#382 CD4046/VCO

Audio range voltage-controlled oscillator using the CD4046 PLL/VCO IC.

Notes

The 4046 includes a VCO sub-system in addition to phase comparators and other components.

As noted in the Art of Electronics (p291, Second Edition):

When shopping for VCO chips, don’t overlook the ICs known as phase-locked loops (PLL), which contain both a VCO and a phase detector. An example is the popular CMOS 4046 (and its faster cousin, the 74HC4046).

The VCO produces a square wave with 50% duty cycle, and a frequency range of approaching 0 Hz to over 1 MHz (1.3 MHz at 9V VDD). The VCO frequency is determined by the voltage at VCO IN (pin 9). Voltage-to-frequency linearity is about 1%.

The VCO can be used independently. This project is inspired by Ray Marston’s “CMOS Clock Circuits” article, and simply demonstrates using the VCO sub-system as a voltage-controlled signal source with a frequency range in the audio spectrum of about 20 Hz to 20 kHz.

Output

The datasheet I have doesn’t include specific ratings for the VCO output, but seems to imply nothing greater than ~8mA. The device does not really appear to be desinged to drive any significant load.

For that reason, I’m using the output to switch a load via a MOSFET. While testing, I just had an LED indicator wired up. Since I’m aiming for an audible range, this could be a speaker output.

Determining Minimum and Maximum Frequency

External R/C components determine the minimum and maximum frequency of the VCO.

• C1 - across pins 6 and 7. C1 ≥ 50 pF
• R1 - connected to pin 11. R1 ≥ 10kΩ
• R2 - connected to pin 12. R2 ≥ 10kΩ

Minimum frequency (Fmin) is determined by C1 and R2 time constant.

Although not mentioned in the datasheet, a “rule of thumb” estimate is 2 x time constant:

fmin = 2/(C1R2)
fmax = 2/(C1(R1||R2))

With R1=10kΩ, R2=10MΩ, C1=10nF, these produce the following estimates:

• fmin = 20Hz
• fmax = 20kHz

These are reasonably close to the 30Hz - 22.4kHz range I’ve measured in practice.

Test Results

Measured for various combinations of R1, R2, C1:

Scope: R1=10kΩ, R2=10MΩ, C1=10nF

Measured minimum frequency:

Measured maximum frequency:

Construction

Credits and References

• CD4046 Datasheet
• CMOS Clock Circuits - Ray Marston. Radio-Electronics, November 1984.
• Practical Oscillator Handbook - includes a page on CD4046 VCO
• Practical Oscillator Circuits - apparently has good coverage of CD4046 VCO (I haven’t read it)
• The Art of Electronics