downconverter from 39 mhz with 12.5 mhz sampler

[h_rafii]

Hi
I have a 3 mhz interested bandwidth in 39 mhz if frequency,
i am doing a down-convert to get this 3 mhz into the Base band:

Fs=(2Fc-B)/M===> (2*39-3)/6=12.5Mhz

so i have a 3 bandwidth in base band .

Question:
Do i need to a LPF or its a clear 3 Mhz base band data for me??
if i do need a LPF Why???

 

[FvM]

Can you sketch/describe the spectrum of the input signal? Are you performing single channel or quadrature sampling?

 

[h_rafii]

Can you sketch/describe the spectrum of the input signal? Are you performing single channel or quadrature sampling?

I have sampling just the real part of the signal and ( single Channel) and i just need to calculate the signals power.
input signal is my Transmitters signal that i toke it down into IF band which is 29 mhz, total bandwidth is 3 mhz and i sampling it with 12.5 mhz, mathematically i will have my signal at base band from 0-3 mhz and 3-6 will be zero( because of analog IF sampling).
so my Question:
in Down Converting you have some mirrors of our band ( for my project in 0-3 and 12.5 - 15.5 and 25-28 and 37.5-40.4 Mhz) , I have 12.5 mhz sampling so it should not have any effects of upper mirrored signals in my samples. is it right??
if i but the samples to spectrum analyzer i should have only 0-3 mhz . is it right?

 

[FvM]

I presume, you mean the IF center frequency is 39 MHz, occupied band 37.5 - 40.5 MHz.

Firstly IF components below 37.5 MHz are mirrored in a non-quadrature sampler, depends on the IF filter characteristic if this matters.

Secondly, what's the spectrum analyzer sampling frequency? If it's 12.5 MHz, there's no need for additional filters. If a rate conversion takes place, you should have a decimation filter. Even if there's no risk of getting signal aliasing , it's still useful to improve signal-to-noise ratio.

 

[h_rafii]

Thanks FvM

is there any book or paper or Site that explained it well that when we need LPF filter and when we not?