What's the different of IF stage and BPF ?

Hello,

An IF stage usually consists of a gain block (linear amplifier) and of a band pass filter. Normally a band pass filter is passive and lossy (can be LC, crystal or SAW filter).
The reason to use 2 or more IF stages is to distribute the total gain requested from the overall IF amplifier. It is better to use medium or lower gain stages for a particular frequency to keep regeneration/self-oscillation effects under control (to achieve a high gain with one IF stage, though it is possible, but needs significant input/output decoupling or screening, this may increase manufacturing cost).
Also, at a multistage IF chain the problem of controlling the overall gain electronically is much easier to solve because the control range is also distributable.
To eliminate the image frequency: this is usually eased by a clever selection of the IF frequency with respect to the input frequency range and solved by the out of passband attenuation of the INPUT filter or (filters). These filters are usually also band pass filters (though could low or high pass or a combination too) and are placed at the antenna input and/or after the RF amplifier, just in front of the mixer.

rgds
unkarc

present day technology in HF is 1 if and then i/q demodulator with software defined radio ... or direct i/q demodulation from rf .. for simple kit see e.g. http://www.amqrp.org/kits/softrock40/

the if stage consists of an amplifier and a BPF

Thanks Unkarc, Cancel and Yahia Muhammad.

hi
u r bit confuse between image freq &if stage
the image freq is found at rf stage
&to eliminate it we use lpf before rf
if stage is to remove spurs i.e spurious harmonics or basically tune o/p to if