Cascode Cross-Coupled LC VCO for 5G

• Cascode cross-coupled LC VCO topology is a differential oscillator design integrating stacked NMOS devices with an LC resonant tank to enhance tuning range and phase noise performance.
• It employs a cascode configuration that effectively doubles negative transconductance, achieving a tuning span of 5.1 GHz compared to conventional VCOs.
• Targeted for 5G millimeter-wave applications, the topology delivers high sensitivity (~8.0 GHz/V) and low phase noise, critical for advanced wireless systems.

A cascode cross-coupled LC voltage-controlled oscillator (VCO) is a differential oscillator topology that integrates a stacked architecture using cross-coupled and cascode NMOS devices in conjunction with an LC resonant tank. The configuration is engineered to simultaneously enhance frequency tuning range, VCO gain, and phase noise performance beyond those achievable by conventional single-level cross-coupled LC VCOs. Recent research specifically characterizes this topology for 5G millimeter-wave applications, emphasizing its analytical operation, design equations, and comparative advantages in advanced wireless systems (Bui et al., 11 Nov 2025).

1. Topological Structure and Functional Principle

The cascode cross-coupled LC VCO comprises two stacked NMOS transistor pairs arranged differentially. Transistors M1/M3 function as the primary cross-coupled tail pair, with their drains coupled to the supply VDD through parallel LC tanks and bias resistors. Cascode transistors M2/M4, biased in the triode region, are inserted between the cross-coupled pair and ground. This intermediate stage raises the impedance at the virtual-ground node, suppressing direct gate-drain feedthrough and enhancing the oscillator's negative resistance.

The resonant tank features paired inductors (LP), metal-insulator-metal capacitors (Cp), diffusion capacitors (CL), and a differential varactor pair (Cvar) whose capacitance is modulated by the tuning voltage ($V_{\rm tune}$). This arrangement increases the total tank capacitance and modulates oscillation frequency, thereby extending the achievable tuning range.

Key circuit characteristics:

• M1/M3: Cross-coupled, operating in saturation, provide primary negative resistance.
• M2/M4: Cascode, operating in triode, increase output impedance and suppress parasitic paths.
• Tank: Differential, comprised of LP, Cp, CL, Cvar, all contributing to $C_{\rm eq}$.

2. Small-Signal Modeling and Negative Resistance

The half-circuit small-signal model replaces M1/M3 with their transconductance ($g_{m1}$), output resistance ($r_{o1}$), and gate-source capacitance ($C_{gs1}$); M2/M4 are reduced to a small-source resistance $r_{ds2}\approx 1/g_{m2}$ and gate-source capacitance $C_{gs2}$. Analyzing the input admittance at resonance, the circuit presents a negative real part due to the regenerative feedback in the cross-coupled pair.

The negative resistance of the stacked structure is analytically determined:

$R_{\rm neg} \approx -\frac{2}{g_{m1}}$

This value is approximately double that of a standard cross-coupled pair, as the cascode configuration effectively enhances the negative transconductance presented to the tank (Bui et al., 11 Nov 2025).

3. Start-up Condition and Stability Criterion

Sustained oscillation necessitates that the magnitude of generated negative conductance exceeds losses in the LC tank. The oscillation build-up criterion is:

$g_{m,\rm eff} \geq \frac{1}{R_p}$

Given $R_{\rm neg}\approx -2/g_{m1}$, this condition specifies:

$2g_{m1} \geq \frac{1}{R_p}$

Where $R_p$ is the equivalent parallel resistance of the tank at resonance. This threshold ensures start-up and maintenance of oscillatory behavior.

