Low Phase Noise and Wide-Range Class-C VCO Using Auto-Adaptive Bias Technique

Abstract

This paper proposes a new structure of 24-GHz class-C voltage-controlled oscillator (VCO) using an auto-adaptive bias technique. The VCO in this paper uses a digitally controlled circuit to eliminate the possibility of start-up failure that a class-C structure can have and has low phase noise and a wide frequency range. To expand the frequency tuning range, a 3-bit cap-bank is used and a triple-coupled transformer is used as the core inductor. The proposed class-C VCO implements a 65-nm RF CMOS process. It has a phase noise performance of −105 dBc/Hz or less at 1-MHz offset frequency and the output frequency range is from 22.8 GHz to 27.3 GHz, which consumes 8.3–10.6 mW of power. The figure-of-merit with tuning range (FoM_T) of this design reached 191.1 dBc/Hz.

1. Introduction

Voltage-controlled oscillator (VCO) is a fundamental component used in communication systems requiring a frequency synthesizer. VCO with high output frequency is mainly used in 5G communication system or sensor system. In VCO design, there are four main factors that affect performance—power consumption, phase noise, output frequency and its range. Among these, phase noise is the most important factor in VCO performance. Therefore, design efforts should be made in the direction of improving the phase noise of the oscillator [1,2,3,4,5,6,7,8,9].

Figure 1 shows the structure of the LC VCO. It is an oscillation circuit that receives the input of the control voltage V_C and determines the output frequency with the center tap inductor L and varactor C_var. Figure 1a is a classical class-B VCO using a structure that differentially cross-couples two NMOSs. And the basic structure of class-C VCO is shown in Figure 1b. Since the gate bias voltage, V_B, of the core transistor NMOS must be provided independently, it adds a capacitor to the core to cut the DC. Figure 1c is a class-C VCO using a transformer structure using an inductor without using a relatively large capacitor. Again, V_B is provided independently.

The phase noise shows a sensitive characteristic in the zero-crossing point of the waveform of the VCO. Class-C VCO can improve the phase noise performance by reducing the gate bias of the cross-coupled NMOS and shortening the time passing through the zero-crossing point by giving the current flowing in the drain in a pulse shape [2,5].

The class-C VCO achieves about 2 dB to 4 dB lower phase noise than the typical class-B VCO at the same power dissipation [6,10]. However, there are two major design issues, as the class-C VCO is well known in previous works. One is that VCO start-up problems can occur and the other is how to control the gate bias of the core transistor, which determines phase noise when the VCO oscillates at the target frequency. If the gate bias of the core transistor NMOS is lowered in the class-C VCO, the transconductance is affected and the VCO fails to start operation. In order to solve the start-up problem, methods have been proposed to alleviate the initial oscillation condition in class-C mode. The class-B/C hybrid VCO is a class-B cross-coupled pair added to the class-C core [11,12]. The VCO is easily oscillated by the class-B core and the oscillation is maintained by the class-C core after the initial oscillation. However, the parasitic capacitance of the class-B core influences the LC resonance tank, which has the disadvantage of outputting the wrong frequency. Moreover, since the gate bias voltage point of the VCO core changes when the target output frequency changes, it is difficult to find that point. So, it is not easy to design a class-C VCO with wide frequency tuning range.

Another start-up solution is to directly adjust the gate bias of the class-C core using an amplifier [13,14]. Using the output voltage of the amplifier as the gate bias, the VCO starts at the high gate bias and changes to the low gate bias corresponding to class-C. However, the use of the amplifier degrades the phase noise performance of the VCO and the stabilization time is long due to the narrow bandwidth of the analog feedback loop. In addition, the above methods set the gate bias of the class-C core depending on an externally applied reference voltage, so that the bias cannot be automatically corrected when the oscillation fails.

This paper presents a startup loop to alleviate the startup problem and sets the VCO operating in the optimal phase-noise state. And the proposed VCO is designed to provide consistent output in various PVT conditions with auto-adaptive bias technique.

2. Proposed Class-C VCO Description

2.1. Design of the Class-C VCO Core

Figure 2a shows a proposed class-C VCO core, which consists of varactors, a 3-bit capacitor bank, a triple-coupled transformer and NMOS (M1, M2) cross-coupled pair. The varactor determines the slope of the output frequency according to the input voltage of the VCO (commonly referred to as K_VCO). It can be seen from Leeson’s rule that the phase noise of VCO is also influenced by K_VCO. K_VCO and phase noise are inversely proportional but the smaller the slope, the narrower the tuning range of the VCO. The 3-bit capacitor bank is used to further expand the VCO output frequency tuning range. The capacitor is doubled in size and stacked in 3 stages, with one switch in each stage. The size of the capacitor is determined by modeling the target frequency with data including layout. The transformer has three differential inductors (L1, L2 and L3) with two center taps. The gate biases of the core transistors are applied through the center-tap of L3. This center-tap biasing can eliminate the unnecessary DC-blocking capacitors. The supply voltage V_DD is also applied through the center-tap of L1. The inductor L2 is connected to the output buffer. The inductors have magnetically coupled each other with coupling factors (K₁₂, K₂₃ and K₁₃). Figure 2b shows 3-dimenional view of the transformer. The inductors L1, L2 and L3 are implemented with metal layers of M9, M8 and M7, respectively. Figure 2c is a graph plotting the values of L and Q-factor for the transformers as a frequency change. The main parameters of the VCO are summarized in

Table 1. This table shows the proposed VCO parameter values.

