DS335 Synthesized Function Generator
DS335 Synthesized Function Generator
DS335 Synthesized Function Generator
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5-6 Circuitry<br />
<strong>Function</strong> Selection (<strong>DS335</strong>M6)<br />
The DPDT relay (U603, an HS212) selects between the filtered (and perhaps attenuated) waveform DAC<br />
output, and the output of the square wave generator. If the square wave output is not being used, then the<br />
square wave amplitude will be set to zero (SW_AMPL=-5 Vdc) so as to reduce crosstalk in the function select<br />
relay. The selected function is passed to the differential output amplifier.<br />
Output Amplifier (<strong>DS335</strong>M7)<br />
The output amplifier is a high speed, low distortion, discrete transistor differential amplifier. The gain is x-10<br />
for the inverting input, and x12 for the non-inverting input, and the 3 dB bandwidth is 35 MHz, it has a phase<br />
linear response to about 175 MHz, and a THD of about 0.05% to 50 kHz. The amplifier has a very low offset<br />
drift, as its dc characteristics are stabilized by an external compensated op-amp.<br />
The amplifier has a very symmetric design. This, together with it class A operation and high open loop<br />
bandwidth, keeps it distortion very low. In this circuit description, only the 'top-half' will be described, as the<br />
'bottom-half' of the amplifier operates in a completely symmetrical and complimentary manner.<br />
The signal is applied to the input differential pair (Q701 and Q702, 2N5770's). The input pair run in a constant<br />
current configuration, with the constant current source (Q706 with R741 and D701). The pnp transistor (Q700)<br />
provides an ac current to absorb the feedback current from R705. Since the input differential pair runs at<br />
constant current, independent of the input signal, their base-emitter voltages are constant, and so are not a<br />
source of distortion.<br />
To understand the operation of the amplifier, consider events when a positive input signal is applied. The<br />
base of Q701 goes up, while Q702's base goes down. This increases Q701's collector current, pulling down<br />
the base of the emitter follower, Q709, which pulls the base of Q710 and Q711 down, increasing their<br />
collector currents. This current will cause the collector voltage of Q710 to slew very quickly, and with lots of<br />
gain, because the collector load is a very high impedance. The Darlington pair, Q712 and Q713, buffer this<br />
node to drive the output, which is a 100 Ohm load. The current imbalance at the high impedance node will<br />
cause the output to continue to slew until the feedback current (though R703 and R747) brings the input<br />
differential pair back into balance.<br />
The ac gain is adjusted by setting R703. The ac gain is adjusted to match the dc gain (which is controlled by<br />
the op-amp and its feedback resistor network.) The ac gain is determined by the ratio of the feedback<br />
resistors to the emitter resistors.<br />
The op-amp (U700, a CA3140) and its feedback resistors (R700,701,702 and R723) stabilize the dc<br />
characteristics. The op-amp is externally compensated so that it will only correct low frequency errors, and so<br />
will not affect high frequency performance. The output of U700 is buffered (for extra current drive) by U701A<br />
and U701B.<br />
Output Attenuator (<strong>DS335</strong>M8)<br />
The output attenuator is a relay controlled, 50 Ohm attenuator, which allows attenuation in a binary sequence<br />
of 2 dB steps. When the power is removed (or on RESET) all of the attenuators are switched 'in'. The relays<br />
are controlled by bits written to the 8 bit latch, U800, a 74HC273. Outputs from the latch are buffered by npn<br />
emitter followers (U801, a CA3082), which drive the relay coils. Each 2 dB attenuator can reduce the output<br />
by a factor of 0.794. Output levels between these steps are obtained by adjusting the reference level to the<br />
waveform DAC. The total attenuation is 30 dB, or a factor of 0.0316, which will reduce the 7.94 Vpp level from<br />
the output amplifier to 0.25 Vpp. For ac levels below this, the pre-attenuator will be used (Sheet FG6) to<br />
provide up to 14 dB additional attenuation (for levels down to 50 mVpp).<br />
<strong>DS335</strong> <strong>Synthesized</strong> <strong>Function</strong> <strong>Generator</strong>