I assume we are talking about a radio receiver that will get a signal from an antenna.
I havn't built much equipment for >440MHz so I'm not too hot on the specifics of microwave stuff.
>The signal is narrow band, or one tone signal.
A single rf carrier or an audio tone modulated onto a carrier?
Let's say if I use broadband LNA followed by a mixer to downconver the signal to IF.
This will inevitably result in a receiver with an image reception problem at either
tuned frequency + 2*IF or tuned frequency - 2*IF depending on whether you put the
LO at F+IF or f-IF. This might be acceptable, the receiver will work when there is
not much energy at the image frequency. I imagine you would be most likly to have
a problem when the image frequency is in the 2.4GHz microwave oven band or the 1800/1900MHz
mobile phone bands.
The only options to avoid image reception are either to filter the image frequency before
the mixer or to use a first IF which is higher than the signal frequency.
If you used an IF frequency of say 500MHz you could have a set of 1GHz wide front end
filters that you select depending on where you are tuned. Electronically switching between
filters with enough isolation would be a problem, perhaps you could do some research in
that area, minicircuit's make some **broken link removed**but I can't find a price off hand so they are probably not cheap, alternativly a big
mechanical switch designed for rf use (constant characteristic impedance) or coaxial relays.
Perhaps you could just plug in whichever filter you need.
How about a first IF at around 7GHz, second IF at say 500MHz ?
>So I think it is better to filter the image after the mixer.
You can't. The problem is that the wanted signal and the signals at the image frequency
are shifted to the same frequency after the mixer.
>The conversion gain varies in different frequency
Either use AGC or in an FM system you may get away with running the amps into clipping when the signal is strong.
>Should I use single conversion or double conversion?
Depends on the performance you require.
You will genereally be able to be get better sensativity and selectivity with a double conversion receiver.