Two receiver qualities that are not the same
Which term describes the ability of a receiver to detect the presence of a signal?
Sensitivity.
Which term describes the ability of a receiver to discriminate between multiple signals?
Selectivity.
Sensitivity is how weak a signal it can hear at all. Selectivity is how well it separates that signal from its neighbours.
They are independent, and a receiver can be excellent at one and poor at the other. A very sensitive receiver with poor selectivity hears everything at once — which is the actual failure mode of cheap wideband receivers, and why adding a preamplifier to a crowded band often makes reception worse rather than better.
The blocks inside
| Block | Function |
|---|---|
| Transceiver | combines a receiver and transmitter |
| Oscillator | generates a signal at a specific frequency |
| VFO | sets the receive and transmit frequency |
| Mixer | converts a signal from one frequency to another |
| Modulator | combines speech with an RF carrier — the process is modulation |
| RF power amplifier | increases the transmitted output power |
| Transverter | converts the RF input and output of a transceiver to another band |
The mixer is the one worth understanding. Every superheterodyne receiver works by mixing the incoming signal with a local oscillator to shift it to a fixed intermediate frequency, where the filtering and amplification happen at one convenient frequency instead of across the whole tuning range. It is the trick that made tunable radio practical, and it is a hundred years old.
A transverter is the same trick applied to a whole station: feed it a signal on a band your radio covers, and it converts to a band your radio does not. It is how a 2 metre radio operates on 23 centimetres or on 6 metres.
Controls with specific meanings
What is the function of a transceiver’s PTT input?
Switches transceiver from receive to transmit when grounded.
Grounded, not powered — the convention across essentially all amateur equipment.
What is the function of the switch which selects either SSB or CW-FM on some VHF power amplifiers?
Set the amplifier for proper operation in the selected mode.
The two mode families make very different demands. SSB is intermittent with high peaks; CW and FM are continuous carrier. The switch adjusts bias and, on many amplifiers, the transmit-receive switching delay to match.
Adding an amplifier
What can be added to the output of a transceiver to increase the transmitted output power?
An RF power amplifier.
Worth pairing with the decibel arithmetic from the foundations track: going from 50 W to 500 W is 10 dB — a genuine improvement, and about one and a half S-units at the far end. Fixing a lossy feed line and putting up a better antenna often buys more, and unlike an amplifier it helps on receive too.
Check yourself
- Your receiver hears a weak station but a strong nearby signal makes it unusable. Which quality is lacking?
- What does a mixer do, and why does a receiver need one?
- You want to operate 6 metres with a 2 metre radio. What device?
Answers
- Selectivity — the ability to discriminate between multiple signals. Sensitivity is fine; it heard the weak one.
- It converts a signal from one frequency to another, so filtering and amplification can happen at one fixed intermediate frequency instead of across the whole tuning range.
- A transverter.