Sine Synthesizer And Function Selector Switch - HP 202A Operating And Service Manual

Sect.
III
Page 4
Figure 3-4. Simplified Linear Integrator
the average level of the output can be adjusted to
exactly the voltage midway between the "reference"
levels. This control then serves adequately to adjust
the triangular wave balance which in turn equalizes
the on-off time of the squarewave. The signals
appearing a t the plates of the f i r s t tube V15, a r e
180" out of phase and nearly equal in magnitude.
These signals a r e also very nearly the difference
between the inputs on the two grids. Since there
is no signal on the left grid, the only signal into
the amplifier
is
that a t the junction o f R and C, which
is
the condition originally required. The second
stage
is
a push-pull amplifier employing the signals
from the plates of the previous stage. Again the
common cathode resistance
is
very high, but there
is
very little degeneration of the push-pull input.
The gain of the system to changes common to both
grids
is
about one-half while the gain to voltages
appearing between the input grids is something over
250. Finally C
is
fed back to the signal grid from
the cathode
of
V17A which is 180" out of phase with
the signal input.
The cathode follower
is
used a s an isolation stage
between the integrator and the bi-stable circuit.
This completes the oscillating loop with its inherent
production of both square and triangular functions.
3-4 SINE SYNTHESIZER A N D F U N C T I O N
SELECT O R S W I T C H
The triangular wave from the linear integrator is
connected to R94.
In the SINE position of the FUNC.-
TION selector switch (53) the other end of R94 is
connected to the sine synthesizing diodes and
to
R93B, one half of the dual AMPLITUDE potentio-
meter. The synthesized sinewave signal appears
a s the difference signal between points C and D,
but an e r r o r signal which appears a t D with respect
to B- also appears a t C with respect to B-. This
composite signal
is
applied to a differential amplifier
in the output circuit.
The plus and minus switching references in the
bi-stable unit a r e adjusted s o that the ratio of the
triangular wave amplitude to the conduction voltages
of the synthesizer diodes produces the least distortion
of the sinewave. This adjustment also fixes the
average voltage a t C and is equal to the average
of the plus and minus switching references.
The dc voltages at D, and the cathode of V4 a r e
adjusted to be the average of the plus and minus
switching references. Since these voltages a r e equal
there is no change in DC level applied to the Output
Amplifier
as
the AMPLITUDE control is varied.