GW Instek GFG-3015 - Manual (Page 53)

GFG-3015 p.49
(3) V.C.F
Basically, the Voltage Control Frequency unit is to transform voltage into
frequency. It consists of a main frequency D/A, a duty cycle D/A, a
positive/negative constant current source, a diode switch unit, a buffer, a level
detector and an integral capacitor, and etc.
The MPU puts a specific value to the main frequency D/A which will generate a
corresponding voltage for the reference of the duty cycle D/A, then two different
polar voltages for current source unit will be generated by the duty cycle D/A, one
is a positive voltage from the positive duty cycle D/A and the other is a negative
voltage from the negative duty cycle D/A.
The current source block will transform two different polar voltages of the duty
cycle D/A output into two different polar correspond constant current. These
currents will be charged or discharged by the diode switch unit to the integral
capacitor C
T
, and the voltage of the C
T
will become a continuous symmetrical
triangle waveform.
The diode switch movement is controlled by the level detector and the voltage of
level detector from the triangle waveform, so does the triangle wave oscillator.
If the positive and negative duty cycle D/A has different values, then the current of
charge and discharge will be different. Therefore, the triangle waveform should
have unsymmetrical duty.
(4) Frequency setting
The same status as the VCF above, the MPU puts a specific value to the main
frequency D/A, the VCF unit will generate a correspond frequency (Symmetrical
triangle wave), then, input a desire frequency value to the instrument, the MPU
will set the frequency accordingly.
(5) Description of every kind of Waveform
The GFG-3015 provides many kinds of waveforms including Sine, Triangle,
Square, Ramp and Pulse. Please refer to the following for details:
1. Triangle and Ramp Waveform
The same status as the VCF above, the output voltage of the VCF unit is a
symmetrical triangle wave which can pass the waveform choice, the output
amplifier, the ATT, and the output through the Main Output terminal.
Regarding the Ramp wave, if the positive and negative duty cycle D/A has
different value, then the triangle waveform should have unsymmetrical duty,
that is the positive and negative Ramp waveform.
2. Sine Waveform
The procedure of generating triangle waveforms is similar to that of
generating the sine waveforms, except that the triangle signal can pass a sine
wave shaper circuit between the VCF unit and the output amplifier, and the
shaper circuit can change the waveform type from triangle to sine.
3. Square and Pulse Waveform
The procedure of generating the triangle waveforms is similar to that of
generating the square waveforms. except that the triangle signal can pass a
square wave shaper circuit (Comparator circuit) between the VCF unit and the
output amplifier.
Regarding the Pulse waveform, if the positive and negative duty cycle D/A
has different value, then the square waveform should have unsymmetrical
duty, that is the positive and negative Pulse waveform.