The shortcomings of the TRF receiver became very obvious as the number of radio stations on the air increased. Edwin Armstrong came up with the idea of converting all incoming frequencies to a common frequency. Designing a stable, high gain amplifier of a specific bandwidth would be a relatively simple task and the performance of such a receiver could be significantly better than the TRF designs. Armstrong called this new receiver the superheterodyne. Heterodyne is an electrical engineering term relating to the non-linear mixing of signals and the mixer certainly was the key circuit in the superheterodyne.
The superheterodyne receiver functions by mixing the incoming signal from the antenna with another RF carrier generated inside the receiver by a circuit known as the local oscillator (LO). The output of the mixer is the difference of the LO signal and the incoming signal. This new difference signal is called the intermediate frequency (IF) signal. The IF signal is amplified by one or more IF amplifiers. A detector is used after the final IF amplifier to recover the original audio signal, which is then amplified by the audio amplifier portion of the receiver and sent to the speaker or other output device.
Let us look at a specific example. An FM radio is tuned to a station operating at 89.9 MHz This signal is mixed with an LO signal at a frequency of 100.6 MHz. The difference frequency at the output of the mixer is 10.7 MHz. This is the IF signal. If the FM radio is tuned to a different station at 107.9 MHz, the LO frequency is also re-tuned to 118.6 MHz. The mixer once again produces an IF signal of 10.7 MHz. In fact, as the FM radio is tuned across the band from 87.9 to 107.9 MHz, the local oscillator (LO)is tuned from 98.6 10 118.6 MHz. No matter what frequency the radio is tuned to (in its operating range), the mixer's output will be 10.7 MHz.
The superheterodyne overcomes the variable sensitivity and selectivity problems of the TRF, by doing most of the amplification at the intermediate frequency, where the gain and selectivity can be carefully controlled. However, the superheterodyne introduces some new challenges: first, the LO signal must always differ from the input signal by exactly the IF frequency, regardless of what input frequency is selected. This is known as "tracking" and second, there are two different frequencies that can mix with the LO signal to produce the IF signal. One of those frequencies is our input signal frequency; the other is known as the "image frequency". The image, input, IF and LO frequencies are related as follows:
IF = LO - Input IF = Image - LO Image = Input + 2*IF.
Here is an example that references our earlier discussion about the FM radio. When the receiver is tuned to 89.9 MHz, the 89.9 MHz signal can mix with the LO signal of 100.6 MHz to create a 10.7 MHz IF signal. However, a signal (or noise) at 111.3 MHz can also mix with the LO signal to create a 10.7 MHz IF. In the next sections we will consider these issues (and some others) in more detail when we look at AM receiver systems.
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