Draw the block diagram of a super heterodyne receiver. Explain the function of each block. Explain the significance of the name super hetero dyne. How RF sections suppress the image channel? Why is up – conversion used in super heterodyne receive? Define image Rejection Ratio.

Super heterodyne receivers was proposed as an alternative to TRF receiver which suffers from selectivity problem .The basic super heterodyne receiver is most widely used receivers at present. Super heterodyne principal is also used in television and radar receiver. In the super heterodyne receivers, the received RF signal voltage is combined with the local oscillator voltage and is converted into a signal of lower fixed frequency. This frequency is called the intermediate frequency (IF).The intermediate frequency signal however contains the same modulation as the original carrier and can be subsequently amplified and demodulated to reproduced the original information .the additional units need in a super heterodyne receiver (in comparison to TRF receiver) include local oscillator ,mixer, and an intermediate frequency (IF) amplifier. A constant frequency difference is maintained between the local oscillator and the RF stage, through capacitance tuning in which the capacitance are ganged together and operated by a common control knob. the IF amplifier usually contains a number of transformer each consisting of a pair of mutually coupled tuned circuits .with this large number of double-tuned circuit operating at a specially chosen frequency, the IF amplifier provide most of the gain and bandwidth requirements of the receiver. The IF amplifier determines the sensitivity and selectivity of super heterodyne receivers .The output of the IF amplifier is demodulated by using an AM detector .The intelligence signal from the detector output is finally given sufficient audio amplification to drive the speaker .The automatic gain control allows the receiver to maintain to a constant level output irrespective of the variation of the input signal strength.

Fig.1 Block diagram of super heterodyne receiver

RF section: It generally consists of a pre – selector and an amplifier stage. They can be a separate circuit or can be a combined circuit. The pre selector is a band pass filter with an adjustable centre frequency that is tuned to the desired carrier frequency. The primary purpose of the pre selector is to provide enough initial band limiting to prevent a specific unwanted carrier frequency called the image frequency from entering the receiver.

Mixer: The mixer is non linear device and converts the radio frequency to the intermediate frequency. Heterodyning takes place here and radio frequencies down converted to intermediate frequencies. The intermediate frequency used in AM radio receiver is 455 KHz.

IF section: The IF section consists of a series of amplifiers and band pass filter. Most of the receiver gain and selectivity are achieved in the IF section. The IF centre frequency and bandwidth are constant for all stations. The centre frequencies are chosen so that their frequency is less than any of the RF signal to the receiver.

Detector section: this section converts the IF signals to audio signals. The detector can be a simple envelope detector or a more advance detector such as balanced modulator and PLL.

Audio amplifier section: this section consists of several cascading audio amplifiers. The power output depends on the requirement.

Image Frequency

The Supper heterodyne receiver is superior to TRF receiver in many respect but suffers from a major problem called Image frequency problem which is inherent to the system. This Image frequency problem arises because of the use of heterodyning arrangement. The frequency conversion process performed by the mixer and L.O combination sometimes will allow an undesired frequency in addition to the desired station frequency to fed to the IF amplifier and subsequent stages.

The local oscillator frequency f_LO = f_s + f_i                         ....(i)

where f_s is the signal frequency and f_i is the intermediate frequency. Thus the intermediate frequency is the difference between the local oscillator frequency and the signal frequency and is the only one which is passed and amplified by the IF stage. If a frequency somehow manages to reach the mixer, such that

f_si = f_LO + f_i                                ....(ii)

Then this frequency will also produce a difference frequency which mixed with local oscillator frequency f_i. Now eliminating f_LO from equation (ii)

f_si = f_s + 2f_i                                ....(iii)

The frequency f_si is a spurious signal which cannot be distinguished by the IF amplifier and will be treated in the some manner as the desired signal. This will lead to interference and result in two stations received simultaneously. The spurious frequency is called the image frequency and is equal to the signal frequency plus twice the intermediate frequency.

Rejection of Image Frequency

The rejection of an image frequency by a single tuned circuit is defined as the ratio of the gain at the signal frequency to the gain at image frequency and is given by

α = √(Q² + ρ²)                                ....(iv)

where ρ = f_si / f_s - f_s / f_si and Q is the quality factor of the tuned circuit in the loaded condition. If the super heterodyne receiver has an RF stage, them there are two tuned circuit both tuned to the same desired station frequency. The image rejection of each state can be calculated using Equation (iv). The overall rejection will be the product of the two. The image rejection capability of a super heterodyne receiver is determined by the selectively of the from-end of the receiver. The rejection must be achieved before the IF stage. Once a spurious frequency is allowed to enter the first stage of the IF amplifier, it becomes impossible to remove it from the desired signal. It can be seen from the equation (iv) and (v) that the image frequency rejection is better if f_si / f_s is large.