VCO Parameters
Ccap-bank	11–101 fF	Qcap-bank	24–40
L1	185 pH	K12	0.68
L2	215 pH	K23	0.56
L3	276 pH	K13	0.53
QL1	20.35	Cvar	171 fF
QL2	7.89	WM1	32 μm
QL3	3.98	WM2	32 μm

. Parameters for the cap-bank are described when the control code is zero to full code. The value of each parameter is a condition when the operating frequency is 24-GHz and is a simulation result determined after modeling the transformer by extracting layout data.

Class-C VCO requires some minimum gate bias voltage to start oscillation. However, to obtain good phase noise, a slightly higher gate bias is required than when starting oscillation. Figure 3 shows the difference in phase noise at 1-MHz offset frequency according to the additional bias voltage through the simulation and is the result of the entire code of the cap-bank. As a result, the point with the optimum phase noise performance is determined when the bias is further increased by 15 mV at the lowest oscillation condition. As the gate bias of the core transistor is increased, the oscillation amplitude also increases as the current increases. However, until the gate bias increases by 15 mV from the minimum oscillation point, the oscillation amplitude increases less steeply than the rate at which the thermal noise increases, so it becomes the lowest point with low phase noise performance [10]. Therefore, the proposed class-C VCO uses a digital to analog converter (DAC) to set the bias to the optimal point.

2.2. Proposed Auto-Adaptive Bias Controller

Figure 4a shows the block diagram of the proposed class-C VCO that digitally controls the auto-adaptive bias controller (AABC). The AABC prevents start-up failure and holds the gate bias point (in this figure, V_B) with optimal phase noise. AABC consists of VCO mode selector (VMS), digital code generator (DCG), 5-bit DAC and oscillation detector (OD).

The VMS has two functions—class-C mode to find gate bias from DAC and class-B mode to provide V_DD/2 (500 mV). The DAC outputs 405 mV at the lowest code 0 and 250 mV at the maximum code 31, 5 mV for 1 LSB. If the VCO does not oscillate sufficiently, V_DET, the output of the OD, acts as a trigger to change the mode of the VMS. VMS repeats class-C and class-B modes according to V_DET until V_B satisfies the minimum oscillation condition. When the VCO finds a V_B with sufficient oscillation conditions, it outputs an end signal from the OD and adds 15 mV verified through simulation to the V_B to find the optimal phase noise point. At this time, DCG changes the code whenever the trigger signal V_DET is HIGH.

Figure 4b is a schematic of OD implemented using the square-law characteristic of a MOSFET operating in the saturation region [15]. The gates of the input NMOS M5 and M6 of OD are connected to Vgp and Vgn of VCO, respectively. NMOS M7 is used as a MOS capacitor and serves to low pass filter the input signal and its harmonics. The bias between M3 and M4 and the bias connected to M5 and M6 were all applied as V_{B_DET}, which has a value of V_DD/2. The differential signal detected by the OD has a peak-to-peak voltage of 0.8-V and a target frequency of 24-GHz.

Figure 4c is a flowchart of the simple operation of ABBC. When ABBC starts operating, C code is set to 31, which is full code and V_B initializes V_DD/2 so that it can perform a class-B operation. After that, the class-B and class-C modes are repeatedly operated and the C code is lowered one by one and the V_B value is set according to the C code value to find the oscillating point. If the oscillation continues even though the mode is changed from class-B to class-C, the V_B at that time becomes the point where ΔV_B is 0, that is, the minimum oscillation voltage. Then, 15 mV is added to V_B according to the END signal rising edge to find the optimal phase noise point and the ABBC operation ends.

Figure 5 shows the transient simulation results of the proposed class-C VCO with AABC. This is the result of the VCO output and its frequency, the gate bias, V_B, the OD output, V_DET and END signal and the code of DAC output. When the first VCO starts-up, V_B is brought to V_DD/2 to start the oscillation.

