Download Radio Frequency Osc.

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
XL = Xc
(1-17)
or when
(1-18)
2fL 
1
2fC
RF oscillators (cont’d)
 Solving Eq. 1-18 for the frequency f, we obtain an expression for the frequency
of oscillation of an RF oscillator, which is
f 
where
1
2 ( LC ) 1 / 2
f = frequency of oscillation
L = total inductance of the phase-shift network
C = total capacitance of the phase-shift network
(1-19)
 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. 1-5.
 As can be seen. the phase-shift network contains a tapped
inductor consisting of sections L1 and L2 and an adjustable
capacitor to vary the frequency of oscillation. The feedback
factor P, is given as
 As can be seen. the phase-shift network contains a tapped
inductor consisting of sections L1 and L2 and an adjustable
capacitor to vary the frequency of oscillation. The feedback
factor P, is given as
 L1

L2
(1-20)
 The negative sign means there must be a 180o phase shift across
the amplifier. This is accomplished by connecting the amplifier in
the inverting configuration as shown in Fig. 1-5.
Fig. 1-5 Basic Hartley oscillator
 As we stated earlier, the circuit must satisfy the Barkhausen
criterion which states that A
may be written as
A
1 to sustain oscillation. This
 
1

(10-21)
 Substituting Eq. 1-20 into Eq. 1-21 gives us
L2
A 
L1
(1-22)
Equation 1-22 states that the gain of the amplifier must be greater than or equal
to the ratio of L1 to L2 to sustain oscillation. Using the equation for the gain of
an inverting amplifier yields
Rf
(1-23)
A
We can determine the value of either
Rf or
R Ri given the value of the, other.
i
EXAMPLE 1-4
 Determine the frequency of oscillation and the minimum
value of Rf to sustain oscillation for the Hartley oscillator
shown in Fig. 1-6.
Fig. 1-6 Hartley Oscillator circuit
Solution
 The frequency if oscillation is determined from Eq. 1-19 as
1
f
2 [( L1  L2 )C ]1 / 2
1

2 [( 280 H ) (0.001 F ]1 / 2

1
 300 Hz
7
(2 ) (5.29 x 10 )
The minimum gain of the amplifier is computed using Eq. 122 as
Amin = L2 270H

L1

10H
  27
Using Eq. 1-23, we can compute the value of the feedback
resistor Rf as
R f  AR L
 (27) (15k)  405 k
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 0.013 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.