Functional Description
The relationships between the major circuit elements are shown in the block diagram.
The summing amplifier sums the voltages from the dial, the frequency lock DAC, sweep input and
the internal sweep circuit and its output controls the magnitude of the complimentary current
source and current sink. This current varies from approximately 4·3uA to 4·3mA for a 1000:1
frequency change (·005 to 5·0). The symmetry control adjusts the ratio between the current
source and current sink.
The diode gate steers current into and out of the range multiplier capacitor and is controlled by
the comparator output. When the comparator output is high the charge on the capacitor will rise,
linearly, producing the positive going triangle slope. When the comparator output is low the
charge on the capacitor will fall linearly producing the negative going triangle slope.
The triangle amplifier has unity gain and buffers the triangle wave on the multiplier capacitor to
drive the comparator and output circuits.
The comparator operates as a window detector with fixed limit points set to the triangle peaks.
One of its two outputs drives the TTL/CMOS output. The other output is level shifted to drive the
diode gate and squarewave shaper. When the comparator output to the diode gate is high the
triangle wave is positive-going until this reaches approximately +1V, when the comparator output
switches low. When the comparator output is low the triangle wave is negative-going until it
reaches approximately –1V, when the comparator goes high, and the cycle is repeated. This
basic function generator loop is shown by the double arrows in the block diagram. Triangle and
squarewave are generated simultaneously as shown.
To achieve the 500Hz, 50Hz and 5Hz ranges, the 5k range capacitor is multiplied by 10, 100 or
1000 by the capacitor multiplier circuit.
The sinewave converter uses the non-linear characteristics of a transistor pair to convert the
triangle wave into a sinewave. The selected function is amplified by the pre-amp and applied to
the amplitude control directly or via the amplitude modulator. The signal is summed with the
voltage from the DC offset control at the output amplifier, which amplifies the signal up to 20V
peak to peak to drive the main output socket.
The microcontroller measures and displays frequency directly down to 0·2Hz; below this the
microcontroller’s A to D converter measures the voltage at the summing amplifier and displays the
corresponding frequency. Amplitude and offset are measured by the A to D converter too.
The sweep generator provides a triangle wave with variable amplitude and speed that is summed
with the dial at the summing amplifier.
External Count In is amplified by a comparator and measured by the microcontroller.
The power supply converts the incoming AC line voltage to the DC rails required by the
instrument.
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