Circuit Descriptions
Power Supply - DC Regulation
Diode bridge BR2 rectifies the full output of the transformer; C56 and C57 are the reservoir
capacitors which supply the ± 15V regulators. Fine adjustment is provide by VR23 and VR24.
The ± 12V rails are derived from the ± 15V rails with D31 and D32.
The +6V rail is derived from the +15V rail with D33.
IC39 is a 2.5V reference; IC41B generates +5VA, fine adjusted by VR38.
IC41C and D provide ± 12VA.
Diode bridge BR1 rectifies the lower output of the transformer; C131 and C132 are the reservoir
capacitors which supply the ± 5V regulators.
Waveform Generation - Summing Amplifier and Current Sources
The dial and sweep voltages are summed by IC36B; the output range of the amplifier is
approximately –4mV to –4V for a 1000:1 frequency change. Q1 gives increased current capability
to drive R24 and R76. When the symmetry button is depressed R24 and R76 are increased by a
factor of 10 with the symmetry control in mid-position, decreasing the output frequency by a factor
of 10. This allows a symmetry range of 10:1.
IC2C refers the output of IC36B to the +12V rail; IC4 and Q4 mirror this into the diode gate.
IC2A inverts the output of IC36B, IC2B refers it to the –12V rail, IC22 and Q18 mirror this into the
diode gate.
Waveform Generation - Range Selection and Triangle Amplifier
Ranges 5kHz to 5MHz use decade steps of range capacitor. The lower three ranges use a
capacitance multiplier. The capacitance multiplier steals current from the capacitor via R101.
The triangle amplifier uses Q7 as a source follower and Q8 as a current source; Q7 and Q8 are
Idss matched. Q9 and Q10 are emitter followers to give two low impedance outputs. The stage
has unity gain and the offset is trimmed by VR14.
Waveform Generation - Comparator and Aux Output
IC5 operates as a window detector and determines the peak to peak amplitude of the triangle
wave on the range capacitor, which is approximately ± 1V. Capacitors C15 to C20 compensate for
comparator delay ensuring that the triangle wave amplitude remains constant with increase in
frequency. The two internal NAND gates in IC5 are wired as a flip-flop to ensure positive
switching of the comparator.
Q6 level shifts the output of IC5 to be about ground, to drive the diode gate. IC6 drives the Aux
Output with a TTL/CMOS compatible signal.
Waveform Generation - Waveform Shaping
The sine shaper comprises a monolithic transistor array IC17 which is driven by the triangle
amplifier. The circuit has two non-linear stages. A pair of emitter followers A and D round the
peaks of the triangle; the output of this stage is impressed across R103 and R108 in series and
applied to the second stage. This comprises a differential pair B and C driven almost into cut-off
and converts the clipped triangle into a sinewave. VR10 adjusts the gain of this pair to bring them
close to cut off, minimising sinewave distortion. VR11 adjusts the dc operating point of the
sinewave converter to give symmetrical operation on both positive and negative halves of the
waveform.
The squarewave shaper is a diode bridge D36 to D39, driven by a level shifter Q26, which steers
current into and out of R91 and C32. This gives a clean waveform which is symmetrical about
ground. When squarewave is not selected SW10A gates the signal off at IC6B.
Preamplifier
The waveform selected by IC14 passes to the preamplifier IC18. The 1·8V pk to pk output is
connected to the Amplitude control via the AM switch.
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