GaN HEMT-based Voltage Controlled Oscillators
Main performance parameters of voltage controlled oscillators (VCO) are:
1. Output power.
2. Tuning range.
3. Tuning linearity.
4. Phase noise level.
Depending on its functions, oscillator could have high output power, low phase noise and wide tuning range.
In the current work, the following voltage controlled oscillators have been designed:
1. Standard design 50–1000MHz VCO (Fig. 1,2).
2. Hybrid VCO with 2–4GHz tuning range (Fig. 3,4).
3. Monolithic VCO with 8–12.5 tuning range (Fig. 5, 6).
Designed oscillators feature GaN HEMT with 250-µm gate width as an active device.
The use of a GaN HEMT as an active device in oscillators is one of the emerging trends in the semiconductor electronics. Parameter comparison of various stabilized oscillators is shown in Table 1. GaN HEMT-, GaAs pHEMT- and HBT-based VCOs comparison is shown in Table 2. It could be observed that AlGaN/GaN VCOs surpasses its AlGaAs/GaAs counterparts by output power performance and phase noise spectral density (PNSD).
1. Transistor with a field plate and source through-vias near the gate (Fig. 9).
2. Transistor with a field plate and source through-vias made on transistor periphery (Fig. 10).
3. Transistor without a field plate and with source through-vias made on transistor periphery (Fig. 11).
Presented curves show that transistor design has a strong influence on the phase noise of the device, which requires further study.
Apart from transistor design, phase noise levels are dependent on the quality of used semiconductors and dielectrics, as well as manufacturing technology.
Within the framework of our research, we designed and produced VCO evaluation sample with frequency range 70.80MHz based on GaN transistor (Fig. 12). Measured performance was in a good agreement with simulated performance (within 10 % error). Phase noise level values of this VCO are shown in Fig. 13. Noise level values at tuning range 100kHz correspond to the results of foreign analogues.
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