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

[John Orlando]

On Fri, Oct 23, 2009 at 7:08 PM, Matt Ettus <address@hidden> wrote:
> John Orlando wrote:
>>
>> 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?
>
>
> 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 Orlando
www.epiq-solutions.com