Input gain of a passive mixer

I have designed a single-balanced passive mixer having an ideal current source
as input.After the switches,as seen in Fig.1,there are LPF's with a cut-off
frequency of 10 MHz.I get the expected output however I cannot understand
why when I plot the input gain,its form is like the one shown in Fig.2 .Shouldnt it
be the similar to the LPF? (I run PSS/PAC analysis to get the results).

Fig.1

Fig.2

Does anyone like RF ?:P

For those who are interested in the solution the product of downconversion and upconversion voltages
creates a bandpass filter to the RF input which gives the respective response..:)