Tektronix 2715, Service Manual

Page: 148 / 682

Theory of Operation
3- 90
2715 Spectrum Analyzer Service Manual
NOTE
. For instruments with serial numbers B030000 or above
,
a dead time
control (DTC) R669 has been added which will be used to set the maximum high
voltage measured at the –2.75KV test point by varying the voltage at pin 1 of
U260. New values for C262 and R652 have been chosen to lower the effect of the
leakage current from pin 4 of U260. Any pin 4 leakage current through R652 will
reduce Power Supply performance. Supply regulation for U260 has been
improved by lowering the resistance of R261
.
The low impedance, fast switching of the MOSFETs is slowed down by series
inductors L241 and L250. As each MOSFET turns on, the drain inductor,
transformer leakage inductance, and transformer reflected capacitance resonate.
This is damped by CR740, CR640, and R240. Also, when each MOSFET turns
on, CR250, CR260, C151, and R152 limit the peak voltage excursion to
approximately 120 V. The series resistor capacitor (RC) networks across the
inductors absorb some energy in both conditions.
Frequency Lock. The inverter frequency is approximately 26 kHz and the
pre--regulator operates at 3 times this rate. The D flip flop U670A is clocked at
78 kHz with the D input driven at 26 kHz through clamp diode CR661. The
Q output is RC filtered and modulates pin 6 of the PWM IC to maintain the
3:1 frequency ratio. When R680 is adjusted for lock, one sample of the D input
is low, the second is high, and the third is at the edge of the 26 kHz waveform.
When the third sample is high, the filtered output to the PWM will reduce its
frequency, and increase the frequency when low. This lock prevents low
frequency IM between the two frequencies that could be audible. The lock can
occur at various ratios with 3:1 chosen so the predominant harmonic of the
inverter coincides with the PWM fundamental.
The +5 V potentiometer, R937, is used to 5 V
1
, the reference voltage.
This reference voltage is compared with the +5 V supply by U630B, and an error
voltage is generated. Feedback through the optoisolator U281 controls the pulse
width of U280 on the primary side. The center tap voltage on the primary side of
the power transformer is adjusted this way to correct small changes in +5 V.
Other secondary DC voltages are determined by transformer winding ratios and
degree of loading. Three secondary windings (5 V, 10 V, and 50 V) have series
RC damping networks to circulate energy stored in the power transformer
leakage inductances.Choke/capacitor (lc) filter networks at the output of each
rectifier reduce ripple to acceptable levels for instrument power.
Most of the available power flows to the +5 V, +10 V, and --11 V supplies. The
+20 V supply is lightly loaded and generated by voltage doubling. The fan
supply is --11 V. The +50 V and +115 V supplies are used for Z axis control,
CRT adjustments, and deflection amplifiers. The --5 V
1
is a stable supply
generated by a three terminal regulator for use in the deflection amplifiers.
Secondary regulation is provided for the high voltage supplies. U630A and Q130
control the peak to peak amplitude of the high voltage AC voltage seen by both
cathode and anode rectifiers. Constant output and post deflection anode ratio is
maintained as internal loading varies. This keeps the display fixed in size.
Secondary