Sect. I11 Page 1
SECTION 111
CIRCUIT DESCRIPTION
1
3-1 GENERAL expressedbyfo 1/2X RIC1.Norinallya 1O:l
range-tuning capacitor-is used for C1 and C2,
The Model 207A Audio Sweep Oscillator is thus the oscillators cover a 1O:l frequency
basically similar to other -hp- RC oscillators, range in a single band.
however the 207A sweeps the entire audio
spectrum in a single frequency band: The
manner in which the 207A accomplishes this
broad single sweep is by means of a synthe-
c
sized RC network in the frequency controlling
bridge which controls the operating frequency
over a 1OOO:l range.
The 207Aoscillator includes a frequency con-
trolling bridge and a balanced push-pull amp-
lifier circuit which constitute the oscillator, an
output circuit which maybe arranged either for
balanced or unbalanced operation, and apo wer
supply circuit. These are shown in block-dia-
gram form in Figure 3-1 and in detail in the
schematic diagram.
3-2 FREQUENCY CONTROLLING BRIDGE
Figure 3 -2 Basic arrangement of RC network
The frequency-controlling network inthe 207A used in -hp- oscillators.
is arranged as a floating bridge, symmetrical
with respect to ground. With no connection to When the frequency characteristic of one of
ground onany terminal of the bridge, stability the RC pairs of the frequency-determining net-
of calibration is assured, since any stray cap- work is plotted on a reactance chart, the plot
acity and leakage to ground present at the will appear as shown in Figure 3-3. At the
bridge output terminals do not shunt either the
frequency or amplitude-controlling arms of the
bridge. The frequency-controlling compo -
nents comprise two arms of the bridge (Fig.
3-1) and consist of special two-terminal im-
pedance elements (Z1 and Z2) and a variable
condenser C8. The network consisting of Z1
and 22 differs from the conventional RC net-
work in that the resistance element of theRC
pairs has been changed to an impedance ele-
ment to effect a 1OOO:l frequency range in a
single band. A comparison of the convention-
al RC network with the 207A synthesized RC
network is described below in order to show
how this 1OOO:l frequency range is effected.
1O:l Range Network
'In the conventional -hp- resistance-capacity
I I I I I
oscillator the frequencyof oscillation is con- f 10 f l02f 103f
trolled by the combination of series and paral-
FREPUENCY
lel RC networks as shown inbasic form in Fig-
ure 3-2. The ratios of R1 to Rz and C1 to C2 Figure 3-3 Reactance plot of one RC pair in
are such that the frequency of oscillation is basic RC oscillator. |
ACEC
