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.

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

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.

The PDFs are out of date.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

ones.

Matt