PLL Synthesizer Architecture
A phase-locked loop (PLL) frequency synthesizer generates precise output frequencies from a stable reference crystal oscillator. The basic components are:
- Reference oscillator: TCXO or OCXO, typically 10–100 MHz
- Phase-frequency detector (PFD): Compares reference and divided VCO
- Charge pump: Converts phase error to current
- Loop filter: Low-pass filter sets loop bandwidth
- VCO: Voltage-controlled oscillator, wide tuning range
- Frequency divider: Integer-N or fractional-N
Integer-N vs. Fractional-N
| Parameter | Integer-N | Fractional-N |
|---|---|---|
| Channel spacing | f_ref / N → limited by ref | Fine (f_ref/2^23 possible) |
| Phase noise in-band | 10·log(N²) degradation | Better for same channel spacing |
| Reference spurs | At f_ref offset | At fractional spurs (complex) |
| Complexity | Low | Requires delta-sigma modulator |
Spur Types and Causes
| Spur Type | Offset Frequency | Typical Level | Cause |
|---|---|---|---|
| Reference spurs | f_ref and harmonics | −50 to −80 dBc | Charge pump leakage |
| Fractional spurs | Fractional offsets | −60 to −90 dBc | Delta-sigma quantization |
| VCO harmonics | 2f₀, 3f₀, ... | −20 to −40 dBc | VCO nonlinearity |
| Crystal harmonics | N·f_xtal | −60 to −80 dBc | Crystal overtones |
Phase Noise Profile
PLL synthesizer phase noise has two regions:
- Inside loop BW (f < f_BW): Dominated by reference oscillator + PLL noise floor. Phase noise tracks reference noise multiplied by 20·log(N)
- Outside loop BW (f > f_BW): Dominated by free-running VCO phase noise (1/f² slope)
Optimal loop bandwidth minimizes total phase noise at the cross-over point where reference and VCO noise contributions are equal.
S-Parameter Role in Synthesizer Modules
While synthesizer output purity is characterized by spectrum analyzers, the output port impedance match is measured as an S-parameter. RF View can load the output port S11 (.s1p) of the synthesizer module to verify:
- Output impedance match to 50 Ω (S11 < −15 dB in operating band)
- Harmonic filter performance: S21 of the output low-pass filter
- Module-to-module isolation in multi-channel synthesizers