# The Colpitts Oscillator as a VFO

**The Colpitts Oscillator: Understanding and Building Your Own Variable Frequency Oscillator**

The Colpitts oscillator, named after its inventor Edwin H. Colpitts in 1918, originally used vacuum tubes. Today, we explore a modernised version that employs NPN bipolar junction transistors instead of vacuum tubes or JFETs. This choice ensures low cost and availability, despite JFETs also being a viable option, albeit more expensive.

#### HOW THE COLPITTS OSCILLATOR WORKS#

Fundamentally, all oscillators consist of three key components: an amplifier, a feedback path, and a filter. The Colpitts oscillator is unique due to its reliance on an emitter follower configuration, which typically provides a gain of less than one. However, it effectively produces oscillations through the voltage gain generated by the LC (inductor-capacitor) tank circuit at the resonant frequency, which compensates for the lower amplifier gain.

#### CIRCUIT DESCRIPTION#

Our Variable Frequency Oscillator (VFO) circuit is specifically designed for operation at 7 MHz, aligning with the 40-Meter Ham Radio Band.

##### KEY COMPONENTS AND CONFIGURATION:#

**Power Supply:**A regulated 9V supply is assumed.**Inductor Design:**The core of the LC tank is a 4 µH air core coil, which can be manually wound using a 10 mm drill bit as a form, creating a self-supporting structure. Alternatively, a T37-6 toroid could be used requiring 36.5 turns.**Base Configuration:**The base of the first NPN transistor connects through a signal line that includes a 5 picofarad base capacitance; the 50 pF Capacitor; a 10K bias resistor; 4 µH inductor; a 100 nF coupling capacitor and the two 300 pF splitting capacitors detailed below.**Impedance and Phase Splitting:**Two 300 picofarad capacitors create an impedance and phase-splitting network, critical for the stability and quality of the output waveform.**Voltage Supply:**A 10K ohm resistor connects the transistor's collector directly to VCC, ensuring proper power distribution.**Capacitance calculation:**In a Colpitts, the tank, or resonant circuit is formed by the sum of all the Capacitors. This includes the two 300 pF capacitors and all other capacitors that come from the signal line that are connected to the base. Since the 100 nF capacitor is very large compared with the rest of these, we do not include this.- Frequency: This circuit has values of L and C designed such that it will oscillate at 7 MHz.

#### CALCULATIONS FOR THE AIR CORE WOUND COIL#

The formula for calculating the inductance L of an air core coil is as follows:

Where:

- L is the inductance,
- r is the radius of the coil in centimetres,
- N is the number of turns,
- l is the length of the coil in centimetres.

For a coil wound on a 10 mm diameter drill-bit as a former to arrive at 4 µH

And this is how we arrive at the number of turns:

Note: This oscillator is not disciplined, so your results may differ from the calculations or simulations. However, You can make a half decent VFO like this.