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.
2. The receiver is designed for an input impedance of 75-ohms
positions of the R.F. RANGE switch except WIDEBAND; in the WIDEBAND
position the input impedance is 2000-ohms.
3. Input signals between 0.98 and 30 Mc/s are via an r.f. amplifier
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
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
according to the setting of VFO-1, any spectrum of signals 1 Mc/s
existing in the range 0.98 to 30 Mc/s cabn be mixed in M1 to produce
accettable to the first i.f. band-pass filter.
4. It should be noted at this stage that the exact setting of
determined by conditions in the second mixer and harmonic mixer
; These restrict the possible settings to position 1 Mc/s apart
42.5, 43.5 Mc/s, etc.).
HARMONIC GENERATOR AND MIXER
5. The output from a 1 Mc/s crystal oscillator is connected to
generator. The harmonics derived from this stage are passed trough
32 Mc/s low-pass filter and mixed with the output from VFO-1 in
mixer. This mixer provides an output at 37.5 Mc/s which is amplified
passing trough a band-pass filter tuned to 37.5 Mc/s with a bandwidth
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
the filter and amplifier. As a result, the first v.f.o. must be
tuned in 1 Mc/s
7. The 40 Mc/s first i.f. signal is mixed in the second mixer
the 37.5 Mc/s output from the harmonic mixer in order to produce
output consisting of a 1 Mc/s spectrum in the frequency range
8. To clarity this method of operation, some examples of dial
and intermediate frequencies corresponding to various incoming
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
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
mixer. Therefore the difference frequency from M2 will remain
10. The 2-3 Mc/s receiver, which follows M2, is preceded by a
bandpass filter. The 2-3 Mc/s output from the filter is mixed
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.
11. The 1.6 Mc/s intermediate frequency is mixed in the fourth
with the 1.7 Mc/s output from the 1,7 Mc/s oscillator/amplifier
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
which provide six alternative bandwidths. Separate signal and
diodes are employed and alternative switched time-constants give
conditions for telegraphy and telephony reception. An additional
is incorporated to give an independent output at 100 kc/s.
13. Two independent audio frequency stages are incorporated for
line output or headphone sockets and internal loudspeaker; each
is provided with a level control (see TECHNICAL SPECIFICATION).
14. A crystal calibrator unit is incorporated to enable the scale
to be checked at 100 kc/s intervals when the V.F.O. switch is
INT. position. These check points are obtained from a regenerative
controlled by the 1 Mc/s crystal oscillator.