Overall Summary: The BB153 hyper abrupt silicon varicap diode is readily available (e.g. Farnell) at low cost. It can result in extremely linear tuning providing correct inductor and resonating and coupling capacitor values are selected. It has a high Q value based on its series resistance rs = 0.65 Ohms. Its capacitance Cd = 38.5 pF at VR = 1.0 V implying a Q value of Qd = 215 at f = 29.6 MHz (VCO center frequency is its LO output is divided by 8 to produce 3.7 MHz - one technique for producing phase quadrature LO outputs with good immunity from RF re-circulation on transmit).
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