DDS vs PLL: Understanding the Key Differences

DDS (Direct Digital Synthesizer) and PLL (Phase-Locked Loop) are both techniques employed in electronic signal generation and frequency synthesis, but they function based on fundamentally different principles. Let’s explore the key distinctions between them.

DDS (Direct Digital Synthesizer)

Figure 1: Internal functional modules of a DDS chip

As illustrated above, a DDS chip primarily consists of a phase accumulator and a phase-to-amplitude converter.

The binary tuning word (N) serves as the input to the phase accumulator. The phase register then generates binary phase information, usually represented with a specific number of bits. This tuning word dictates the output frequency as a function of the reference clock frequency.

The following formula calculates the output frequency of a DDS chip:

F_OUT = M * F_ref / 2^N

Where:

• F_OUT = The output frequency of the DDS
• M = Binary tuning word (e.g., 32 bits of binary information)
• F_ref = Reference clock frequency
• N = Length in bits of the phase accumulator (e.g., 32). This determines the frequency resolution.

Using this equation, the value of M is determined based on desired values of F_OUT, F_ref and N.

PLL (Phase-Locked Loop)

Figure 2: RF Synthesizer

A PLL operates as a feedback control system. It compares the phase of a reference frequency against the phase of the output signal emanating from a Voltage-Controlled Oscillator (VCO).

The PLL adjusts the VCO’s frequency to ensure the phases align, effectively locking the output frequency to a multiple or fraction of the reference frequency.

A PLL’s operation unfolds in three stages:

1. Free-running
2. Capture mode
3. Phase-locked state

The output frequency of a frequency synthesizer based on PLL technology can be calculated using these formulas, depending on its type:

1. F_out = N * (F_r / R), for Integer PLL
2. F_out = (N + F) * (F_r / R), for fractional PLL

Key Differences Between DDS and PLL

Here’s a breakdown of the primary differences between DDS synthesizers and PLLs:

1. Operating Principle:

   • DDS: Generates waveforms directly in the digital domain. A digital accumulator creates a phase accumulator, which is then transformed into an analog signal via a Digital-to-Analog Converter (DAC).

   • PLL: Relies on comparing the phase of a reference signal with a feedback signal. This feedback signal originates from dividing down the output of a Voltage-Controlled Oscillator (VCO). The PLL adjusts the VCO’s frequency to minimize the phase difference between these two signals.

2. Frequency Resolution:

   • DDS: Typically offers high frequency resolution and agility.
   • PLL: Generally provides lower frequency resolution compared to DDS.

3. Spurious Components:

   • DDS: Tends to have fewer spurious components and lower phase noise, resulting in cleaner output signals.
   • PLL: May introduce more phase noise and spurious components, particularly at higher frequencies or when employing frequency dividers.

Conclusion

In essence, DDS excels in applications requiring high resolution and agility, while PLLs are favored for their stability and phase-locking capabilities. The optimal choice depends heavily on the specific needs of the application.

Consider the design of an L-Band RF Synthesizer utilizing a DDS chip alongside other discrete components. The L-Band, spanning from 950 MHz to 1450 MHz, is commonly utilized in satellite communication systems.