Elements of a Communication System: Source, Channel and Receiver
Every communication link, from a phone call to a LoRa sensor sending soil moisture readings, has the same basic job: move information from one place to another with acceptable errors, delay and cost. Engineers describe this with a simple block diagram. Once you understand each block and the problems it solves, topics like modulation, sampling, antennas and serial protocols stop looking like separate subjects and start looking like answers to the same few questions.
This article walks through the blocks, explains why modulation is needed at all, classifies channels and modes of transmission, and finishes with Shannon's capacity formula and a worked example on a telephone line.
The Block Diagram
Information -> Input -> Transmitter -> Channel -> Receiver -> Output -> Destination
source transducer ^ transducer
(voice, data) (microphone) | (speaker)
Noise| Block | Job | Example |
|---|---|---|
| Information source | Produces the message | Human voice, temperature sensor, keyboard |
| Input transducer | Converts the message to an electrical signal | Microphone, thermistor with ADC |
| Transmitter | Prepares the signal for the channel: filtering, encoding, modulation, power amplification | FM broadcast transmitter, UART peripheral, Wi-Fi radio |
| Channel | The physical medium that carries the signal | Copper pair, coaxial cable, optical fibre, free space |
| Receiver | Recovers the message: amplification, filtering, demodulation, decoding | FM radio, UART receiver, Wi-Fi radio |
| Output transducer | Converts electrical signal back to a usable form | Loudspeaker, display |
| Destination | The person or machine using the message | Listener, microcontroller firmware |
Noise is drawn entering at the channel because that is where most of it is added, but in reality every amplifier and resistor in the receiver also contributes noise. The receiver's first stage matters most, which is why good receivers start with a low-noise amplifier.
What the Channel Does to Your Signal
- Attenuation: the signal gets weaker with distance. In cable it grows with length and frequency; in free space it falls with the square of distance.
- Noise: random unwanted signals added to the message, such as thermal noise in resistors, interference from motors and switching supplies, and atmospheric noise.
- Distortion: the channel treats different frequencies differently, so pulses spread out and overlap. In digital links this causes inter-symbol interference.
- Interference: other transmitters using the same or nearby frequencies.
- Fading and multipath: in wireless links, reflected copies of the signal arrive with different delays and add or cancel.
The two limited resources in any link are bandwidth (the range of frequencies you are allowed to use) and power (how strong your signal is compared with the noise, usually given as the signal-to-noise ratio, SNR). Almost every design decision trades one against the other.
Why Modulation Is Needed
Voice occupies roughly 300 Hz to 3.4 kHz. You cannot simply connect a microphone to an antenna and broadcast it, for three reasons.
- Antenna size. An efficient antenna is around a quarter wavelength long. Wavelength is c divided by f, so at 1 kHz a quarter wave is 75 km, while at 1 MHz it is 75 m and at 100 MHz only 75 cm.
- Multiplexing. If every station transmitted raw audio, all signals would overlap in the same band. Shifting each one to its own carrier frequency lets many stations share the spectrum.
- Matching the channel. Some channels only pass certain frequencies, for example optical fibre or a band-pass telephone line. Modulation moves the signal into the band the channel carries well.
Wavelength: lambda = c / f (c = 3 x 10^8 m/s) Quarter-wave length: L = lambda / 4 f = 1 kHz -> lambda = 300 km -> L = 75 km f = 1 MHz -> lambda = 300 m -> L = 75 m f = 100 MHz -> lambda = 3 m -> L = 0.75 m
Modulation means varying some property of a high-frequency carrier, its amplitude, frequency or phase, in step with the message. Analog modulation (AM, FM, PM) varies it continuously. Digital modulation (ASK, FSK, PSK, QAM) switches it between a fixed set of states, each representing one or more bits.
Classifying Communication Systems
| Basis | Types | Examples |
|---|---|---|
| Direction of flow | Simplex, half duplex, full duplex | FM broadcast; walkie-talkie and RS-485; telephone, SPI |
| Nature of signal | Analog, digital | AM radio; UART, Wi-Fi |
| Medium | Wired (guided), wireless (unguided) | Ethernet, fibre; Bluetooth, LoRa |
| Baseband or passband | Baseband (no carrier), passband (modulated carrier) | UART on a PCB trace; FM radio |
Digital systems dominate today because regenerative repeaters can clean up a digital signal completely, error-correcting codes can detect and fix bit errors, and the same link can carry voice, video and data. The price is extra bandwidth and the need for accurate timing.
Channel Capacity: The Shannon-Hartley Limit
Claude Shannon showed that a channel with bandwidth B and signal-to-noise ratio S/N has a maximum error-free data rate, called the capacity. No modulation scheme can beat it, but good coding can approach it.
C = B x log2(1 + S/N) bits per second B = bandwidth in Hz S/N = signal-to-noise ratio as a plain ratio (not dB) S/N (ratio) = 10^(SNR_dB / 10) log2(x) = log10(x) / log10(2) = 3.322 x log10(x)
Worked Example: Analog Telephone Line
A voice telephone channel passes about 300 Hz to 3400 Hz, so B is 3100 Hz. Assume an SNR of 30 dB.
S/N = 10^(30/10) = 1000
C = 3100 x log2(1 + 1000)
= 3100 x 3.322 x log10(1001)
= 3100 x 3.322 x 3.0004
= 3100 x 9.967
= about 30,900 bit/s (30.9 kbit/s)This matches history well: analog voice-band modems topped out around 33.6 kbit/s on lines with slightly better SNR. Doubling the bandwidth doubles the capacity, but doubling the signal power only adds about one bit per second per hertz, because power sits inside a logarithm.
Common Mistakes Students Make
- Putting SNR in dB directly into Shannon's formula. Convert 30 dB to 1000 first.
- Confusing bandwidth (Hz) with data rate (bit/s). They are related through the modulation and coding, not equal.
- Forgetting that noise is added by the receiver too, not only by the channel.
- Assuming half duplex means slower. It means the two ends take turns on the same medium; each direction can still run at full speed.
Summary
A communication system is a chain of source, transducer, transmitter, channel, receiver and destination. The channel attenuates, distorts and adds noise; the transmitter and receiver exist to fight those effects within limited bandwidth and power. Modulation makes antennas practical and lets many users share spectrum. Shannon's formula C = B log2(1 + S/N) sets the ceiling for every link you will design, and the rest of this series looks at how real modulation schemes and protocols work inside that ceiling.