BRIEF TECHNICAL DESCRIPTION



1. This section describes briefly, with the aid of the block diagram
fig 3, the basic theory of operation. For a more detailed explanation
of the receiver, DETAILED CIRCUIT DESCRIPTION, should be consulted.


SIGNAL INPUT

2. The receiver is designed for an input impedance of 75-ohms for all
positions of the R.F. RANGE switch except WIDEBAND; in the WIDEBAND
position the input impedance is 2000-ohms.


FIRST MIXER

3. Input signals between 0.98 and 30 Mc/s are via an r.f. amplifier and
a 30 Mc/s low-pass filter to the first mixer (M1) where they are
mixed with the output from a variable frequency oscillator VFO-1 (MEGACYCLES
tuning). This oscillator has a frequency range of 41.5 to 69.5 Mc/s. The first
i.f. stage is in effect a band-pass filter tuned to 40 Mc/s ±650 kc/s. Thus,
according to the setting of VFO-1, any spectrum of signals 1 Mc/s wide and
existing in the range 0.98 to 30 Mc/s cabn be mixed in M1 to produce an output
accettable to the first i.f. band-pass filter.

4. It should be noted at this stage that the exact setting of VFO-1 is
determined by conditions in the second mixer and harmonic mixer circuit
; These restrict the possible settings to position 1 Mc/s apart (e.g. 41.5,
42.5, 43.5 Mc/s, etc.).


HARMONIC GENERATOR AND MIXER

5. The output from a 1 Mc/s crystal oscillator is connected to a harmonic
generator. The harmonics derived from this stage are passed trough a
32 Mc/s low-pass filter and mixed with the output from VFO-1 in the harmonic
mixer. This mixer provides an output at 37.5 Mc/s which is amplified before
passing trough a band-pass filter tuned to 37.5 Mc/s with a bandwidth of
±150 kc/s.

6. The presence of this filter restricts the setting of VFO-1 to an exact
number of Mc/s plus 37.5 Mc/s in order to give an output acceptaple to
the filter and amplifier. As a result, the first v.f.o. must be tuned in 1 Mc/s
steps.


SECOND MIXER

7. The 40 Mc/s first i.f. signal is mixed in the second mixer (M2) with
the 37.5 Mc/s output from the harmonic mixer in order to produce an
output consisting of a 1 Mc/s spectrum in the frequency range 2-3 Mc/s
(second i.f.).

8. To clarity this method of operation, some examples of dial settings
and intermediate frequencies corresponding to various incoming signals
are tabulated below:

Dial Settings Signal Freq. VFO-1 Xtal harmonic 1st I.F. 2nd I.F.
Mc/s kc/s (fs) Mc/s (fo)Mc/s (nfc)Mc/s

4 1.000 5.0 44.5 7th 39.5 2.0
5 0 5.0 45.5 8th 40.5 3.0
18 600 18.6 58.5 21st 39.9 2.4

9. Frequency drift of VFO-1 within the limits of the 37.5 Mc/s filter
bandwidth, does not affect the frequency stability of the receiver.
A change in this oscillator frequency will alter the first i.f. to the same
extent and in the same sense as the nominal 37.5 Mc/s signal from the harmonic
mixer. Therefore the difference frequency from M2 will remain constant.


THIRD MIXER

10. The 2-3 Mc/s receiver, which follows M2, is preceded by a pre-tuned
bandpass filter. The 2-3 Mc/s output from the filter is mixed in the
third mixer with either the output from the second variable frequency
oscillator VFO-2 or an external signal within the frequency range of 3.6 to
4.6 Mc/s to provide the third intermediate frequency of 1.6 Mc/s.


FOURTH MIXER

11. The 1.6 Mc/s intermediate frequency is mixed in the fourth mixer (M4)
with the 1.7 Mc/s output from the 1,7 Mc/s oscillator/amplifier to
provide the fourth and final intermediate frequency of 100 kc/s.


FOURTH I.F. STAGE

12. The final i.f. stages are preceded by crystal lattice and L-C filters
which provide six alternative bandwidths. Separate signal and a.v.c.
diodes are employed and alternative switched time-constants give the optimum
conditions for telegraphy and telephony reception. An additional i.f. amplifier
is incorporated to give an independent output at 100 kc/s.


A.F. STAGES

13. Two independent audio frequency stages are incorporated for either
line output or headphone sockets and internal loudspeaker; each stage
is provided with a level control (see TECHNICAL SPECIFICATION).


CRYSTAL CALIBRATOR

14. A crystal calibrator unit is incorporated to enable the scale of VFO-2
to be checked at 100 kc/s intervals when the V.F.O. switch is set to
INT. position. These check points are obtained from a regenerative divider
controlled by the 1 Mc/s crystal oscillator.


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