Synthesizer
Understanding Frequency Synthesizers
Frequency synthesizers combine the stability of a crystal oscillator with the tunability of a VCO. The PLL continuously adjusts the VCO frequency to maintain a fixed phase relationship with the crystal reference. The output frequency is f_out = N x f_ref, where N is the PLL divider ratio.
Synthesizer Architectures
- Integer-N PLL: Output frequency is an integer multiple of the reference. Simple but limited frequency resolution (step size = f_ref).
- Fractional-N PLL: Uses a sigma-delta modulator to achieve fractional division ratios. Fine frequency resolution (sub-Hz) with low phase noise.
- Direct Digital Synthesis (DDS): Generates waveforms digitally from a lookup table and DAC. Very fast frequency switching (< 1 us). Limited to lower frequencies (typically < 1 GHz output).
- Hybrid: DDS + PLL combinations for wide tuning with fast switching and fine resolution.
f_out = N x f_ref
Step size = f_ref
Fractional-N PLL:
f_out = (N + F/M) x f_ref
Step size = f_ref / M (very fine)
Phase noise improvement from PLL:
Inside loop BW: phase noise = 20log10(N) + ref noise
Outside loop BW: VCO free-running noise
Frequently Asked Questions
What is a frequency synthesizer?
A synthesizer generates precise output frequencies by locking a tunable VCO to a stable crystal reference using a PLL. It provides the frequency accuracy of a crystal oscillator with the tunability of a VCO. Output frequencies can be set with sub-Hz resolution.
What is the difference between integer-N and fractional-N?
Integer-N PLLs can only output integer multiples of the reference frequency, limiting step size. Fractional-N PLLs use sigma-delta modulation to achieve fractional division, enabling much finer frequency resolution (kHz or Hz steps) while maintaining low phase noise.
How fast can a synthesizer switch frequencies?
PLL synthesizers switch in 10-100 us (determined by loop bandwidth). DDS synthesizers switch in under 1 us. YIG-tuned synthesizers take 1-10 ms. For radar and frequency-hopping applications, fast switching is critical and drives the architecture choice.