Sect. 111 Page 2
frequencywhere the resistance curve R inter- pedance characteristics. Figure 3-1 shows
sects the reactance curve of the capacitor, the how the complete circuit appears when the two
voltage eg out of the network (Figure 3-2) is impedance elements are used.
in phase with the voltage ef applied to the net-
work, and the overall circuit oscillates. As Amplitude Stabilizing Arms
the capacitor is tuned through its 1O:l range, Avoltage divider which includes a thermally-
the reactance moves through a 1O:l range (in- sensitive resistance comprises the other two
dicated by shaded area in Figure 3-3), con- arms of the frequency-controllingbridge. The
sequently the frequency of oscillation moves amplitude is stabilized at such a level that the
through a 1O:l range. amplifier tubes are operated in the substan-
tially linear portion of their characteristics.
1OOO:l Range Network This results in a very pure sine wave oscil-
The frequency-determining network of the 207A lation.
retains the same 1O:l range-tuning capacitor,
but theresistance element R is replaced with
a synthesized impedance element which has a The bridge is fed by the balanced voltage de-
slope between 0 and -1, i.e., between that of veloped at the cathode of V3 and V4 in the out-
a resistance and that of a capacitive reactance. put of the balanced amplifier. The output of
By doing this, the limits of the frequency range the frequency-controlling branch of the bridge
over which the circuit canbe tuned occur where is applied to the control grid of V2 while the
the impedance curve of the new network inter- output of the amplitude-stabilizing branch is
sects the reactance limit lines of the tuning applied to the control grid of V1.
capacitor, as shown in Figure 3-4. Each of
the two impedance elements used in the 207A
has an impedance line whose slope approxi- 3-3 BALANCED PUSH-PULL OSCILLATOR
mates -213 as shown in Figure 3-4, thus the
frequency coverage of the oscillator has been The oscillator is a balanced push-pull circuit
expanded to cover a 1OOO:l range in a single which includes an amplifier stage (Vl, V2) and
band. a cathode-follower stage (V3, V4). The bal-
anced output from the cathodes of V3 and V4
is fed back to the frequency controlling bridge
and also to the primary of the output trans-
former. Criss-cross positive feedback is
used in the cathode-follower stage to provide
an essentially zero output impedance as seen
by the cathode-to-cathode load. Thus the OS-
cillator can be operated into any finite load
impedance with little reaction on the oscillator.
The feedback paths are from the plate of V3
to the screen and control grid of V4, and from
the plate of V4 to the screen and control grid
of V3. The degree of positive feedback is a
function of the load and increases as the load
decreases, thus tending to maintain the out-
put constant regardless of load conditions.
I?
I I I 1
f 103f
3-4 OUTPUT CIRCUIT
FREQUENCY
Transformer coupling provides isolation be-
Figure 3-4 Reactance plot of impedance net- tween the oscillator circuit and the output cir -
work
cuit, and allows the output to be obtained either
balanced or unbalanced. The secondary wind-
ing of coupling transformer T2 supplies a con-
The impedance elements approach a straight ventional bridged ?TIt attenuator, the setting
line characteristic of slope -2/3 by using a of which is adjusted by operation of the AMP-
series of RC networks as shown inFigure 3-1. LITUDE control on the front panel. As the
Each impedance element contains 6 resistors control is turned counterclockwise, the loss
and 5 capacitors to achieve the required im- inserted by the attenuator is increased. The |
ACEC
