Brief Technical Description - Racal Instruments RA-117 User Manual

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
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
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
Mc/s kc/s
4 1.000
5 0
18 600
650 kc/s. Thus, according to the setting of VFO-1, any spectrum
Signal Freq. VFO-1
( ) Mc/s ( )Mc/s (
5.0 44.5
5.0 45.5
18.6 58.5
[14]
150 kc/s.
Xtal harmonic 1st I.F. 2nd I.F.
)Mc/s Mc/s
7th 39.5
8th 40.5
21st 39.9
Mc/s
2.0
3.0
2.4