Amateur radio VFO for 7 MHz. This design used the negative resistance method. The 600 milli -ohms represents the inductor's internal series resistance of the windings.
The approximate equation for the negative input resistance is:
Zin=-gm/((2xpixfo)^2xC1xC2). For this circuit this comes to -3.3 ohms. The coil resistance is 0.6 ohms.
The negative input resistance is about 6 times larger than the winding resistance. Usually a factor of 5 to 10 times larger is used for the value of negative resistance, therefore this value is acceptable.
The 1nF capacitor in parallel with RE is chosen to have a high parallel Q factor with 'RE where Q=RE/XC2. In this case the Q factor is about 100.
The 100 pf capacitor across the inductor represents the tuning capacitance.
The value of gm is given by Ic/0.026, I have used 2 mA as an approximation for the value of Ic. (It is actually 1.62 mA).
I prefer the return ratio method for designing oscillators. It gives much clearer values to use.
