Due to the rapidly growing demands of mobile communication, it is necessary to move up to the higher frequency band of 2 GHz or above to avoid the congested 900 MHz band. At the same time, the reduction of the physical size and electrical power consumption in the application of the mobile communication is mandatory rather than desirable.
In this paper, we suggest a new type of VCO for mobile communications. To satisfy the above points, we designed the VCO circuit by adopting the commercial GaAs MMIC foundry process to get the higher cutoff frequency $f_T$ of the active device and reduce the size dramatically with one external component of resonator. For the external component of the VCO, we devised a new type of multi-layer resonator.
The VCO circuit is cascaded structure between the oscillating part and the buffering amplifier stage with the same type of MESFET in MMIC process to save the power. At the same time, the varactor in the circuit is also adopted as the same type of MESFET by connecting the source and drain to fit in the MMIC process. We use the same MMIC foundry to complete the layout of the VCO circuit.
The designed MMIC VCO is 0.92×1.02 mm size and simulation results by MDS of HP guarantees excellent VCO performance for the application of the mobile communications.
In order to get the satisfactory performance in VCO, We analyzed the resonator which is partially opened strip-line structure of the 3 layer ceramic PCB in the size of 1.5×2.5 mm both in the analytical way and computational electromagnetic method using CAD software.
The new structure provides a tuning capability of the VCO which is impossible in MMIC scheme and improve the performance of the VCO.
We fabricated the resonator first in 4 types and tested by network analyzer using 1 port measurement method. the resonator test results show good possibility of applying in MMIC VCO as a resonator component.
The resonator is fabricated with Alumina substrates suitable for mounting a MMIC chip on the top of the resonator.
For manufacturing convenience, there is no lead attached to the resonator which could be possible through additional processes with slight increasing the size of the resonator. The commercial SO-8 package of size 5.08×3.56mm is employed to provide the VCO with test leads for RF output, frequency tune and DC supply. The resonator has three layers and the MMIC chip is mounted on the top layer.
The resonator is analyzed with the EM simulator of Momentum and HFSS. Its inductance varies with the length of the second layer. Four patterns are chosen to give slightly higher and lower inductance values from the desired values. Parallel resonance occurs at much higher frequency than 2GHz, and one more resonance occurs because of the additive series inductance generated by via-hole between the patch and the second layer. With the measured result, the circuit is simulated by the harmonic balance method in MDS. The simulation shows that the DC current consumption is about 8mA and the output power is 0 dBm. The frequency tuning range is found to be about 120MHz with the tuning voltage from 0 to 3V. The centre frequency of oscillation is about 2.0GHz. The trimming of the patch pattern changes the oscillation frequency over 50 MHz. This frequency change is sufficient enough to improve the yield of MMIC chips for mass production.
As expected, the test results in the range of 0.5GHz to 10GHz exhibit that the resonator may be viewed as a parallel LC resonant circuit with series L component. The calculated Q, based on the test results is reached up to almost 100, which is sufficient for a resonator. The inductance L changes from 2nH to 3nH by the length of the 2nd layer.
Test board and jig are prepared to test the VCO. The frequency tuning range covers over 110MHz with the control voltage from 0 to 3V. The RF output power varies approximately from -5dBm to -3dBm. The DC bias current of the VCO is about 7mA. The worst value of phase noise is lower than -80dBc/Hz @100KHz when the control voltage is 0V, whereas the best value is lower than -100dBc/Hz @100KHz when the control voltage is 3V. All of the test items are in good agreement with the simulation results. The oscillation frequency is found to be slightly lower than the simulation result due to additional inductance of the bonding wire.
The patch pattern provides the centre frequency tuning capability and may improve the yields of the production. The size is reduced by a factor almost 30, compared to the latest results reported in research and industry. By attaching leads to the ceramic resonator, this MMIC VCO can be directly applied to 2GHz mobile communication handset application.'
