Basically, the Fine Tune design is very similar to the Coarse Tune design. The YFT curve should look like Fig.20. When VFT=10V, the Re[YFT] stays very close to zero, while the Im[YFT] is kind of linear, and more flat (than the case in Coarse Tune.) Since Re[YFT] cannot be zero (an ideal open, in that case), it will load the circuit. That’s why we said we needed to have a ‘good’ Coarse Tune design and working circuit before entering Fine Tune designing process.
Fig.20 Impedance of Fine Tune varactor diode (Ideal Case)
Actually Fine Tune is more experimental than theoretical. The following steps are our tweaks done in lab.
First we had our design like the one (Fig.31) below. But the tuning sensitivity is too high.
We found that we can
lower the tuning sensitivity by reducing the width of the traces. However, it didn’t help much. The tuning sensitivity was still too
high. Then, we decided to move the
varactor to about l/4
from the attach point (Fig.32). We
reasoned that this high impedance line may reduce the coupling effect of the
fine tune branch, because the narrow transmission line serves as 
Indeed, the tuning sensitivity was lowered, but now we had to deal with the linearity. Our goal now was to increase the tuning sensitivity at lower frequency, and decrease the tuning sensitivity at high frequency.
First we made our trace even thinner (Fig.33). This reduced the tuning sensitivity again.
Then we put a piece of triangular foil as our tweak (Fig.34) After a few trials, we had a good fine tune.
The idea of triangular tweaking is that the higher the frequency, the shorter the wavelength. Since we wanted the branch less sensitive at high frequency, we narrowed the trace at the shorter distance. In the meanwhile, we wanted to increase sensitivity at low frequencies, so we widened the farther trace.
| Introduction | Measurements | Fundamental Theory | Transistor Selection |
| Terminating Network Design | Load Network Design | Coarse Tune Design |
| DC Bias Circuit Design | Fine Tune Design | Phase Noise |