LOCAL OSCILLATOR (Unit A2a-Fig. 7-2)

TM 11-6625-3017-14

itors. One switch, SA, on the back panel selects the

where mixing with the local oscillator takes place. The

mains voltage range, and another, SB, the power source

output from the mixer is at an i. f. of 1.5 MHz.

which may be mains or battery.

The normal input range is 4 MHz to 1000 MHz. R.

The mains input feeds straight into a low-pass filter,

F. inputs of over 1000 MHz can be used with reduced

Z1, to prevent r. f. entering the instrument, then into a

sensitivity.

The useful upper limit is governed by

transformer T1, which has a double-wound primary. The

oscillator stability and input sensitivity.

transformer is astatically wound because of the

susceptibility of the oscillator tuning system to magnetic

3.4 LOCAL OSCILLATOR (Unit A2a-Fig. 7-2)

fields. Its two primary halves are connected in series for

240 V, or parallel for 110 V, according to the. position of

VT1 produces 5.5 to 11 MHz directly for range 1,

switch SA. The secondary winding, centre tapped to

and 11 to 22 MHz second harmonic for range 2. Tuned

earth, drives a full-wave rectifier whose output is

by L1, one of the three variable inductors connected to

smoothed by a conventional R-C network to give about

the main tuning, the oscillator is evolved from the Colpitts

30 V. A series regulator follows the smoothing circuit,

circuit. There is no crystal control on this oscillator. C1

using integrated circuit regulator U1 to control the output

and L2 are provided to pre-adjust the range limits of

voltages from series transistor Q1.

frequency to the scale of the oscillator. VT1 is switched

off by the switch SB on ranges 3-4 and 5-8.

The output from this regulator is at -18 V, adjustable

Ranges 3 and 4 use VT2, tuned by L3, for the

by preset potentiometer RV1, and supplies the a. m.

oscillator frequency, which is basically 22 to 44 MHz with

detector, both amplifiers, the peak reading voltmeter, the

44 to 88 MHz second harmonic. The fundamental

discriminator and the -12 V regulator. Further isolation

frequency is used for range 3, and the harmonic for

from mains voltage variations and hum is required for the

range 4. This oscillator can be controlled by switching

remaining units. This is provided by the -12 V regulator

one of three crystals X1, 2, 3 into the circuit of VT2 in

which uses integrated circuit regulator U2 to control the

place of the de-coupling capacitor, L3 still needing to be

output voltage from Q2. Potentiometer R2 sets the -12

tuned for maximum output.

volt level. Resistors R6, R7, and R8 sense the load

current to provide short circuit protection.

Ranges 5 to 8 are derived from the 22 to 44 MHz

oscillator, but the output goes to the multiplier board

24 V applied to the BATTERY terminals (positive

before it reaches the mixer.

earth or isolated) passes through MR3, to avoid risk of

damage through accidental polarity reversal, and goes to

Due to the permeability tuning employed in both

Q1 directly, when SB is in the BATTERY position.

oscillators, outputs cannot be taken from the inductor

coils. The outputs are therefore taken by tapping the

preset tuning capacitors, C1 and C14, by two capacitive

3.3 MIXER (Unit A1--Fig. 7-2)

matching systems - C4, 5 or C10, 11 in series. These

outputs are fed via simple fixed attenuators to switch

The mixer is a conventional, untuned, square law

SB2B except in the case of VT2 on ranges 5-8, when it is

type, using a point contact silicon diode. It is built as a

fed via the doubler and harmonic generator board (A2b).

single, compact screened unit to keep spurious

responses at a low level over the wide frequency range.

Doubler and harmonic generator (Unit A2b-Fig. 7-2)

Short signal paths are provided to allow operation near

high intensity r. f. fields. The mixer is preceded by the r.

VT1 is tuned, rather flatly, by L2 to act as a

f. attenuator which, at an input impedance of 50 Ω, gives

frequency doubler to the output from the oscillator VT2

(on A2a). L2 tunes over the range 44 to 88 MHz, and this

6 to 60 dB attenuation.

signal is applied to the harmonic generator VT2 and VT3

(on A2b), whose output contains all the required

The r. f. input passes via the continuously variable

harmonics of the oscillator fundamental.

attenuator, AT1, into the crystal mixer

3-3