3 1 Characteristics of radio receiver Sensitivity Ability

3. 1 Characteristics of radio receiver Sensitivity � Ability to amplify weak signals. � Minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal. � Broadcast receivers/ radio receivers should have reasonably high sensitivity so that it may have good response to the desired signal � But should not have excessively high sensitivity otherwise it will pick up all undesired noise signals. � It is function of receiver gain and measures in decibels. � Sensitivity of the receiver depends on : 1. Noise power present at the input to the receiver 2. Receiver noise figure 3. Bandwidth improvement factor of the receiver

Selectivity of radio receiver is its ability to differentiate desired signal from unwanted signals. For better selectivity ‘Q’ should be high.

Fidelity � Fidelity is defined as – a measure of the ability of a communication system to produce an exact replica of the original source information at the output of the receiver. � Any variations in the demodulated signal that are not in the original information signal is considered as distortion. � Radio receiver should have high fidelity or accuracy. � Example- In an A. M. broadcast the maximum audio frequency is 5 KHz hence receiver with good fidelity must produce entire frequency up to 5 KHz.

Image Frequency � In radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces. � Image frequency – any frequency other than the selected radio frequency carrier that will produce a cross-product frequency that is equal to the intermediate frequency if allowed to enter a receiver and mix with the local oscillator. � It is given by signal frequency plus twice the intermediate frequency fsi = fs + 2 fi

�The higher the IF, the farther away the image frequency is from the desired radio frequency. Therefore, for better image frequency rejection, a high IF is preferred. �However, the higher the IF, it is more difficult to build a stable amplifier with high gain. i. e. there is a trade-off when selecting the IF for a radio receiver (image frequency rejection vs IF gain and stability)

3. 2 Block diagram of super heterodyne receiver IF=fo- fs fs RF amplifier mixer fo Local oscillator Ganged tuning IF amplifier detector AF amplifier Modulating signal

§ RF section �Consists of a pre-selector and an amplifier �Pre-selector is a broad-tuned bandpass filter with an adjustable center frequency used to reject unwanted radio frequency and to reduce the noise bandwidth. �RF amplifier determines the sensitivity of the receiver and a predominant factor in determining the noise figure for the receiver. § Mixer/converter section �Consists of a radio-frequency oscillator and a mixer. �Choice of oscillator depends on the stability and accuracy desired. �Mixer is a nonlinear device to convert radio frequency to intermediate frequencies (i. e. heterodyning process). �The shape of the envelope, the bandwidth and the original information contained in the envelope remains unchanged although the carrier and sideband frequencies are translated from RF to IF.

§ IF section �Consists of a series of IF amplifiers and bandpass filters to achieve most of the receiver gain and selectivity. �The IF is always lower than the RF because it is easier and less expensive to construct high-gain, stable amplifiers for low frequency signals. �IF amplifiers are also less likely to oscillate than their RF counterparts. § Detector section �To convert the IF signals back to the original source information (demodulation). �Can be as simple as a single diode or as complex as a PLL or balanced demodulator. § Audio amplifier section �Comprises several cascaded audio amplifiers and one or more speakers

§ AGC ( Automatic Gain Control ) �Adjust the IF amplifier gain according to signal level(to the average amplitude signal almost constant). � AGC is a system by means of which the overall gain of radio receiver is varied automatically with the variations in the strength of received signals, to maintain the output constant. � AGC circuit is used to adjust and stabilize the frequency of local oscillator. � Types of AGC – Ø No AGC Ø Simple AGC Ø Delayed AGC

3. 3 Envelop detector using diode

3. 4 Basic FM Demodulator � An electronic circuit in which frequency variations of modulated signals are converted to amplitude variations first, with the help of tuned circuit � And then original information is extracted with the AM demodulation techniques say diode detector. Frequency Variations TUNED CIRUIT Amplitude Variations

SLOPE Detector Output FM FM Source Tank Circuit

Voltage Voltag e Transfe r Curve Output f Slope Detector Transfer Characteristics t Input t

fc fc+ f fc- f t Voltage Voltag e Transfe r Curve Input Output f f Slope Detector Transfer Characteristics t

Limitations of Slope Detector � It is inefficient, as it is linear in very limited frequency range. � It reacts to all amplitude changes. � It is relatively difficult to tune, as tuned circuit must be tuned to different frequency than carrier frequency.

Balanced Slope Detector D 1 + fc+ f IN fc T’ T’’ fc- f - Vo + D 2

15 V 10 V 5 V T’ (+10 to +15 V) fc fc- f -5 V Output of T’ at fc- f (+5 to +10 V) (-5 to -10 V) T’’ +Ve fc+ f Output of T’’ at fc+ f -10 V 15 V Output of T’ at fc+ f Output of T’ at fc- f (-10 to -15 V) -Ve

Combined Transfer Curve Vo Useful Range fc- f fc fc+ f

Phase Discriminator C VIN C 1 L 1 b L 2 a a’ + D 1 R 3 C 3 L 3 o - V a’b’ R 4 C 4 + b’ D 2

� And from the phasor diagram given below : V ao Vb o � That as Vao=Vbo, hence discriminator output is zero.

• And from the phasor diagram given below : V ao Vb o That as Vao>Vbo, hence discriminator output is positive.

• And from the phasor diagram given below : V ao Vb o • That as Vao<Vbo, hence discriminator output is negative.

Vo Useful Range extends upto half -power points of tuned transformer. Useful Range fc Beyond which o/p falls due to frequency response of transformer.

C VI C 1 L 1 b L 2 N a a’ + D 1 R 3 C 3 R 5 L 3 o V o C 5 + C 4 R 6 b’ D 2 SUM Ratio-Detector

Change 1: Diode D 2 is reversed so that now sum of Vao & Vbo appears across points a’ and b’ instead of difference. Change 2: A capacitor C 5 with large time constant is connected across a’-b’ in order to keep Vao+Vbo constant. Change 3: Output is taken from o-o’ as the difference of Vao + Vbo appears there. Ground is shifted to O’.

Operation Above Resonance � So output voltage in this case will be positive as shown in vector diagram: V ao Vb o Output

Operation Below Resonance �So the output in this case will be negative. V ao Vb o Output

3. 5 Block diagram of FM Receiver � Uses FM Discriminator to detect small frequency deviations � FM receivers use IF limiter amplifiers; no AGC needed � Squelch detects high-frequency noise and gates audio Mixer Local Oscillator IF amplifier Discriminator AF amplifier Squelch circuit

3. 6 Double Conversion Receivers n n For good image rejection, relatively high IF is desired. However, for a high gain selective amplifiers that are stable, a low IF is necessary. The solution fro above constrain is to use 2 intermediate frequencies, i. e. by using double conversion AM receiver. q q The 1 st IF is a relatively high frequency for good image rejection. The 2 nd IF is a relatively low frequency for good selectivity and easy amplification.

3. 7 Squelch Circuit n n A squelch circuit, or muting circuit, is found in most communications receivers. The squelch is used to keep the receiver audio turned off until an RF signal appears at the receiver input. In AM systems such as CB radios, the noise level is high and can be very annoying. Squelch circuits provide a means of keeping the audio amplifier turned off during the time that noise is received in the background and enabling it when an RF signal appears at the input.

Automatic Gain Control Circuits The gain of a bipolar transistor amplifier is proportional to the amount of collector current flowing. n Two methods of applying AGC are as follows: 1. The gain can be decreased by decreasing the collector current. This is called reverse AGC. 2. The gain can be reduced by increasing the collector current. A stronger signal increases AGC voltage and base current and, in turn, increases collector current, reducing the gain. This method of gain control is known as forward AGC. n