30 MHz LPF on LFRX daughterboard?

GNU Radio
1

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 O.

2

John O. wrote:

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?

The caps on each leg are 82pF, or 41pF in series. That gives a 3dB
bandwidth of about 38 MHz.

Matt

3

On Fri, Oct 23, 2009 at 7:08 PM, Matt E. [email protected] wrote:

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?

The caps on each leg are 82pF, or 41pF in series. That gives a 3dB
bandwidth of about 38 MHz.

Thanks Matt…I’m looking at the LFRX schematic found here:
http://gnuradio.org/trac/browser/usrp-hw/trunk/basic-lf/lfrx.pdf

Is there a more up-to-date version I should be looking at? This one
still shows the 20 pF caps instead of the 82 pF caps.

Regards,
John O.

4

The PDFs are out of date. I’m going to update all of them and post new
ones.

Matt