Brombaugh
Subject: Re: [Discuss-gnuradio] High pass frequency response with LFRX boards
Date: Thu, 04 Nov 2010 16:47:57 -0700
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101027 Fedora/3.1.6-1.fc13 Thunderbird/3.1.6

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.


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