A In a superheterodyne receiver, the radio frequency (RF) signal picked up at the antenna must be converted to a lower frequency prior to demodulation. This conversion takes place in the mixer stage of the receiver when the RF signal is mixed with another signal generated by the local oscillator (LO). This mixing process produces sum and difference signal frequencies. The difference frequency is amplified and becomes the Intermediate Frequency, or IF (see Figure 2). The IF is usually high enough to still be considered RF, but it may be substantially lower than the signal at the antenna. For example, FM receivers commonly convert to an IF of 10.7 MHz. AM broadcast receivers often use an IF of 455 kHz. The exceptions are so-called “up conversion” receivers that use IFs that are higher than the highest received signal frequency. To complicate matters further, superhet designs may also include more than one mixer/IF section (Figure 3).
It’s interesting to note that in a direct conversion receiver the RF conversion takes place in one huge step—mixing the signal from the antenna with a local oscillator signal at nearly the same frequency. This puts the difference frequency in the audio range for immediate demodulation.
Figure 2—A basic block diagram of a superheterodyne receiver.
Figure 3—Block diagram of a double-conversion superhet with two IF sections.
From QST December 2000
