Local Oscillator

The local oscillator must meet exacting requirements in frequency coverage, frequency stability, constant output, and correct, tracking. The local oscillator may use any of the fundamental oscillator circuits. The modified Hartley and the tuned grid are commonly used. Most VHF and UHF receivers use crystal-controlled local oscillators. To maintain frequency stability, the plate voltage of the oscillator is often regulated.

Methods of Mixer Injection

MIXER

MIXER

Pentagrid Circuit

J AMPLIFIER

J AMPLIFIER

OSCILLATOR

Another problem of stability is the effect that the other radio frequencies present have on the local oscillator. The oscillator tends to synchronize its oscillation with the other radio frequencies. The stronger these other RF signals are and the closer their frequency is to the oscillator frequency, the greater is the tendency for the oscillator to synchronize with these RF signals. A change in oscillator frequency caused by these RF signals is called oscillator pulling. Oscillator pulling may be reduced by isolating the oscillator as completely as possible from the other radio frequencies. This isolation is accomplished not only by proper shielding of oscillator components, but also by using appropriate means for coupling the oscillator signal to the 1st detector. Oscillator voltage may be introduced into the 1st detector by inductive, ca-pacitive, or electron coupling It may be injected at the cathode, control grid, screen grid, or suppressor grid. Other types of injection make use of pentagrid tubes, discussed later in this chapter. These special purpose tubes are designed to isolate the oscillator circuit more effectively from the RF signal frequencies. For that reason they are helpful in reducing oscillator pulling.

Mixer

Notice the various mixers and various methods of injection shown in the illustration on page 32. Both pentodes and triodes can be used as mixer tubes.

At A, the output of a tuned grid oscillator is inductively coupled to the cathode circuit of the mixer. At B, a modified electron coupled Hartley oscillator is capacitively coupled to the control grid of the mixer. The oscillator uses a pentode instead of a triode because with the pentode there is less likelihood of pulling. At C, a tuned grid oscillator is conductively coupled to the screen grid of the mixer. At D, the output of the oscillator is taken from the oscillator control grid and connected directly to the suppressor grid of the mixer.

An advantage of suppressor grid injection is that the screen grid acts as a shield between the oscillator signal and the RF input signal (applied to the mixer control grid). This reduces pulling. However, with suppressor grid injection, the suppressor grid of the mixer is at the potential of the oscillator control grid. This puts negative voltage on the suppressor grid and lowers the gain of the mixer tube.

Pentagrid Mixer

A multielement tube called the pentagrid may be used as a mixer or as a converter. The five grids of the pentagrid tube are shown in the pentagrid mixer diagram below. Counting up, the first and third grids are the first and second control grids. The second and fourth grids, joined within the tube, are the inner and outer screen grids. The fifth grid, joined to the cathode within the tube, is the suppressor grid. The oscillator voltage is injected at the second

Typical Pentagrid Mixer 33
Pentagrid Converter

control grid, which is isolated from the first control grid and from the plate by screens. Thus, pulling on the oscillator is kept to a minimum. The RF signal voltage is introduced at the first control grid. Both control grids affect the flow of current from cathode to plate so that the signals of both grids are reproduced in the mixer output.

Pentagrid Converter

For low frequencies, where electrode interaction is relatively less important, the pentagrid can be used as a converter. In this use, it combines mixer and oscillator functions in a single tube, as shown in the diagram above. The oscillator section of the tube is composed of cathode, first control grid, and the combined screen grids (as the anode). Feedback to maintain oscillation is provided by the autotransformer action of tapped coil L1-L2. Current flowing to the cathode through LI induces the feedback in L2. The oscillator voltage appearing on the first control grid affects the flow of current through the tube. The RF signal voltage is applied to the second control grid.

Conversion Gain

The efficiency of a conversion stage is calculated in terms of the ratio of IF output in the plate circuit of the mixer to the RF signal voltage input on the grid of the mixer. The conversion gain is usually about 0.3 of the normal gain of the tube when used as an IF amplifier.

