RFI considerations with aliasing

I have a narrowband receiving system (250KHz to 750KHz wide, depending
on local requirements) which receives at the low-end of VHF.

I’m using a Basic_RX card, which is fed with:

o a crossed-dipole antenna
o a 23.5MHz 3rd order high-pass filter (components I had lying

around–nothing magical about 23.5MHz!)
o 20dB of gain
o a 5th order 48MHz low-pass filter
o 20dB of gain
o another 5th order 48MHz low-pass filter

The initial high-pass is to attenuate lower-frequency signals (broadcast
band in particular, but also HF broadcasters and amateur radio), and
keep them out of the gain stages. The 48MHz filters are designed as
anti-alias filters–since I’m using a USRP2 which samples at 100Msps,
I should be able to “see” up to 50MHz alias free. One of my main
frequencies is 38.125MHz, which puts the first alias at 61.875Mhz, which
is part of on-air TV channel 3, and I do see RFI in the evenings, but
it isn’t consistent with a TV carrier, and it comes up and down over a
several-tens-of-seconds cycle.

I’m wondering if I have “enough” low-pass anti-alias filtering in this

How good is the filtering that happens during digital downconversion in
the FPGA?

Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium