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Re: [Discuss-gnuradio] DC component and daughter-boards

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] DC component and daughter-boards
Date: Thu, 28 Apr 2005 10:48:33 -0700
User-agent: Mozilla Thunderbird 1.0 (X11/20041206)

Damien B. wrote:

> Matt in a previous thread you were talking about making d/b that let
> DC component pass:
>>If there is enough interest, I might make such a set of boards.  If someone 
>>is interested in the design, the BasicRX/TX are a great starting point, and I
>>can offer guidance.

I have seen very little interest in that.

> When i'm inspecting the my FIR output, it's really difficult to
> measure a 30ns (one sample) delay if the signal is not clean.

This is unrelated to the DC component issue.

> Would it
> be possible to make a quick but not perfect hack of the basicTX and
> RX, something like:
>    - removing transformer
>    - Short-cut pin 3 to 6 and 1 to 4
> the resulting impedance would be 40 Ohms (50 // 50+200+50), but is it
> really bad for low freq square wave < 2MHz ?

Sort of.  You would remove the transformer first.  Then you have 2
options --

1>  Feed your signal to the + input, and send the reference voltage
(Vdd/2, 1.65V, available on the board) to the - input.  The problem here
is that the ADC expects the signals to be +/- 1V from the reference,
1.65V.  Your signal will need to be properly biased.

The better option is:

2>  Put an AD8132 or similar differential amplifier in the circuit.  You
can find the appropriate connections in the AD8132 datasheet.  This is
what I would do if I did make a DC version of the BasicRX


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