How to use Electronic Frequency Control to Enhance Crystal Oscillator Design

Want to know what's better than a crystal oscillator? A crystal oscillator combined with Electronic Frequency Control (EFC). Of course, it really comes down to your specific application and what requirements you're looking for to determine if EFC would be a good addition to your crystal oscillator circuit design, and if so, which method is best for you.

There are 4 options to choose from when selecting an Electronic Frequency Control method for your crystal oscillator. These 4 options are

1. Pulse Width Modulation & Low Pass Filter
2. Reference RF Signal & Phase Locked Loop (PLL)
3. Voltage Divide
4. Digital-to-Analog Converter (DAC)

In this post, let's take a closer look at each option and compare the pros and cons between them.

Option A: Pulse Width Modulation & Low Pass Filter

Pulse Width Modulation is a way to control analog devices with a digital output. The signal can be varied to determine how long a signal is "high" in analog form. While a PWM can only be high (typically 5V) or low (ground) at a given time, you can adjust the amount of high vs. low time over a consistent time interval. This will allow you to achieve an average operating voltage that is emitted.

If the pulse is driven high 50% of the time, we call this a 50% duty cycle. The term duty cycle is used elsewhere in electronics, but in every case, duty cycle is a comparison of high vs low. 

Pulse Width Modulation can be used in quite a few applications including sophisticated control circuitry.  Their main use is for controlling DC motors but it can also be used to control valves, pumps, hydraulics, and other mechanical parts.

Additionally, a Low Pass Filter is commonly used to smooth the PWM output before heading to a quartz crystal oscillator. A Low Pass Filter will convert your PWM output into a voltage corresponding to the percentage of the PWM waveform.

Option B: Reference RF Signal & Phased Locked Loop (PLL)

A reference RF signal is any outside reference frequency. This typically includes cesium, rubidium, or a superior performing crystal oscillator (AKA Master Reference Oscillator).

The reference RF signal is then sent to a Phased Locked Loop (PLL). A PLL consists of 3 components

1. Phase Detector (PD)
2. Low-pass filter (LPF)
3. Voltage Controlled Oscillator (VCO)

Although Phase Detector is common terminology, it's actually a Phase Difference Detector. The PD accepts two periodic input signals and produces an output signal that represents the phase difference between the two.

That output is then transferred to the Low-pass filter. We already chatted about LPFs above, so no real need to explain it again here. Although it is worth mentioning that the output from the PD isn't a straightforward analog signal that's proportional to the phase difference. The LPF helps eliminate unnecessary high-frequencies and transforms the signal into an output that controls a voltage controlled oscillator.

Next in line is the Voltage Controlled Oscillator. It's an oscillator, well, controlled by a voltage. More specifically, the output frequency of the oscillator is precisely controlled by a voltage. Therefore, the VCO is a variable-frequency oscillator that allows an external voltage to influence its frequency of oscillation. In this case of a PLL, the external voltage comes from the Phase Detector and Low-pass filter. 

Option C: Voltage Divider

A Voltage Divider is a simple circuit that turns a large voltage into a smaller one. You can use just two series resistors and an input voltage to create an output voltage that is a fraction of the input. Voltage Dividers are considered to be one of the most fundamental electronic circuits.

You can use the voltage divider equation to determine the output of a voltage divider. For this equation you'll need to know

1. Input voltage (V_in)
2. Both resonator values (R₁ and R₂)

Being one of the most fundamental circuits, Voltage Dividers are used in seemingly endless applications. Some common ones include potentiometers, sensors, and level shifters.

While there are almost endless applications for Voltage Dividers, they should not be used as a load's power supply.

Option D: Digital-to-Analog Converter (DAC)

Shockingly enough, a Digital-to-Analog Converter (DAC) converts digital signals into analog signals... so it's a frequency converter... (WHAAA?!)

Anyway, there are many DAC architectures that are suitable for different applications. These designs are determined by metrics such as resolution and the maximum sampling frequency.

Digital-to-analog conversion can degrade signals, so a DAC should be specified that has very little errors in terms of the application.

The most specialized DACs are implemented as integrated circuits (ICs) because of the complexity and the need for precisely matched components. Discrete DACs would typically be extremely high speed low resolution power hungry types, as used in military radar systems. Very high speed test equipment, especially sampling oscilloscopes, may also use discrete DACs.