Oscillators — Making the Radio Frequency

What Does an Oscillator Do?

An oscillator generates a continuous AC signal at a specific frequency. It's the heartbeat of every radio — your transmitter needs one to generate the carrier, and your receiver needs one (the local oscillator) to convert signals to the IF.

How Oscillators Work

An oscillator is just an amplifier where some of the output is fed back to the input in phase (positive feedback). For it to work, two conditions must be met — these are called the Barkhausen criteria:

  1. Loop gain ≥ 1 — the fed-back signal must be at least as strong as what's needed at the input
  2. Phase shift = 0° (or 360°) — the fed-back signal must arrive in phase to reinforce the original

Think of it like a PA system that's feeding back (howling). That howl IS an oscillator — the microphone picks up the speaker, the amp boosts it, the speaker plays it louder, and round it goes.

Common Oscillator Types in Radio

Colpitts Oscillator

Uses two capacitors and one inductor. The two capacitors form a voltage divider for the feedback. Very popular in VFOs (Variable Frequency Oscillators) because it's stable and easy to tune by varying one of the capacitors.

\( f = \frac{1}{2\pi\sqrt{L \cdot \frac{C_1 C_2}{C_1 + C_2}}} \)

Hartley Oscillator

Uses one capacitor and a tapped inductor (or two inductors). The tapped inductor provides the feedback. Less common than Colpitts in modern equipment.

Crystal Oscillator

Uses a quartz crystal as the frequency-determining element. The crystal has an extremely high Q (10,000+), making it incredibly stable. Used for:

Radio connection: When you tune your HF radio, you're usually not changing an oscillator directly. Modern radios use a PLL synthesiser — a crystal oscillator provides the stable reference, and the PLL multiplies it to the frequency you want.

PLL Frequency Synthesiser

The PLL (Phase-Locked Loop) is how modern radios generate precise, tunable frequencies. Here's how it works in simple terms:

  1. A crystal reference provides an accurate, stable signal (e.g., 10 kHz)
  2. A VCO (Voltage-Controlled Oscillator) generates the output frequency
  3. A divider (÷N) divides the VCO output down
  4. A phase detector compares the divided output with the reference
  5. If they don't match, it adjusts the VCO until they do
\( f_{out} = N \times f_{ref} \)

Example: Reference = 10 kHz, N = 1450 → output = 14.500 MHz (right on 20 metres!). Change N to 1451 and you step to 14.510 MHz. That's how your tuning knob works in 10 kHz steps.

Mixers — Combining Frequencies

A mixer takes two signals and produces their sum and difference frequencies:

\( f_{out} = f_1 + f_2 \quad \text{and} \quad f_{out} = f_1 - f_2 \)

This is how receivers convert the signal you want to the IF, and how transmitters shift the generated signal to the operating frequency.

The best type is the double-balanced mixer — it suppresses both input signals at the output, leaving just the sum and difference products. This makes filtering much easier.

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