Here are two new solutions to the problem of independent frequency and fill factor adjustment in RC rectangular pulse generators.
Previously-known schemes for RC rectangular pulse generators with a regulated duty cycle had an ineradicable disadvantage: changing the pulse fill factor (D) inevitably changed the frequency of generation. I have described elsewhere generator schemes with separate frequency and fill factor control (references 1-3). Here are two new solutions to the problem of independent frequency and fill factor adjustment in RC generators.
The generator in Figure 1 works as follows: When the device is switched on, capacitor C1 is discharged, so the output of U1A (U1) is high. This high output rapidly charges capacitor C1 through the D1-R4 chain until C1 reaches the logic element’s low-to-high switching voltage threshold, at which point the output of U1A goes low. When U1A’s output goes low, the capacitor starts discharging through the parallel combination of R1 and R2+R3, following an exponential curve (Figure 2). When C1 reaches the high-to-low switching threshold, the output of U1A goes high, and the cycle repeats.
Figure 2 The pulse train that the generator creates is independent of the output duty cycle.
Hysteresis in the switching thresholds of U1A help dictate the resulting pulse train’s frequency. Using a supply voltage (Vdd) of 15V and the formula given on the CD4093 data sheet (page 4), the switching frequency is roughly:
f (in kHz) = 1,152/(Rs x C1),
where Rs = ((R1 x R2) + (R1 x R3))/(R1 + R2 + R3).
Because of the way the potentiometer is connected, the resistance R3 that the circuit sees is variable and so the potentiometer can be used to set the frequency. The generator will operate reliably in the frequency range 200 to 1100 Hz and remains operational up to a frequency of at least 500 kHz.
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The input pins of the logic element U1B are also connected to the capacitor C1 as well as to the middle pin of the potentiometer R1. Adjusting this potentiometer, which is acting as a voltage divider, allows you adjust what fraction of the voltage on C1 U1B input pin 6 sees. A large fraction means that the input will reach the switching threshold more quickly, while a small fraction will reach threshold more slowly. Adjusting R1, then, allows you to smoothly change the moment in the cycle that logic element U1B will switch and, accordingly, allows you to adjust the width of its output pulses (Uout). Duty cycles from near 0% to 100% are achievable. Further, this duty cycle adjustment does not affect the frequency of generation.
The alternative rectangular pulse generator shown in Figure 3 operates in the frequency range 27 to 1000 Hz and also allows you to separately adjust the frequency and fill factor. The device consists of a sawtooth pulse generator (U1 – CD40106) and a comparator (U2 LM339) with an adjustable switching threshold.
The sawtooth pulse generator uses Q1 to provide a stable current source for the RC generator created by the U1A CD40106 element. The combination creates a sawtooth-shaped voltage at capacitor C1, the shape of which does not change when the frequency changes (Figure 4). Potentiometer R2 controls the current source and thus the charging rate of C1, which determines the pulse frequency. Potentiometer R6 controls the comparator’s (U2A LM339) switching threshold and, accordingly, the output (Uout) waveform’s fill factor. Resistors R5 and R7 set limits for adjusting the pulse fill factor D.
Figure 4 The linear rise time of the sawtooth ensures that the duty cycle is also a linear function of the potentiometer setting.
Because the voltage on capacitor C1 changes linearly with time, the width of the output signal also changes linearly when adjusting potentiometer R6. This is in contrast to the first generator, which has a non-linear adjustment for the fill factor.
This article was originally published on EDN.