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[Discuss-gnuradio] 30 MHz LPF on LFRX daughterboard?

From: John Orlando
Subject: [Discuss-gnuradio] 30 MHz LPF on LFRX daughterboard?
Date: Fri, 23 Oct 2009 17:59:39 -0500

Hi all,
I was just reading the specs/schematic on the LFRX RF daughterboard,
and noticed that the literature indicates that there is a 30 MHz
low-pass filter after the differential amplifiers.  Looking at the
schematic, there is a 50 ohm resistor + 20 pF capacitor on each leg of
the differential amplifier output.  I believe this would mean that,
for each of the differential amplifiers, the effective resistance is
really 100 ohms (two 50 ohms in series) with 10 pF (two 20 pF in
series).  If it really is 100 ohms + 10 pF, the passband of the filter
is much larger than 30 MHz.  Even taking into account some input
capacitance on the A/D converter on the USRP (10-20 pF?), it still
would be larger than 30 MHz.  Am I missing something here?

Thanks much,
John Orlando

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