7-12. Draw the block diagram for a two-way FM radio transmitter and explain itsoperation.The figure shown is a four-channel unit that operates in the 150 to 174-MHz frequencyband. The channel selector switch applies power to one of the four crystal oscillator modulesthat operates at a frequency between 12.5 and 14.5 MHz, depending on the final transmitcarrier frequency. The oscillator frequency is temperature compensated to ensure a stability of+0.0002%. The phase modulator uses a varactor diode that is modulated by the audio signal atthe output of the audio limiter. The audio signal amplitude is limited to ensure that thetransmitter is not over deviated. The modulated IF carrier is amplified and is then multiplied by12 to produce the desired RF carrier frequency. The RF signal is further amplified and filteredprior to transmission.(Fig. 7-28)7-14. Describe the operation of an electronic push-to-talk circuit.The electronic push-to-talk (PTT)circuit is used rather than a simple mechanical switchto reduce the static noise associated with contact bouncein mechanical switches. Keying thePTT applies DC power to the selected transmit oscillator module and the RF power amplifiers. Keying the PTT switch grounds the base of Q1 , causing it to conduct and turn off Q2.With Q2off, VCCis applied to the transmitter and removed from the receiver. With the PTT switchreleased, Q1shuts off , removing VCCfrom the transmitter, turning on Q2, and applying VCCto thereceiver. (Fig. 7-29)7-16. Briefly explain how a composite FM stereo signal is produced.The L and R audio channels are combined in a matrix network to produce the L + R andL – R audio channels. The L – R audio channel modulates a 38-kHz sub-carrier and produces a23 – 53-kHz L – R stereo channel. Because there is a time delay introduced in the L – R signalpath as it propagates through the balanced modulator, the L + R stereo channel must be
Foster Seeley Discriminator or FM Detector
- details of FM demodulators - Foster Seeley discriminator or demodulator and the Ratio demodulator or detector.
FM demodulation tutorial includes
The Foster Seeley is a common type of FM detector circuit used mainly within radio sets constructed using discrete components.
The Foster Seeley circuit is characterised by the transformer, choke and diodes used within the circuit that forms the basis of its operation.
Invented in 1936 by Dudley E. Foster and Stuart William Seeley, it was widely used until the 1970s when ICs using other techniques that were more easily integrated became more widely available.
Foster-Seeley FM discriminator basics
The Foster Seeley detector or as it is sometimes described the Foster Seeley discriminator has many similarities to the ratio detector. The circuit topology looks very similar, having a transformer and a pair of diodes, but there is no third winding and instead a choke is used.
The Foster-Seeley discriminator / detector
Like the ratio detector, the Foster-Seeley circuit operates using a phase difference between signals. To obtain the different phased signals a connection is made to the primary side of the transformer using a capacitor, and this is taken to the centre tap of the transformer. This gives a signal that is 90 degrees out of phase.
When an un-modulated carrier is applied at the centre frequency, both diodes conduct, to produce equal and opposite voltages across their respective load resistors. These voltages cancel each one another out at the output so that no voltage is present. As the carrier moves off to one side of the centre frequency the balance condition is destroyed, and one diode conducts more than the other. This results in the voltage across one of the resistors being larger than the other, and a resulting voltage at the output corresponding to the modulation on the incoming signal.
The choke is required in the circuit to ensure that no RF signals appear at the output. The capacitors C1 and C2 provide a similar filtering function.
Both the ratio and Foster-Seeley detectors are expensive to manufacture. Wound components like coils are not easy to produce to the required specification and therefore they are comparatively costly. Accordingly these circuits are rarely used in modern equipment.
Foster-Seeley detector advantages & disadvantages
As with any circuit there are a number of advantages and disadvantages to be considered when choosing between the various techniques available for FM demodulation.
As a result of its advantages and disadvantages the Foster Seeley detector or discriminator is not widely used these days. Its main use was within radios constructed using discrete components.
By Ian Poole
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