The simplest method for generating FM directly is to vary the frequency of an oscillator. A capacitance microphone or a varactor diode may be used as part of the oscillator’s frequency determining network. The capacitor microphone’s capacitance varies in response to the intensity of the sound waves striking it, making the oscillator’s frequency vary as the amplitude of the sound varies. The varactor diode's capacitance depends on the voltage across it. Audio signals placed across the diode cause its capacitance to change, which in turn, causes the frequency of the oscillator to vary.
A second method of direct FM generation is to use a reactance modulator. A reactance modulator is a circuit in which a transistor is made to act like a variable reactance. The reactance modulator is placed across the LC circuit of the oscillator and as the modulator’s reactance varies in response to an applied audio signal, the oscillator frequency varies as well.
The third technique is to use a voltage controlled oscillator (VCO). The VCO’s output frequency is proportional to the voltage of the input signal. If audio is applied to the input of a VCO, the output is an FM signal.
All three of these methods suffer from a serious drawback. There is no way to prevent drift of the carrier frequency. It is necessary that the carrier frequency stay constant so that the FM signal does not drift out of its assigned channel. Although a crystal oscillator is very stable, it is not possible to directly frequency modulate a crystal oscillator because the circuit Q is too high. To address this issue, the Crosby modulator was developed. The Crosby circuit incorporates an automatic frequency control (AFC). The circuit operation is as follows:
1. The FM output signal is sampled and converted to a low frequency (~ 2 MHz) by a mixer. The mixer output is applied to a discriminator, which is a frequency controlled voltage source (the opposite of a VCO). The output of the discriminator is exactly 0 when the carrier is on its assigned frequency. If the carrier drifts, the discriminator outputs an error voltage, which is fed back to the modulator to compensate for the drift. The discriminator circuit has a sufficiently long time constant that it does not respond to the frequency variations due to modulation, but only to the slow drift of the carrier.
The maximum deviation possible with direct FM is typically 5 KHz, which is too small for wideband FM. To overcome this difficulty, frequency multiplication is used. A frequency multiplier is an amplifier that operates class C (non-linearly), whose output network is tuned to a multiple of the input network (the multiple is usually 2 or 3). A non linear amplifier has an output rich in harmonics of the input signal. The output network is tuned to select one of these harmonics. The efficiency of a multiplier decreases as the multiplier rises, so typically multipliers are designed to be doublers or triplers. Several multipliers can be placed in series to reach higher frequency multiples. For example, a frequency can be multiplied 12 times by feeding it through two doublers and a tripler.
A frequency multiplier multiplies all frequencies in its input pass band. Consider a frequency tripler whose input is an FM signal with a carrier frequency of 5 MHz and a deviation of 5 KHz. The input frequency varies from 4.995 to 5.005 MHz. At the output of the frequency multiplier, everything is tripled, so the output frequency varies from 14.985 to 15.015 MHz. The carrier is now 15 MHz, so the deviation has tripled to 15 KHz. This technique can be used to generate a high frequency wideband FM (WFM) signal from a low frequency narrowband FM (NBFM) signal.
Let us look at an example. An FM station operates at 106.5 MHz with a maximum deviation of 75 KHz. The FM signal is generated by a reactance modulator that operates at 3.9444 MHz, with a maximum deviation of 2.7778 KHz. The resulting FM signal is fed through 3 frequency triplers, multiplying the carrier frequency and deviation 27 times. The final carrier frequency is 27*3.9444 = 106.5 MHz and the final deviation is 27*2.7778 = 75 KHz.
It is important to remember that frequency multiplication multiplies both the
carrier frequency and the deviation.
It is important to remember that frequency multiplication multiplies both the carrier frequency and the deviation.
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