Frequency modulator is a circuit that varies carrier frequency in accordance with the modulating signal. The carrier is generated by either an LC or a crystal oscillator circuit, and so a way must be found to change the frequency of oscillation. In an LC oscillator, the carrier frequency is fixed by the values of the inductance and capacitance in a tuned circuit, and the carrier frequency can therefore be changed by varying either inductance or capacitance.
The idea is to find a circuit or component that converts a modulating voltage to a corresponding change in capacitance or inductance. When the carrier is generated by a crystal oscillator, the frequency is fixed by the crystal. However, keep in mind that the equivalent circuit of a crystal is an LCR circuit with both series and parallel resonant points. Connecting an external capacitor to the crystal allows minor variations in operating frequency to be obtained.
Again, the objective is to find a circuit or component whose capacitance will change in response to the modulating signal. The component most frequently used for this purpose is a varactor. Also known as a voltage variable capacitor, variable capacitance diode, or varicap, this device is basically a semiconductor junction diode operated in a reverse-bias mode.
A junction diode is created when P- and N-type semiconductors are formed during the manufacturing process. Some electrons in the N-type material drift over into the P-type material and neutralize the holes there [see Fig. 6-1(a)], forming a thin area called the depletion region, where there are no free carriers, holes, or electrons.
This region acts as a thin insulator that prevents current from flowing through the device. If a forward bias is applied to the diode, it will conduct. The external potential forces the holes and electrons toward the junction, where they combine and cause a continuous current inside the diode as well as externally. The depletion layer simply disappears [see Fig. 6-1(b)]. If an external reverse bias is applied to the diode, as in Fig. 6-1(c), no current will flow. The bias increases the width of the depletion layer, with the amount of increase depending on the amount of the reverse bias.
The higher the reverse bias, the wider the depletion layer and the less chance for current flow. A reverse-biased junction diode acts as a small capacitor. The P- and N-type materials act as the two plates of the capacitor, and the depletion region acts as the dielectric. With all the active current carriers (electrons and holes) neutralized in the depletion region, it functions just as an insulating material. The width of the depletion layer determines the width of the dielectric and, therefore, the amount of capacitance. If the reverse bias is high, the depletion region will be wide and the dielectric will cause the plates of the capacitor to be widely spaced, producing a low capacitance. Decreasing the amount of reverse bias narrows the depletion region; the plates of the capacitor are effectively closer together, producing a higher capacitance. All junction diodes exhibit variable capacitance as the reverse bias is changed.
However, varactors are designed to optimize this particular characteristic, so that the capacitance variations are as wide and linear as possible. The symbols used to represent varactor diodes are shown in Fig. 6-2. Varactors are made with a wide range of capacitance values, most units having a nominal capacitance in the 1- to 200-pF range. The capacitance variation range can be as high as 12 :1. Fig. 6-3 shows the curve for a typical diode.
A maximum capacitance of 80 pF is obtained at 1 V. With 60 V applied, the capacitance drops to 20 pF, a 4 :1 range. The operating range is usually restricted to the linear center portion of the curve.
Fig. 6-4, a carrier oscillator for a transmitter, shows the basic concept of a varactor frequency modulator. The capacitance of varactor diode D1 and L1 forms the parallel-tuned circuit of the oscillator. The value of C1 is made very large at the operating frequency so that its reactance is very low. As a result, C1 connects the tuned circuit to the oscillator circuit. Also C1 blocks the dc bias on the base of Q1 from being shorted to ground through L1. The values of L1 and D1 fix the center carrier frequency. The capacitance of D1 is controlled in two ways, through a fixed dc bias and by the modulating signal.
In Fig. 6-4, the bias on D1 is set by the voltage divider potentiometer R4. Varying R4 allows the center carrier frequency to be adjusted over a narrow range. The modulating signal is applied through C5 and the radio frequency choke (RFC); C5 is a blocking capacitor that keeps the dc varactor bias out of the modulating-signal circuits. The reactance of the RFC is high at the carrier frequency to prevent the carrier signal from getting back into the audio modulating-signal circuits. The modulating signal derived from the microphone is amplified and applied to the modulator. As the modulating signal varies, it adds to and subtracts from the fixed-bias voltage. Thus, the effective voltage applied to D1 causes its capacitance to vary. This, in turn, produces the desired deviation of the carrier frequency. A positive- going signal at point A adds to the reverse bias, decreasing the capacitance and increasing the carrier frequency. A negative-going signal at A subtracts from the bias, increasing the capacitance and decreasing the carrier frequency.
Example 6-1 The value of capacitance of a varactor at the center of its linear range is 40 pF. This varactor will be in parallel with a fixed 20-pF capacitor. What value of inductance should be used to resonate this combination to 5.5 MHz in an oscillator? Total capacitance CT = 40 + 20 = 60 pF.
The main problem with the circuit in Fig. 6-4 is that most LC oscillators are simply not stable enough to provide a carrier signal. Even with high-quality components and optimal design, the frequency of LC oscillators will vary because of temperature changes, variations in circuit voltage, and other factors. Such instabilities cannot be tolerated in most modern electronic communication systems, where a transmitter must stay on frequency as precisely as possible. The LC oscillators simply are not stable enough to meet the stringent requirements imposed by the FCC. As a result, crystal oscillators are normally used to set carrier frequency. Not only do crystal oscillators provide a highly accurate carrier frequency, but also their frequency stability is superior over a wide temperature range.
