RADIO RECEIVER STAGES.pptx
- 2. LESSON OBJECTIVE At the end of the lesson, the student should be able to correctly explain the principle operations of AM radio receiver stages.
- 3. LESSON SCOPE •RF amplifier •Mixer or converter •Local oscillator •IF amplifier •Detector •Audio stage •Automatic Gain Control(AGC)
- 4. RF Amplifier The RF amplifier is the first stage of the receiver and performs the following: •Selects the desired signal by rejecting all unwanted signals. •Amplifies the RF signal to suitable high level. •Improves the signal to noise ratio(SNR) by improving the gain of the incoming signal to withstand the noise introduced by the mixer stage
- 5. RF AMPLIFIER
- 6. RF AMPLIFIER OPERATION • Refer to the diagram above. The antenna receives the transmitted signal and voltage is induced in the primary winding of the input transformer,T1.The primary winding coil is also known as the antenna coil. This voltage is inductively coupled to the base of the transistor,Q1 which works as an RF amplifier. The gang capacitor,CG1 selects the desired signal by tuning the LC cct to the desired frequency. This capacitor is part of the gang capacitor.
- 7. RF AMPLIFIER------ The RF amplifier is a tuned voltage amplifier ,whose collector is tuned to the selected signal frequency with the help of CG2,which is ganged with CG1.This amplifier amplifies the voltage of the selected signal and is inductively coupled to the mixer stage through a step down transformer,T2.This transformer matches the high collector impendence to the low base- to – emitter impendence of the mixer stage. The step down transformer also is step up transformer for the current, and so provides a high gain to the mixer stage.
- 8. RF AMPLIFIER -------- •The primary coils of the transformers T1 and T2 are shunted with variable capacitors ,CT1 and CT2,respectively.These are called trimmers and are used to tune the receiver.The resistor RC and capacitor CB form the decoupling network to block any RF signal from reaching other stages through VCC supply.
- 9. MIXER STAGE The mixer stage has two inputs namely: • Signal frequency fs • Local oscillator frequency flo The mixer operates in the non linear region of an active device such as a transistor and produces the sum and the difference frequencies of fo and fs. It also produces higher harmonics of the sum and difference frequencies. All the frequency components except the difference or IF signal are rejected.
- 10. MIXER STAGE
- 11. MIXER Two methods used to produce an IF are: - Mixer - Converter In a mixer cct two active devices are used ,one each for frequency mixing and local oscillator.In the converter ,only one device is used for for both the local oscillator and the mixer.A converter is economical when compared to a mixer because it uses only one active device. Therefore ,a converter is generally used in this stage.
- 12. MIXER
- 13. CONVERTER
- 14. IF AMPLIFIER---- An IF amplifier stage consists of two stage tuned voltage amplifier tuned to IF frequency as shown below:
- 15. IF AMPLIFIER
- 16. IF AMPLIFIER OPERATION • Transistor C45 works the first IF amplifier C46 works as second IF amplifier.The input to C45 comes from the mixer stage through IF T1.This input is applied to the base of C45.The amplified signal is coupled to next amplifier stage through the second IF transformer,IFT2 which is also tuned to the IF frquency. • The second IF amplifier consists of transistor C46.The final amplified IF signal is available at its collector.The collector has the third IF transformer,IFT3.The amplified IF signal is finally applied to the detector stage.
- 17. DETECTOR STAGE
- 18. DETECTOR • The detector stage demodulates the AM signal by using the envelope detection technique.Generation of AM signals ,to recover the original modulating signal. • The detector cct consists of a detector diode,D and a filter cct consisting of a capacitor,C and resistor R. The detector diode rectifies the IF signal and the filter removes the carrier signal from the rectified signal.The output of the filter cct is the original modulating signal.This audio signal is applied to the audio amplifier through the volume control.
- 20. AUDIO STAGE LM386 is a low voltage audio amplifier and frequently used in battery powered music devices like radios, guitars, toys etc. The gain range is 20 to 200, gain is internally set to 20 (without using external component) but can be increased to 200 by using resistor and capacitor between PIN 1 and 8, or just with a capacitor. Voltage gain simply means that Voltage out is 200 times the Voltage IN. LM386 has a wide supply voltage range 4-12v.
- 21. AGC STAGE Automatic Gain Control or AGC is a circuit design which maintain the same level of amplification for sound or radio frequency. If the signal is too low the AGC circuit will increase (amplify) the level and if is to high will lower it to maintain a constant level as possible. The Automatic Gain Control principle is widely use in AM receivers and sometimes AGC is called an compressor-expander because it acts just like one.
- 22. AGC CCT
- 23. Q&A OK, what did I not make perfectly clear?
- 24. LESSON OBJECTIVE AT THE END THE OF THE LESSON,THE STUDENT SHOULD BE ABLE TO CORRECTLY EXPLAIN;- 1. WHAT ARE RECEIVER STAGES 2. WHAT IS RECEIVER RF AMPLIFIER STAGE 3. WHAT IS IF AMPLIFIER 4. WHAT IS MIXER STAGE 5. WHAT IS DETECTOR 6. WHAT IS AUDIO AMPLIFIER