29/09/2026
Nearly every radio you've ever owned has a little signal generator of its own inside — and without it, you couldn't tune a thing. 📻
Beginners #9. Last time: the mixer. Feed it two frequencies and out come their sum and their difference.
Today, the second ingredient: the signal your radio makes for itself. It's called the local oscillator, or LO. "Local" because it's generated inside the radio, not picked up by the antenna.
Same numbers as last time, no maths panic:
• Wanted station: 14.000 MHz
• Local oscillator: 23.000 MHz
• Difference out of the mixer: 9.000 MHz, the IF
Now the clever bit. Want 14.200 instead? The radio doesn't retune its sharp IF filters. It moves the oscillator up to 23.200 MHz, and the difference comes out at exactly 9.000 again.
Every station you tune gets delivered to the same fixed frequency, where the good filtering lives. So when you turn the tuning k**b, you're really steering the local oscillator.
On older rigs the k**b literally turned a variable-frequency oscillator, the famous VFO. On modern radios it tells a frequency synthesiser what to make (that's lesson 16).
Why it matters on air:
Stability. If the LO drifts with heat, your signal drifts with it; on SSB, even a small drift makes a voice sound squeaky or growly. It's why owners of older valve rigs let them warm up before getting on air.
Cleanliness. A noisy LO smears strong neighbouring signals across the weak one you're trying to hear. That's why independent receiver test tables include an "LO noise" column.
One for the history buffs: in WW2, the story goes that Allied troops weren't allowed superhet receivers, because a receiver's own oscillator can leak and the enemy could listen for it. Hence the improvised "foxhole radio", which had no oscillator to give it away.
Next time, it all bolts together: mixer + local oscillator + IF = the superheterodyne. Questions so far? Drop them below and we'll answer every one 👇
Everything for your shack: https://www.moonrakeronline.com