Oscillator-Mixer Tracking

Notice that in all the frequency conversion circuit diagrams shown before the variable capacitor of the oscillator tuned circuit is ganged with a variable capacitor of an RF tuned circuit. This makes the oscillator tuned circuit track with the RF tuned circuit. The difference frequency is the IF. In addition to the ganged variable capacitors, the pentagrid converter circuit has small variable padder and trimmer capacitors. These can be adjusted to assure that the ganged circuits maintain the correct difference frequency throughout the tuning range. The trimmer, a parallel capacitor, has its greatest effect at the high end of the band. The padder, a series capacitor, has its greatest effect at the low frequency end of the band.

MIXER

MIXER

COMPONENT

COMPONENT SIZE

CAPACITY RATIO

FREQUENCY RATIO

CI

40-340 MMF

1:3

C2

40-360 MMF

1*

C3

500 MMF (variable)

C2IC3

1:2.36

Padder in Oscillator Tracking

You can see how the padder capacitor is important from the above circuit diagram and the accompanying chart. CI and C2 are the ganged capacitors. Each has the same range of capacitance (40-360 mmf). However, each does not tune across the same range of frequencies. The oscillator circuit tracks higher than the RF circuit by a frequency difference equal to the IF. Therefore, if the frequency coverage of the receiver is 1 to 3 mc and the IF is 465 kc, then the oscillator covers from 1.465 mc to 3.465 mc. The tuning ratio of the RF circuit, low to high frequency, is 1:3. The tuning ratio of the oscillator is 1:2.36. The capacity ratio of both ganged capacitors is the

i

CAPACITY RATIO

FREQUENCY RATIO

CI

40-360 MMF

1:9

1:3

C2

40-360 MMF

C3

30 MMF

C2AC3

EFFECTIVE VALUE 70-390 MMF

ABOUT 1:5.6

ABOUT 1:2.36

Trimmer in Oscillator Tracking same 1:9.

Since frequency is inversely proportional to the square root of the capacitance f I-^ —V

\2TT\LCI

there is a match between a tuning ratio of 1:3 and a capacity ratio of 1:9 for capacitor CI. However, for capacitor C2, the tuning ratio of 1:2.36 does not match the capacity ratio of 1:9. Therefore C3 is added as a padder. The addition of capacitor C3, 500 mmf (variable), makes the range of C2 and C3 together vary between 37-210 mmf. This is a capacity ratio of 1:5.6. The combination is a good match for the tuning ratio of the oscillator of 1:2.36.

COMPONENT

COMPONENT SIZE

CAPACITY RATIO

FREQUENCY RATIO

CI

40-360 MMF

1:9

1:3

C2

40-360 MMF

(variable)

C4

500 MMF (variabU)

C2, C3, C4

1:2.3

Padder and Trimmer in Oscillator Tracking

Padder and Trimmer in Oscillator Tracking

Note that adding the padder affects the high frequency end of the band (the low capacity end) very little. It merely changes the capacity from 40 mmf to 37 mmf. At the low frequency (high capacity) end. however, it affects the capacity considerably. It changes the capacity from 360 mmf to 210 mmf.

Now examine the circuit diagram and chart showing the use of a trimmer. CI and C2 are the same capacitors shown in the padder circuit. C3 is the trimmer in parallel with C2. Its value is 30 mmf. The parallel combination of C2 and C3 has the effective range of 70-390

mmf. Thus the capacity ratio is 1:5.6, which matches the frequency ratio of the oscillator tank circuit of 1:2.36. Note that, proportionately, the trimmer capacitor affects the low frequency (high capacity) end much less than it affects the high frequency end. It changes the high frequency (low capacity) end from 40 mmf to 70 mmf.

To see how both padder and trimmer are used together, examine the circuit and chart at the left. Adding C3, a trimmer of 3 mmf (variable), and C4, a padder of 500 mmf (variable), produces a combination with an effective range from 40 mmf to 210 mmf. This is a capacity ratio of about 1:5.2 which matches the frequency ratio of 1:2.3 for the oscillator tuned circuit. Trimmers and padders are adjustable to permit tracking at both ends of the frequency range.

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