TTI TG501, Руководство по обслуживанию, Страница: 12 / 33

Waveform Generation - Waveform Shaping
The squarewave shaper is a diode bridge which steers current from either R89 or R90 into R91.
This provides a squarewave with controlled rise/fall times, and thus no overshoot and ringing, which
is symmetrical about ground. The drive signal comes from Q17.
The sine shaper comprises a monolithic transistor array IC18 which is driven by the triangle
amplifier. The circuit has two non linear stages. A pair of emitter followers round the peaks of the
triangle which are adjusted by VR18 for the bottom of the waveform and VR22 for the top of the
waveform. The output from this stage is impressed across R78 and R81 in series and applied to the
second stage. This comprises a long tailed pair driven almost into cut off and converts the clipped
triangle wave into a sinewave. VR19 adjusts the gain of the long tailed pair to bring them close to
cut off and therefore minimum sinewave distortion. VR23 adjusts the dc operating point of the
sinewave convertor to give symmetrical operation on both positive and negative halves of the
waveform. VR20 adjusts the dc output of the convertor and VR21 output amplitude.
Output Preamplifier
The selected waveform passes to the preamplifier. Q20 and Q21 form a long- tailed pair and are in
thermal contact with each other to reduce dc drift. Q22 and Q23 are two cascaded emitter
followers; feedback is via R98. The preamplifier is an inverting virtual earth type. DC offset is
trimmed by VR26. Sine, square and triangle waveform amplitudes are set for 10V peak to peak into
50 Ohm at the 50 Ohm output by VR21, VR24 and VR25 respectively.
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