Racal Instruments RA-117 User Manual page 17

FIRST MIXER (M1)
6. The outputs from the 30 Mc/s low-pass filter and the variable frequency oscillator
VFO-1 are fed to the signal grid of the mixer stage (V7) which produces a signal at
40 Mc/s. The signal is then passed to a 40 Mc/s band-pass filter which forms the
anode load of this stage.
40 MC/S BAND-PASS FILTER
7. The 40 Mc/s band-pass filter consists of eight over-coupled tuned circuits connected
in cascade and is tuned by the trimming capacitors C21, C33, C43, C53, C61, C70,
C79 and C88. This filter, which has a passband of 40 Mc/s
only the required 1 Mc/s spectrum of signals is passed to the second stage. This filter
is deliberately set to a slightly wider passband than is theoretically required, to allow
for possible drift in VFO-1.
1 MC/S CRYSTAL OSCILLATOR/AMPLIFIER
8. The frequency of the crystal oscillator V1 may be set precisely to 1 Mc/s by adjusting
the trimming capacitor C2A. The crystal XL1 which is connected between the control
grid and the screen grid is electron coupled to the anode. The anode coil L2 is adjusted
to resonate at 1 Mc/s by means of a dust iron core. The fixed capacitors C9, C10 and
C11 complete the tuned circuit. When an external signal is applied to socket SK3, the
valve operates as an amplifier.
9. The output from V1 is capacitance-coupled to the harmonic generator V2 and via
SK2 to a "T" adptor for feeding a 1 Mc/s input into the L.F. converter and also the
control grid of the mixer valve V13.
HARMONIC GENERATOR
10. The 1 Mc/s signal is fed via coupling capacitor C8 to the control grid of the harmonic
generator V2. The H.T. is fed to the screen grid via R12 and is decoupled by C8A.
Harmonics produced at this stage are passed to a 32 Mc/s low-pass filter.
32 MC/S LOW-PASS FILTER
11. The megacycle harmonics are fed through a 32 Mc/s low-pass filter circuit to prevent
harmonics other than those required from passing to the harmonic mixer (V4). Limited
control over the cut-off frequency is provided by C7 which is adjusted to equalize the
output from yhe filter at the frequencies corresponding to 28 and 29 Mc/s on the
MEGACYCLE dial.
HARMONIC MIXER
12. The outputs from the 32 Mc/s low-pass filter and VFO-1 are mixed in the harmonic
mixer by applying the filtered megacycle harmonics to the suppressor grid and the
output from the VFO-1 to the control grid. The 37 5 Mc/s output is selected by the
tuned anode load, consisting of a fixed capacitor C50 and an inductance L28 which
may be adjusted by means of a dust iron core, and coupled by C51 to V6. R36 is grid
stopper.
2-STAGE 37.5 MC/S AMPLIFIER (1)
13. The anode load of V6 is a tuned circuit consisting of a fixed capacitor C67 and an
inductor L33 Which is tuned to 37.5 Mc/s. Frequency adjustment is by the dust iron
Detailed Circuit Description 17
650 kc/s, ensures that