7. 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 can be mixed in M1
to produce an output acceptable 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 through 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 through 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
(
f
s
) Mc/s
(
f
o
)Mc/s
(
nf
c
)Mc/s
Mc/s
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
[14]
Summary of Contents for RA-117
Page 1: ...Racal RA 117 Radio Receiver...
Page 24: ...Detailed Circuit Description 24 at maximum is the correct calibration maintained...
Page 32: ...32...
Page 70: ...Illustrations 70...
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