AGC and Power Supplies
Automatic Gain Control (AGC)
Imagine tuning across the band. One station is barely audible, the next blasts your ears off. Without AGC, that's what every QSO would be like.
AGC automatically turns the gain up for weak signals and down for strong ones, keeping the audio level roughly constant.
How It Works
- The receiver measures the strength of the incoming signal
- This measurement creates a control voltage
- The control voltage adjusts the gain of the RF and IF amplifier stages
- Strong signal → less gain. Weak signal → more gain.
Fast vs Slow AGC — Which to Use?
| Setting | Attack | Release | Best for | Why |
|---|---|---|---|---|
| Fast | Quick | Quick | CW, digital modes | Quick recovery between elements |
| Slow | Quick | Slow | SSB voice | Doesn't "pump" between syllables |
Practical tip: If you hear SSB signals "pumping" (volume surging between words), try switching to slow AGC. If CW signals seem to have a slow fade-in on each character, switch to fast AGC. Most radios default to a good middle ground.
Power Supplies for Radio
Most amateur HF transceivers need a regulated 13.8V DC supply. There are two main types:
Linear Power Supply
Simple, reliable, and electrically quiet (very little RF noise). Works by dropping excess voltage across a regulator — the waste becomes heat.
- Efficiency = Vout / Vin (e.g., 13.8V / 18V = ~77%)
- Heavy due to large mains transformer
- Preferred by many operators for HF because of low noise
Switch-Mode Power Supply (SMPS)
Much more efficient (80-95%) and lighter, but can generate RF interference.
- Works by rapidly switching DC on and off (50-500 kHz), which inherently creates RF noise
- Good SMPS designs include filtering to keep noise low
- Cheaper and lighter than linear supplies of the same rating
EMC consideration: If you hear a buzzing or whining on your receiver that changes with tuning, your SMPS might be the culprit. Try ferrite chokes on the DC power leads and keep the supply away from your antenna feedline.