The interference-fighting controls
A modern HF receiver has four distinct tools, and each addresses a different problem. Using the wrong one wastes effort.
| Control | Fixes | How |
|---|---|---|
| Notch filter | a carrier in the passband | narrow notch at one frequency |
| Noise blanker | impulse noise | reduces receiver gain during a noise pulse |
| Noise reduction | broadband hiss | DSP processing |
| Attenuator | overload from strong signals | reduces front-end level |
What is the purpose of the notch filter found on many HF transceivers?
To reduce interference from carriers in the receiver passband.
A single heterodyne — a carrier whistling away in your passband — is what a notch removes. It does nothing for hiss or for another SSB signal.
How does a noise blanker work?
By reducing receiver gain during a noise pulse.
It gates the receiver off for the microseconds a pulse lasts. Excellent for ignition noise and electric fence clicks; useless for continuous noise, and it can make matters worse in the presence of strong signals, which it mistakes for pulses.
What happens as a receiver’s noise reduction control level is increased?
Received signals may become distorted.
DSP noise reduction guesses which parts of the signal are noise, and the harder you push it the more of the wanted signal it discards. Voices acquire a hollow, underwater quality. Use the least that helps.
What is the purpose of using a receive attenuator?
To prevent receiver overload from strong incoming signals.
Counterintuitive but real: reducing signal level can improve what you hear, by keeping the front end out of the region where it generates intermodulation. On a crowded low band in the evening, switching in 10 dB of attenuation frequently reveals signals that were buried in the receiver’s own products.
What is the benefit of using the opposite or “reverse” sideband when receiving CW?
It may be possible to reduce or eliminate interference from other signals
Demodulating on the other sideband flips the passband about the carrier, so an interfering signal that was inside it may now fall outside. Free, and often effective.
Dual VFO
Which of the following is a common use of the dual-VFO feature on a transceiver?
To transmit on one frequency and listen on another.
Split operation — essential for working a DX station that listens away from its transmit frequency, and for any pile-up.
Amplifier tuning
Tube amplifiers still populate HF stations, and their two controls are asked directly.
What is the effect on plate current of the correct setting of a vacuum-tube RF power amplifier’s TUNE control?
A pronounced dip
TUNE resonates the output network. Sweep it and plate current falls sharply at resonance — the dip is the indicator, and it is unmistakable.
What is the correct adjustment for the LOAD or COUPLING control of a vacuum tube RF power amplifier?
Desired power output without exceeding maximum allowable plate current
LOAD sets how tightly the tube couples to the load. Tune for the dip, load for output, re-dip, repeat. Both limits matter: enough output, without exceeding plate current.
ALC, and when to switch it off
Why is automatic level control (ALC) used with an RF power amplifier?
To prevent excessive drive.
The amplifier feeds a control voltage back to the exciter, which reduces drive before the amplifier is overdriven. A protection loop.
And the exception that catches people:
Why should the ALC system be inactive when transmitting AFSK data signals?
The ALC action distorts the signal.
ALC is designed for the peaks of speech. A constant-amplitude data signal makes it act continuously, and its action is a non-linear amplitude change — which produces exactly the intermodulation sidebands the digital-modes lesson taught you to look for on a waterfall.
For digital modes: switch ALC out of circuit and set drive manually, below the level at which anything limits.
Amplifier sequencing
What is the purpose of delaying RF output after activating a transmitter’s keying line to an external amplifier?
To allow time for the amplifier to switch the antenna between the transceiver and the amplifier output
Relays take milliseconds. Send RF before they have finished moving and you arc the contacts — destroying the relay and, often, the amplifier’s input circuit. The delay is not a convenience; it is what keeps hot-switching from happening.
Antenna tuner, stated precisely
What is the purpose of an antenna tuner?
Increase power transfer from the transmitter to the feed line.
Read that carefully. To the feed line — not to the antenna. It matches at the tuner, and the standing wave beyond it is unchanged. The General antennas module makes the same point from the SWR side.
Electronic keyer
What is the function of an electronic keyer?
Automatic generation of dots and dashes for CW operation.
You supply the rhythm from a paddle; the keyer supplies the timing.
Check yourself
- A steady whistle sits in your SSB passband. Which control?
- Ignition noise from passing cars. Which control?
- You are about to run FT8 through a linear amplifier. What must you do about ALC?
Answers
- Notch filter — it removes a single carrier in the passband.
- Noise blanker — it gates the receiver off during each pulse.
- Switch it out of circuit and set drive manually. ALC action on a constant-amplitude signal distorts it and generates intermodulation sidebands.