HP 3312A Service Manual

Model 3312A
4-22. When the diode switch is reverse biased there is a
diode capacitance which distorts the peak of the triangle
(see Figure 4-4). This distortion is removed by the level
shifting circuit of Q401 and C404. The square wave at the
collector of Q401 is passed through the ac voltage divider
of C404 and C205 and added to the triangle wave to
eliminate the “drop-off” caused by diode capacitance.
3312 AB - 4035
| Figure 4-4, Level Shifter,
4.23. integrating Capacitance. The integrating capacitors
are C205, C201, C202, C203, C204, and C206. Because the
current charging and discharging is constant, except for
direction, the voltage across the integrating capacitance will
be triangular. The resulting waveform is varied in frequency
by changing the current and capacitance with connections
through the RANGE Hz switches and the FREQUENCY
dial.
4-24. Buffer Amplifier. The Buffer Amplifier (Figure 7-2)
has a gain of one and consists of a FET input and a
push-pull output. The high input impedance of the FET
provides isolation, and the emitter followers Q202 and
Q203, form a push-pull circuit. Diodes CR209 and CR211
are temperature compensating diodes.
4-25. Comparator. While the integrating capacitor is charg-
ing, the positive-going ramp is applied to pin 4 of U201
(Figure 7-3). The positive-going ramp is compared in
magnitude, to the square wave on pin 2. Upon coincidence,
the Comparator changes state, reversing the biasing of the
gating diodes, and the integrating capacitance discharges.
Coincidence of the negative-going ramp with the square
wave now at its negative limit, switches the Comparator
back to the original state, completing one cycle of
operation,
4.26. The square wave output of U201 pin 5 is processed
by a +5 V diode clamping network to provide the square
wave Output.
4-27. Sync Generator. The Sync Generator is a simple
divider network which attenuates the output of the £5 V
Section ÍV
clamp. The sync output is, therefore, always a square wave
in phase with the main generator (180° out of phase with
the square wave output due to output amplifier inversion)
and under the control of the Symmetry Switch.
4-28. Sine Shaper. The triangle wave from the Buffer
Amplifier (Figure 7-3) is connected through R257 to the
sine synthesizing diodes. The diodes of the sine network are -
reverse biased by the resistive dividers made up of R259,
R262, R263, etc. As the triangle ramp goes positive, the
bias is overcome and the diodes in the lower half turn on
to produce a non-linear loading effect on the triangle wave.
When the ramp reaches a negative level, the upper half of
the sine shaping network begins to load the triangle wave
thus shaping the negative half of the wave. The diodes in
this section are selected for their soft turn-on characteristics
to insure a smooth sine-shaped curve.
4.29. Transistors Q216, Q217, Q218 and Q219 compen-
sate for thermal changes. This insures that the proper bias
level is maintained over a wide temperature range. In the
Model 3312A, the sine shaper is followed by a low-pass LC
filter that has a sharp cutoff at 14 MHz. This filter
attenuates third harmonics substantially at generator fre-
quencies above 5 MHz, enabling the instrument to produce
sine waves with all harmonics more than 30 dB below the
fundamental up to its maximum frequency.
4-30. Output Section.
4-31. The output section consists of the Output Amplifier
and the Output Attenuator (Figure 7-4).
4.32. The amplifier system has a voltage gain of - X16 and
has been impedance matched on both sides. The ac signal is
coupled through Q501 to Q502 and Q503. To obtain a
frequency response that extends to zero, the main output is
direct coupled to U501 and then to the complementary
stage of Q504 and Q507. The dc offset is summed into pin
2 of U501 via R504. In the CAL position, R504 is opened
and no offset is summed into the output amplifier.
4-33. The output is dc coupled to the output attenuator.
The step attenuator consists of resistive attenuator sections
of 20 dB steps with an impedance of 50 ohms,
4-34. Modulation Generator.
4-35. Signal Generator. Basic to this circuit are the inte-
erator U301 and the comparator U302 (Figure 4-5). By
integrating the square wave at its input the integrator
generates a triangle wave. The level of the triangle is then
compared to the square wave and when the voltage at the
input node to the comparator equals zero [R,
(E, - E, /R; + R,) =E,] the comparator output switches
states. The square wave output from the comparator is then
inverted, clamped and fed back to the inverter to control
the generation of the triangle wave.
+3

HP 3312A Specifications

General

Brand	HP
Model	3312A
Category	Inverter
Language	English