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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 14:47:50 -0700
User-agent: Mozilla Thunderbird 1.0 (X11/20041206)

> Count me as interested, and actually wondering why your diff-amp solution 
> wasn't the default?  I wonder what the cost differential between the AD8132 
> and the transformer used?

Actually, the differential amps are _cheaper_ than the transformers.  I
went with the transformers because they allow for very high IF
frequencies -- the ADCs are spec'ed to 400 MHz, and I have done direct
sampling as high as 1.2 GHz (with about 15-20 dB of loss).  The
differential amps don't perform well past 70 MHz or so.  Even under 70
MHz, you'll get cleaner signals with the transformers, since they have
no distortion or noise.

> Speaking of, what is the usable low-frequency cutoff for those transformers 
> according to spec?

I think they have a 3dB cutoff of around 100 kHz or so.  If you need a
lower cutoff you can switch them out for other Minicircuits transformers
which have cutoffs as low as 10 kHz (like the T1-1T), at the expense of
a lower frequency response on the high end.  Most should be pin compatible.


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