Tektronix TSG 95, Service Manual

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Theory of Operation
TSG 95 Service Manual
3–5
A programmable sine wave generator, U12, generates audio tones. U12 receives
a serial data stream from the microprocessor (U2) through pins 6, 7, and 8. This
data sets the tone frequency. Pin 3 of U12 is used by U2 to power down the tone
generator. A 3.375 MHz input clock is applied to U12–16 from the ASIC (U26).
A reference voltage is applied by a digitally controlled potentiometer (U16) to
U12. Pins 1, 2, and 7 of U16 interface with the
m
P (U2). The 2.5 V reference
(U20) sets the high voltage of U16, and potentiometer R53 sets the low voltage.
The wiper, U16–5, is connected to the reference input of U12. The minimum
tone level is set with R53, then the output levels are set through the keypad input
to the
m
P.
The tone output of U12 is filtered through R50, L10, and C58. C12 is used to
AC-couple the tone to avoid DC offset.
U13 provides output amplification for the channel 1 tone. U13A is an inverting
amplifier with a gain of 1.3 set by R22, R36, and R18. Q3 pulls the input of the
amplifier to ground when a high level is applied, by U22, to the base of Q3. Q3
is used to apply an identifying click to the signal. Q10 causes an attenuation of
the signal allowing output of –10 dBu. Fine tuning of the –10 dBu level is done
through the keypad using U16. U13B is an inverting amplifier that inverts the
output of U13A to create the other half of a balanced output. R20 and R21 form
a 50
Ω
balanced output impedance. U13 is a special op-amp that goes into a low
power “sleep” mode when either a low-load current is required, or the input volt-
age is low. R54 is used to keep U13 from going into this sleep mode for high
impedance loads.
U17 provides output amplification for the channel 2 tone in the same manner.
Power is applied to the TSG 95 either through a DC input jack on the side of the
instrument, or through batteries in the battery compartment. The input voltage
should be limited to 15 V to avoid a higher than suggested charge rate for the
NiCad battery pack.
The battery compartment is designed to accept eight alkaline batteries, or a Ni-
Cad battery pack. The NiCad pack applies its positive voltage and accepts its
charge from special contacts in the battery compartment. The special contacts are
used to prevent the application of charging current to alkaline batteries. The +
side of the Nicad pack uses a special contact to prevent the application of a re-
verse voltage in the event that a battery pack is installed incorrectly.
The charging of the NiCad pack is done through Q6. The current through Q6 is
set by resistors R57 and R58. The voltage across R57/R58 is the voltage of CR3
minus the base-emitter voltage of Q6. CR3 and R56 set the base bias voltage
for Q6.
The battery voltage passes through the DC input connector J5 before it is applied
to the power supply. This allows J5 to disconnect the batteries from the power
Tone Generation
Power Supply