Choice of Intermediate Frequency of a receiving system is usually a compromise , since there are reasons why it is neither low nor high, nor in a certain range between the two.
The following are the major factors influencing the choice of the Intermediate Frequency in any particular system.
- If the IF is too high poor selectivity and poor adjacent channel rejection results unless sharp cut-off filters(crystal/mechanical filters) are used in the IF stage.
- A high value of Intermediate Frequency(IF) increases tracking difficulties.
- If we chose IF as low frequency, image frequency rejection becomes poorer. i.e, if is more IFRR(image Frequency Rejection Ratio) has been improved, which requires a high Intermediate Frequency(). Similarly when is more IFRR becomes worst.
- Average Intermediate Frequency(IF) can make the selectivity too sharp cutting of the side bands.This problem arises because the Q must be low when the IF is low, unless crystal or mechanical filters are used and hence gain per stage is low. Thus a designer is more likely to raise Q rather than increasing the number of IF amplifiers.
- If IF is very low , the frequency stability of local oscillator must be made correspondingly high.
- IF must not fall in the tuning range of the receiver or else instability occurs and hetero dyne whistles (noise) will be heard.
- Standard AM broadcast receivers tuned to (540 KHz-1650 KHz) or(6 MHz-18 MHz) and European long wave band (150 KHZ- 350 KHz) uses IF in the range (438 KHz- 465 KHz). 455 KHz is the most popular value used.
- FM receivers using the standard (88 MHz -108 MHz) band have an IF which is almost always 10.7 MHz.
- TV Receivers in the VHF band (54 MHz-223 MHz),UHF band (470 MHz-940 MHz) uses IF between (26 MHz-46 MHz) and the popular values are 36 MHz and 46 MHz.
- AM-SSB Receviers employed for short-wave reception in the short wave band / VHF band uses IF in the range (1.6 MHz to 2.3 MHz).