Download The Circuit The circuit of the Wobbulator W11 appears in Fig. 2

The Circuit
The circuit of the Wobbulator W11 appears in Fig. 2. In this diagram the 50 c/s modulating
voltage is provided by the secondaries of the mains transformer. A 50 c/s output for the
oscilloscope Y plates is obtained from one half of the h.t. secondary via R15 and VR2. VR2,
the ‘Scan Width’ control, varies the amplitude of the 50 c/s voltage passed to the
oscilloscope. The 50 c/s modulating voltage for the frequency modulated oscillator is
provided by the heater secondary, being passed, via R14 and VR3, to V2B, which together
with its grid and cathode components, provides the waveshape correction referred to earlier.
VR3, ‘Sweep Width’ , controls the amplitude of the modulating voltage applied to the
correction circuit, and, thence, to the frequency modulated oscillator; in consequence it
controls the frequency deviation of that oscillator. The on-off switch, S1, is ganged with
VR3.
The corrected modulating voltage is applied to the variable capacity diode D2. D2 is a silicon
diode which, when reverse-biased (i.e. biased in the manner which does not allow current to
flow), exhibits a varying capacity for varying bias voltage. With the particular diode
employed a change in reverse bias from -1 to -10 volts causes a change in capacity of
approximately 4pF. The relationship between voltage and capacity is non-linear; the change
in capacity for a voltage change from -1 to -2 being much greater than for a voltage change
from -9 to -10 volts. It is this non-linearity which the correction circuit around V2B is
designed to counteract, the correction circuit causing a change in modulating voltage
waveform which compensates for the non-linearity in the diode.
V1B is the frequency modulated oscillator, the coil in its tuned circuit being tuned by the
variable capacity diode D2 (and by valve and stray capacities). The coils presented to V1B
cause it to oscillate at a centre frequency of 10 Mc/s on Range 1, and at a centre frequency of
150 Mc/s on Ranges 2 and 3. The output of V1B is fed, via C9, to the mixer V1A.
V2A is the variable frequency, or tuneable oscillator. The valve functions in a conventional
Colpitts circuit, and tuning is carried out by means of the twin-gang condenser CV10A and
CV10B, and the trimmer CT. CV10A/CV10B are adjusted by the ‘Tuning’ control on the
front panel.
On Range 1, the coil presented to V2A causes it to tune over the range 10-12 Mc/s. The coils
presented on Ranges 2 and 3 cause V2A to tune over 150-190 Mc/s and 190-230 Mc/s
respectively. Suppression of output voltage during the retrace period is achieved by causing
V2A to cut-off, and therefore cease oscillation, during alternate half-cycles of the 50 c/s
modulating voltage. Cut-off is obtained by applying a relatively high a.c. voltage, derived
from the h.t. secondary of the mains transformer, to the earthy end of grid leak R2. The diode
D1 prevents the application of positive voltage to the grid of V2A during scan period, whilst
permitting large negative excursions to take place during retrace periods. The output of V2A
is fed, via C4, to the grid of the mixer V1A.
V1A functions as a grid leak biased additive mixer, the outputs of both oscillators (plus
marker input frequencies via C5) being fed to its control grid. The output of V1A is built up
across its anode load R7, and the potentiometer VR1. VR1 the ‘Attenuator’ control, varies the
amplitude of the r.f. output passed to the equipment under test.