4. Resonance Frequency and Tuning Mechanism

The oscillation frequency is governed by the resonant properties of the LC tank. The composite tank capacitance, uniquely affected by the cascode stack and varactor arrangement, is:

$$C_{\rm eq} = 0.875\,C_{gs1} + C_p + C_L + \frac{C_{\rm var}}{2}$$

The corresponding angular frequency is:

$\omega_0 = \frac{1}{\sqrt{L C_{\rm eq}}}$

And the oscillation frequency in hertz:

$$f_0 = \frac{1}{2\pi\sqrt{L \left(0.875\,C_{gs1}+C_p+C_L + C_{\rm var}/2\right)}}$$

Tuning is achieved by modulating $C_{\rm var}$ via $V_{\rm tune}$, resulting in a frequency span from 21.0 GHz to 26.1 GHz (5.1 GHz range) for the cascode topology, a substantial extension over conventional VCO configurations (22.6–26.8 GHz, or 4.2 GHz span) (Bui et al., 11 Nov 2025).

5. VCO Gain and Sensitivity

The gain or sensitivity ($K_{\rm VCO}$) quantifies the frequency change per unit control voltage. It derives from differentiating the oscillation frequency with respect to $V_{\rm tune}$:

$$K_{f} = \frac{\partial f_0}{\partial V_{\rm tune}} = -\frac{f_0}{2\,C_{\rm eq}} \frac{d\,C_{\rm var}}{dV_{\rm tune}}$$

Simulated results at mid-tune ($V_{\rm tune}=0.5$ V) yield $K_f \approx 8.0$ GHz/V for the cascode topology, compared to 5.3 GHz/V for the baseline cross-coupled pair, underscoring the increased frequency agility facilitated by the augmented tank capacitance and enhanced negative resistance (Bui et al., 11 Nov 2025).

6. Phase Noise Characteristics

Phase noise is analyzed using the Leeson model:

$$L(\Delta\omega) = 10\log_{10}\left[\frac{kT}{2 V_{\max}^2 R_p} \left(1+\left(\frac{\omega_0}{2Q\Delta\omega}\right)^2\right) \right]$$

Where:

• $kT$ is thermal noise power,
• $R_p$ is the tank’s parallel resistance,
• $Q = R_p \sqrt{C_{\rm eq}/L}$ is the unloaded tank quality factor,
• $P_{\rm sig} = V^2_{\max} / 2R_p$ is the signal power.

The cascode arrangement enhances $R_p$ and $C_{\rm eq}$, lowering the noise floor without detriment to the Leeson shaping term. Simulated phase noise at 800 MHz offset is –155.7 dBc/Hz for cascode and –154.4 dBc/Hz for the conventional case; at 10 MHz offset, –116.26 dBc/Hz and –116.53 dBc/Hz, respectively.

7. Comparative Performance and Design Trade-offs

The following table summarizes principal performance metrics:

Metric	Cascode Cross-Coupled VCO	Conventional VCO
Frequency Tuning Range	21.0–26.1 GHz (5.1 GHz)	22.6–26.8 GHz (4.2 GHz)
VCO Gain (@0.5 V tune)	8.0 GHz/V	5.3 GHz/V
Phase Noise @10 MHz offset	–116.26 dBc/Hz	–116.53 dBc/Hz
Phase Noise @800 MHz offset	–155.7 dBc/Hz	–154.4 dBc/Hz

Reported trade-offs include increased tank capacitance and negative resistance (beneficial for tuning range and phase noise), with a minor penalty in the form of additional device capacitance in the feedback path. The slight reduction in maximum oscillation frequency for a fixed inductor L is offset by the expanded tuning bandwidth and improved noise properties (Bui et al., 11 Nov 2025).

8. Application Context and Technical Significance

The cascode cross-coupled LC VCO architecture is explicitly targeted for wideband, low-phase-noise millimeter-wave oscillators in 5G transceiver front-ends. By substantially increasing both negative resistance and tank capacitance via device stacking, this topology achieves performance levels that directly address requirements for wideband frequency synthesis and low-jitter clocking in next-generation wireless systems. A plausible implication is that further research will optimize the trade-off between maximum frequency, tuning range, and phase noise for specific technology nodes or integration constraints.

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Principal reference: "Wide Tuning Range and Low Noise Voltage Control Oscillators for 5G Technology" (Bui et al., 11 Nov 2025).