Radio Frequency Osc.
2- RADIO-FREQUENCY OSCILLATORS Radio-frequency (RF) oscillators must satisfy the same basic criteria for oscillation as was discussed in Section 1-4 for audio oscillators. That is the Barkhausen criteria must be satisfied.
(RF) oscillators The phase-shift network for RF oscillators is an inductance- capacitance (LC) network. This LC combination, which is generally referred to as a tank circuit, acts as a filter to pass the desired oscillating frequency and block all other frequencies.
RF oscillators (cont’d) The tank circuit is designed to be resonant at the desired frequency of oscillation. An LC circuit is said to be resonant when the inductive and capacitive reactances are equal, that is, when X L = X c (1-17) or when (1-18)
RF oscillators (cont’d) Solving Eq for the frequency f, we obtain an expression for the frequency of oscillation of an RF oscillator, which is (1-19) where f = frequency of oscillation L = total inductance of the phase-shift network C = total capacitance of the phase-shift network
There are a number of standard RF oscillator circuits in use: the most popular are the Colpitts oscillator and the Hartley oscillator shown in Fig As can be seen. the phase-shift network contains a tapped inductor consisting of sections L 1 and L 2 and an adjustable capacitor to vary the frequency of oscillation. The feedback factor P, is given as
(1-20)
The negative sign means there must be a 180 o phase shift across the amplifier. This is accomplished by connecting the amplifier in the inverting configuration as shown in Fig Fig. 1-5 Basic Hartley oscillator
As we stated earlier, the circuit must satisfy the Barkhausen criterion which states that A 1 to sustain oscillation. This may be written as (10-21)
Substituting Eq into Eq gives us (1-22) Equation 1-22 states that the gain of the amplifier must be greater than or equal to the ratio of L 1 to L 2 to sustain oscillation. Using the equation for the gain of an inverting amplifier yields (1-23) We can determine the value of either R f or R i given the value of the, other.
EXAMPLE 1-4 Determine the frequency of oscillation and the minimum value of R f to sustain oscillation for the Hartley oscillator shown in Fig Fig. 1-6 Hartley Oscillator circuit
Solution The frequency if oscillation is determined from Eq as
The minimum gain of the amplifier is computed using Eq as A min = Using Eq. 1-23, we can compute the value of the feedback resistor R f as
1-6 RADIO-FREQUENCY GENERATORS Radio-frequency (RF) generators are designed to provide an output signal over a wide range of frequencies from approximately 30 kHz to nearly 3000 MHz. The term generator is generally used for an instrument that is capable of providing a modulated output signal.
Cont’d Laboratory-quality RF generators contain a precision output attenuator network that permits selection of output voltages from approximately 1 to nearly 3V in precise steps.
Cont’d The output impedance of RF generators is generally 50 . Few generators cover the entire RF spectrum but many cover a very wide range of frequencies. A frequency range exceeding 100 MHz is fairly commonplace in RF generators.
Cont’d Fig. 1-7 Basic RF signal generator
Cont’d The circuit is that of a very stable, multiple-band. RF oscillator. The frequency range is selected with the band selector.
Both the modulation frequency and the percentage of modulation can be adjusted by a vernier control. The amplifier output is applied to a step attenuation network. The output of the attenuator is monitored by the output meter, which indicates the signal level for the user. The entire instrument is contained in a shielded cabinet. Many laboratory-quality RF generators provide shielding for the oscillator plus shielding for the entire instrument.
The exact frequency is selected with the vernier frequency selector. The frequency stability of the generator is limited by the stability of the LC oscillator circuit. Therefore, considerable attention should be directed toward design of the master oscillator. The minimally distorted sinusoidal output of the oscillator is applied to a broadband amplifier that amplifies the signal and provides buffering between the oscillator and any load connected to the output terminal. If so desired, the RF signal is modulated at the amplifier by the modulation oscillator.
Both the modulation frequency and the percentage of modulation can be adjusted by a vernier control. The amplifier output is applied to a step attenuation network. The output of the attenuator is monitored by the output meter, which indicates the signal level for the user. The entire instrument is contained in a shielded cabinet. Many laboratory-quality RF generators provide shielding for the oscillator plus shielding for the entire instrument.
Hewlett-Packard Model 8640A RF signal generator. This laboratory-quality instrument covers the frequency range from 500 kHz to 512 MHz in ten bands. The calibrated and metered output is adjustable from V to 2 V (-145 to +19 db). Fig. 1-8 Laboratory-quality RF signal generator
The instrument provides internal AM and FM modulation as well as external AM. FM, and pulse modulation. The metered and calibrated modulated signal is variable in frequency from 20 Hz to 600 kHz.