Theory of Operation
3-90 2715 Spectrum Analyzer Service Manual
NOTE. For instruments with serial numbers B030000 or above,adeadtime
control (DTC) R669 has been addedw hichw ill beused to set the maximum high
voltage measuredat the –2.75K V testpoint by varying the voltage at pin 1 of
U260. New values for C262 and R652 have been chosen to lower the effectof the
leakage currentfrom pin 4 of U260. Any pin 4 leakage current through R652 will
reduce Power Supply performance. Supply regulationfor U260 has been
improved by lowering the resistanceof R261.
The low impedance, fast switching ofthe MOSF ETsis slowed down by series
inductors L241 and L250. As each MOSFET turns on, the drain inductor,
transformer leakageinductance, 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 energyin both conditions.
Frequency Lock. The inverter frequencyis approximately 26 kHz and the
pre-- regulatoroperates at 3 times this rate. The D flip flop U670A is clocked at
78 kHz with the D input driven at 26 kHz through clampdiode CR661. The
Q output is RC filtered and modulates pin 6 of the PWM IC to maintainthe
3:1 frequency ratio. WhenR 680 is adjustedfor 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 filteredoutput to the PWM will reduce its
frequency,and increase the frequency when low. This lock preventslow
frequency IM betweenthe two frequencies that could be audible. The lock can
occur at various ratioswith 3:1 chosen so the predominant harmonic of the
inverter coincides with theP WMfundamental.
The +5 V potentiometer, R937, is used to adjust +5 V1, the reference voltage.
This referencevoltage is compared with the +5 V supply by U630B, and an error
voltage is generated. Feedbackthrough the optoisolator U281 controls the pulse
width of U280 on the primary side. The centertap voltage on the primary side of
the power transformeris adjusted this way to correct small changes in +5 V.
Other secondary DC voltages aredetermined 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 energystored in the power transformer
leakage inductances.Choke/capacitor(lc) filter networks at the output of each
rectifier reduce ripple to acceptable lev els for instrument power.
Most of the available power flows to the +5 V, +10 V,and --11V supplies. The
+20 V supply is lightly loaded and generatedby voltage doubling. The fan
supply is --11V. The +50 V and +115 V supplies are used for Z axis control,
CRT adjustments, and deflectionamplifiers. The --5V1is a stable supply
generated by a three terminalregulator for use in the deflection amplifiers.
Secondary regulation is providedfor the high voltage supplies. U630A and Q130
control the peak to peak amplitudeof the high voltage AC voltage seen by both
cathode and anode rectifiers.Constant output and post deflection anode ratio is
maintained as internal loadingvaries. This keeps the display fixed in size.
Secondary