Detailed Circuit Description - Racal Instruments RA-117 User Manual

8. Detailed Circuit Description

1. Reference should be made to the circuit diagram at the end of this handbook.
AERIAL CIRCUIT
2. A 75-ohms unbalanced aerial source is connected to the tuned R.F. amplifier through
a three-section 30 Mc/s low-pass filter and a five- position attenuator covering a range
of 0 to 40 dB. Switch S2 selects wide- band 75-ohms or wideband (high impedance) or
any one the five double-tuned aerial coils L4-L8 for tuned operation. These aerial coils
are aligned by means of dust iron cores. The aerial is tuned by a capacitor C18A/B
which is switched out of circuit in both wideband positions.
R.F. AMPLIFIER
3. The incoming signal is fed via C28 and grid stopper R25 to the grid of V3B; the
R.F. stage (V3) employs a variable-mu, low-noise double- triode; the two halves of the
valve are connected in cascode so as to utilize the low-noise high-gain properties of
the valve. A delayed A.V.C. voltage, derived from a shunt diode network, is applied to
the grid of V3B when the signal level is approximately 10 V. The capacitors C40 and
C41 ensure that the cathode is adequately decoupled over the wide frequency range.
Ferrite beads have been fitted to the heater lead, connected to pin 4, the anode of V3A
and the cathode of V3B adjacent to C41, to prevent parasitic oscillations occurring.
30 MC/S LOW-PASS FILTER
4. The amplified signal is passed to a 30 Mc/s low-pass filter which has a substantially
flat responseover the frequency range. L27, C47 and R28 constitute the first 'L half
Section' of the filter. The signal is then fed at low impedance (680-ohms) through the
coupling capacitor C74 and the grid stopper R45 to the control grid of V7, the first
mixer stage. The input capacitance of V7 forms the capacitance to chassis betweeenL15
and L17 required to the filter network.
NOTE:This capacitance is not critical, therefore no adjustment will be necessary should
V7 be changed.
FIRST VARIABLE FREQUENCY OSCILLATOR (VFO-1)
5. This circuit comprises a cathode-coupled Hartley oscillator stage (V5) which may be
continously tuned over the frequency range of 40 5 to 69 5 Mc/s. The frequency deter-
mining components are an inductance L36 and a variable capacitance C76. Alignment
is accomplished by adjusting aluminium core of L36 and the trimming capacitor C77.
The variable capacitor C76 is coupled to the Mc/s dial which is calibrated from 0 to
29 Mc/s. The anode load consists of L20, a compensating inductance which is wound
on a 470-ohm resistor R18. The oscillator is coupled via C85 to the signal grid of the
first mixer stage V7 and also via C42 to the control grid of the harmonic mixer V4.
NOTE:The Mc/s dial calibration may be affected if V5 if changed. The necessary correc-
tion may be made by adjusting C77 with the Mc/s dial set to 29 Mc/s.
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