Instruction manual

TM 11-6625-3017-14
itors. One switch, SA, on the back panel selects the
mains voltage range, and another, SB, the power source
which may be mains or battery.
The mains input feeds straight into a low-pass filter,
Z1, to prevent r. f. entering the instrument, then into a
transformer T1, which has a double-wound primary. The
transformer is astatically wound because of the
susceptibility of the oscillator tuning system to magnetic
fields. Its two primary halves are connected in series for
240 V, or parallel for 110 V, according to the. position of
switch SA. The secondary winding, centre tapped to
earth, drives a full-wave rectifier whose output is
smoothed by a conventional R-C network to give about
30 V. A series regulator follows the smoothing circuit,
using integrated circuit regulator U1 to control the output
voltages from series transistor Q1.
The output from this regulator is at -18 V, adjustable
by preset potentiometer RV1, and supplies the a. m.
detector, both amplifiers, the peak reading voltmeter, the
discriminator and the -12 V regulator. Further isolation
from mains voltage variations and hum is required for the
remaining units. This is provided by the -12 V regulator
which uses integrated circuit regulator U2 to control the
output voltage from Q2. Potentiometer R2 sets the -12
volt level. Resistors R6, R7, and R8 sense the load
current to provide short circuit protection.
24 V applied to the BATTERY terminals (positive
earth or isolated) passes through MR3, to avoid risk of
damage through accidental polarity reversal, and goes to
Q1 directly, when SB is in the BATTERY position.
3.3 MIXER
(Unit A1--Fig. 7-2)
The mixer is a conventional, untuned, square law
type, using a point contact silicon diode. It is built as a
single, compact screened unit to keep spurious
responses at a low level over the wide frequency range.
Short signal paths are provided to allow operation near
high intensity r. f. fields. The mixer is preceded by the r.
f. attenuator which, at an input impedance of 50
Ω
, gives
6 to 60 dB attenuation.
The r. f. input passes via the continuously variable
attenuator, AT1, into the crystal mixer
where mixing with the local oscillator takes place. The
output from the mixer is at an i. f. of 1.5 MHz.
The normal input range is 4 MHz to 1000 MHz. R.
F. inputs of over 1000 MHz can be used with reduced
sensitivity. The useful upper limit is governed by
oscillator stability and input sensitivity.
3.4 LOCAL OSCILLATOR
(Unit A2a-Fig. 7-2)
VT1 produces 5.5 to 11 MHz directly for range 1,
and 11 to 22 MHz second harmonic for range 2. Tuned
by L1, one of the three variable inductors connected to
the main tuning, the oscillator is evolved from the Colpitts
circuit. There is no crystal control on this oscillator. C1
and L2 are provided to pre-adjust the range limits of
frequency to the scale of the oscillator. VT1 is switched
off by the switch SB on ranges 3-4 and 5-8.
Ranges 3 and 4 use VT2, tuned by L3, for the
oscillator frequency, which is basically 22 to 44 MHz with
44 to 88 MHz second harmonic. The fundamental
frequency is used for range 3, and the harmonic for
range 4. This oscillator can be controlled by switching
one of three crystals X1, 2, 3 into the circuit of VT2 in
place of the de-coupling capacitor, L3 still needing to be
tuned for maximum output.
Ranges 5 to 8 are derived from the 22 to 44 MHz
oscillator, but the output goes to the multiplier board
before it reaches the mixer.
Due to the permeability tuning employed in both
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
matching systems - C4, 5 or C10, 11 in series. These
outputs are fed via simple fixed attenuators to switch
SB2B except in the case of VT2 on ranges 5-8, when it is
fed via the doubler and harmonic generator board (A2b).
Doubler and harmonic generator
(Unit A2b-Fig. 7-2)
VT1 is tuned, rather flatly, by L2 to act as a
frequency doubler to the output from the oscillator VT2
(on A2a). L2 tunes over the range 44 to 88 MHz, and this
signal is applied to the harmonic generator VT2 and VT3
(on A2b), whose output contains all the required
harmonics of the oscillator fundamental.
3-3