It is possible to vary the frequency of a crystal oscillator by changing the value of capacitance in series or in parallel with the crystal. Fig. 6-5 shows a typical crystal oscillator. When a small value of capacitance is connected in series with the crystal, the crystal frequency can be “pulled” slightly from its natural resonant frequency. By making the series capacitance a varactor diode, frequency modulation of the crystal oscillator can be achieved. The modulating signal is applied to the varactor diode D1, which changes the oscillator frequency.
It is important to note that only a very small frequency deviation is possible with frequency-modulated crystal oscillators. Rarely can the frequency of a crystal oscillator be changed more than several hundred hertz from the nominal crystal value. The resulting deviation may be less than the total deviation desired. For example, to achieve a total frequency shift of 75 kHz, which is necessary in commercial FM broadcasting, other techniques must be used.
In NBFM communication systems, the narrower deviations are acceptable. Although it is possible to achieve a deviation of only several hundred cycles from the crystal oscillator frequency, the total deviation can be increased by using frequency multiplier circuits after the carrier oscillator. A frequency multiplier circuit is one whose output frequency is some integer multiple of the input frequency. A frequency multiplier that multiplies a frequency by 2 is called a doubler, a frequency multiplier circuit that multiplies an input frequency by 3 is called a tripler, and so on. Frequency multipliers can also be cascaded. When the FM signal is applied to a frequency multiplier, both the carrier frequency of operation and the amount of deviation are increased. Typical frequency multipliers can increase the carrier oscillator frequency by 24 to 32 times. Fig. 6-6 shows how frequency multipliers increase carrier frequency and deviation.
The desired output frequency from the FM transmitter in the figure is 156 MHz, and the desired maximum frequency deviation is 5 kHz. The carrier is generated by a 6.5-MHz crystal oscillator, which is followed by frequency multiplier circuits that increase the frequency by a factor of 24 (6.5 MHz x 24 = 156 MHz). Frequency modulation of the crystal oscillator by the varactor produces a maximum deviation of only 200 Hz. When multiplied by a factor of 24 in the frequency multiplier circuits, this deviation is increased to 200 x 24 = 4800 Hz, or 4.8 kHz, which is close to the desired deviation. Frequency multiplier circuits are discussed in greater detail in Next Article.
Oscillators whose frequencies are controlled by an external input voltage are generally referred to as voltage-controlled oscillators (VCOs). Voltage-controlled crystal oscillators are generally referred to as VXOs. Although some VCOs are used primarily in FM, they are also used in other applications where voltage-to-frequency conversion is required. As you will see, their most common application is in phase-locked loops, discussed later in next article.
Although VCOs for VHF, UHF, and microwaves are still implemented with discrete components, more and more they are being integrated on a single chip of silicon along with other transmitter or receiver circuits. An example of such a VCO is shown in Fig. 6-7. This circuit uses silicon-germanium (SiGe) bipolar transistor to achieve an operating frequency centered near 10 GHz. The oscillator uses cross- coupled transistors Q1 and Q2 in a multivibrator or flip-fl op type of design. The signal is a sine wave whose frequency is set by the collector inductances and varactor capacitances.
The modulating voltage, usually a binary signal to produce FSK, is applied to the junction of D1 and D2. Two complementary outputs are available from the emitter followers Q3 and Q4. In this circuit, the inductors are actually tiny spirals of aluminum (or copper) inside the chip, with inductance in the 500- to 900-pH range.
The varactors are reverse-biased diodes that function as variable capacitors. The tuning range is from 9.953 to 10.66 GHz. A CMOS version of the VCO is shown in Fig. 6-8. This circuit also uses a cross coupled LC resonant circuit design and operates in the 2.4- to 2.5-GHz range. Variations of it are used in Bluetooth transceivers and wireless LAN applications. (See Chap. 20.)
There are also many different types of lower-frequency VCOs in common use, including IC VCOs using RC multivibrator-type oscillators whose frequency can be controlled over a wide range by an ac or dc input voltage. These VCOs typically have an operating range of less than 1 Hz to approximately 1 MHz. The output is either a square or a triangular wave rather than a sine wave. Fig. 6-9(a) is a block diagram of one widely used IC VCO, the popular NE566. External resistor R1 at pin 6 sets the value of current produced by the internal current
sources. The current sources linearly charge and discharge external capacitor C1 at pin 7. An external voltage VC applied at pin 5 is used to vary the amount of current produced by the current sources. The Schmitt trigger circuit is a level detector that controls the current source by switching between charging and discharging when the capacitor charges or discharges to a specific voltage level. A linear sawtooth of voltage is developed across the capacitor by the current source. This is buffered by an amplifier and made available at pin 4.
The Schmitt trigger output is a square wave at the same frequency available at pin 3. If a sine wave output is desired, the triangular wave is usually fi ltered with a tuned circuit resonant to the desired carrier frequency. A complete frequency modulator circuit using the NE566 is shown in Fig. 6-9(b). The current sources are biased with a voltage divider made up of R2 and R3. The modulating signal is applied through C2 to the voltage divider at pin 5.
The 0.001-μF capacitor between pins 5 and 6 is used to prevent unwanted oscillations. The center carrier frequency of the circuit is set by the values of R1 and C1. Carrier frequencies up to 1 MHz may be used with this IC. If higher frequencies and deviations are necessary, the outputs can be filtered or used to drive other circuits, such as a frequency multiplier. The modulating signal can vary the carrier frequency over nearly a 10 :1 range, making very large deviations possible. The deviation is linear with respect to the input amplitude over the entire range.
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