[ FET or BJT ] Although field effect transistors (FETs) are commonly used at higher frequencies, they are generally noisy. On the other hand, bipolar junction transistors (BJTs) or hetero-junction bipolar transistors (HBTs) are used at lower frequency and have lower noise than FETs.
[ fT ] fT is known as
transition frequency, which means at this frequency,
S21=1. To design a VCO for a frequency range of fL to fH, a transistor whose fT is at least double of fH is needed. The higher fT, the better overall
performance with respect to power. fT should not be greater than
4fL, or else the circuit
has the possibility of oscillating at a second harmonic (2fL). \2fH < fT < 4fL.
For fH = 8.7GHz, fL=7.8GHz, \ 17.4GHz< fT < 31.2GHz. An Infineon BFP420, whose fT=25GHz, is chosen for this project.
[K] K is known as the stability constant of the transistor.
where
D=S11S22
- S21S12
If |K|<1, then the transistor is potentially unstable, and can be used to build an oscillator. If both |K|>1 and |D|>1, then the transistor is unconditionally stable, and it cannot be used to build an oscillator without using a feedback.
Our
requirement is |K|<1 for all
frequencies between fL and fH. To be safe, we have changed our tolerance so
that |K|<1 for all frequencies between 0.9fL and 1.1fH. In our
case, |K| = 0.795, |D|
= 0.114 at 8GHz.
| Introduction | Measurements | Fundamental Theory | Transistor Selection |
| Terminating Network Design | Load Network Design | Coarse Tune Design |
| DC Bias Circuit Design | Fine Tune Design | Phase Noise |