Figure 5a shows the output voltage and frequency of the proposed class-C VCO and it is possible to check whether the VCO oscillates and the peak-to-peak voltage of the output voltage. The output frequency of the VCO does not change when AABC is operating and finding the optimal phase noise point. Figure 5b shows the change in V_B voltage when the proposed class-C VCO operates. To find the lowest oscillation condition, V_B repeats the voltage according to the initial values of V_DD/2 and C code. In other words, make the VCO repeat class-B and class-C operations. Because the slew rate exists, V_B reaches to near V_DD/2. If the VCO maintains the oscillation state at any moment in the V_B that changes by 1 LSB, then this is the moment when the minimum oscillation condition is satisfied. After that, OD gives an END signal and adds 15 mV to become a class-C VCO with optimal phase noise. Figure 5c shows the output signal and END signal of V_DET and Figure 5d shows the C code that changes by 1 LSB for each rising edge of V_DET.

The proposed AABC operates in asynchronous way without external clocked control. The time required for the VCO to find the minimum oscillation condition for class-C mode is <200 ns. The stabilization time depends on PVT variation and capacitor bank code, however, at worst case the settling time is <400 ns.

3. Measurement Results

The proposed 24-GHz class-C VCO with ABBC using triple coupled transformer used a 65-nm TSMC RF CMOS process. Figure 6 shows the die photograph of the produced VCO. The core size excluding the buffer of the proposed VCO is 0.23 × 0.22 mm². Figure 7a shows the output frequency according to the control voltage. The output frequency range is 22.8 to 27.3-GHz and the tuning range is 4.5-GHz, reaching 17.8%. Figure 7b shows the power consumption according to the control voltage. The control voltage means the voltage between varactors and both plots are the results when the cap-bank code is swept from 0 to 7.

Figure 8 shows the phase noise at 24-GHz of the proposed class-C VCO and the offset frequency −105 and −128.2 dBc/Hz at 1-MHz and 10-MHz respectively at 1-V supply voltage. The phase noise that changes when the cap-bank code changes are less than 2-dB. The output frequency and phase noise can be obtained using the Agilent E4440A spectrum analyzer (Agilent Technologies, Santa Clara, CA, USA) and E5052B signal source analyzer (Keysight Technologies, Santa Rosa, CA, USA). Figure 9 shows the result of measuring the phase noise and figure-of-merit with tuning range (FoM_T) at 1-MHz offset according to the frequency. FoM_T according to the phase noise is −185.5 to −192 dBc. The formula of standard FoM is as follows:

$$FoM=L\left(\Delta f\right)+10log\left(\frac{P_{\mathrm{DC}}}{1mW}\right)-20log\left(\frac{f_o}{\Delta f}\right)$$

(1)

and the equation for FoM_T is as follows:

$$Fo{M}_T=L\left(\Delta f\right)+10log\left(\frac{P_{\mathrm{DC}}}{1mW}\right)-20log\left(\frac{f_o}{\Delta f}\right)-20log\left(\frac{FTR}{10}\right)$$

(2)

Here, L(Δf) is phase noise at a specific offset frequency and FTR is frequency tuning range in percent. P_DC is the power consumption of VCO core and the unit is milliwatt.

The proposed class-C VCO adopts a simple but powerful auto-adaptive bias technique, as summarized in

Table 2. Performance comparison with published class-C VCO.

	This Work	[7]	[8]	[14]	[16]	[17]	Unit
Tech.	65	180	65	SiGe HBT	28	180 SiGe	nm
Config.	Class-C	Class-C	Class-B/C	Class-B/C	Class-C	-	-
Freq.	22.8–27.3	4.74–4.85	10.2–11.2	22.7	19.5	22.5–26.2	GHz
FTR	17.8	2.3	9.6	13	12	15.31	%
L(1-MHz)	−105	−125	−107.7	−114	−112	−107.7	dBc/Hz
FoM	−186.2	−193	−185	−189	−185	−186.9	dBc/Hz
FoMT	−191.1	−180.6	−185	−193	−186.6	−190.6	dBc/Hz
PDC	9.2	3.4	2.2	18	20.7	8.2	mW
Area	0.055	0.15	0.07	-	0.07	0.28	mm2

. Although phase noise is not better than other previous studies, this VCO has comparable less power consumption, wide frequency tuning range, low FoM_T and smaller chip area than other class-C VCO studies [7,8,14,16,17].

4. Conclusions

In this paper, a class-C VCO using a digitally controlled auto-adaptive bias technology is proposed with a triple transformer configuration. It can have low phase acquisition, low power consumption, high frequency and a wide frequency range, simultaneously. The NMOS class-C VCO is combined using an 8-port transformer in a small chip area. The proposed VCO shows low phase noise of −105 dBc/Hz at 1-MHz offset frequency at 24-GHz with high FoM_T of 191.1 dBc/Hz. The power consumption of each main block of the proposed class-C VCO is 460 μW for AABC and 8.7 mW for core, a total of 9.2 mW. The frequency tuning range is about 17.8% from 22.8 GHz to 27.3 GHz. The proposed class-C VCO implements 65-nm TSMC RF CMOS process and chip area of the core is 0.23 × 0.22 mm².