Electronic measuring instruments
A measuring instrument turns something invisible (a signal) into a number or a picture. Common ones:
- Multimeter: voltage, current and resistance.
- Oscilloscope: draws voltage against time. You read amplitude, period T and then frequency f = 1/T.
- Frequency counter: counts waves in one second.
- Signal generator: makes a clean test signal of chosen frequency and level.
- Power meter: reads the power of a radio or light signal.
Rule: first guess the size of the signal, set the instrument range a little higher, then read. Always connect the probe ground first.
Example. If one wave takes 2 ms on the screen, f = 1 / 0.002 = 500 Hz.
Transmitter measurement
A transmitter must stay on its own frequency, send the right power, and not over-modulate. We measure:
- Frequency with a counter. A small drift can disturb other stations.
- Output power with a power meter on a dummy load (a resistor that safely takes the power instead of the antenna).
- Modulation depth m = (Emax − Emin) / (Emax + Emin) × 100 %, read from the AM wave on the scope. If m is more than 100 %, the sound is distorted.
Receiver measurement
A receiver is tested with a signal generator. We feed a very weak known signal and watch the output.
- Sensitivity: the weakest input that still gives a clear output. Smaller number means a better receiver.
- Signal-to-noise ratio (S/N): signal level divided by noise level. Bigger is better.
- Selectivity: how well it picks one station and ignores its neighbours.
Microwave and optical measurement
At microwave and light frequencies we talk about loss and gain in decibels (dB). dB = 10 × log10(P_out / P_in). A loss of 3 dB means half the power is left; 10 dB means one tenth; 20 dB means one hundredth.
Because dB are added, a long link is easy to work out: total loss = (fibre loss per km × km) + connector losses.
Tools: optical power meter and light source (for fibre), OTDR (sends a pulse and times the echoes to find breaks), spectrum analyser (shows power at each frequency), and power sensor for microwaves. Example. A 20 km fibre at 0.2 dB/km loses 4 dB. If 10 mW enters, about 4 mW leaves.
Antenna and radio-wave measurement
An antenna is tested for how well it sends power out and how strong the wave is far away.
- Field strength (volts per metre, or dBµV/m) is measured with a field-strength meter and a test antenna. It becomes weaker as the distance grows (about 1/d in open space).
- SWR (standing wave ratio): shows how much power bounces back from a badly matched antenna. 1 : 1 is perfect; above 3 : 1 is poor.
- Directivity: turn the antenna and note the strongest direction.
Try it
1. In the 3D, set the scope slider to 3 waves and write f. 2. Move the receiver slider so the sound just turns clear. What is the smallest signal? 3. On the fibre slider find the length where half the power is left (about 15 km). Predict first, then check.
Key formulas and definitions
- f = 1 / T
- m = (Emax − Emin) / (Emax + Emin) × 100 %
- dB = 10 log10(P_out / P_in)
- Fibre loss (dB) = loss per km × length
- S/N = signal level / noise level
Worked examples
1. One wave on the scope takes 0.5 ms. Find the frequency.
f = 1 / T = 1 / 0.0005 = 2000 Hz = 2 kHz.
2. An AM wave has Emax = 9 V and Emin = 3 V. Find the modulation depth.
m = (9 − 3) / (9 + 3) = 6 / 12 = 0.5, so 50 %.
3. A fibre of 30 km loses 0.2 dB per km. What is the total loss and what part of the power remains?
Loss = 0.2 × 30 = 6 dB. Remaining = 10^(−0.6) = 0.25, so about one quarter.
4. 10 mW enters a cable and 1 mW leaves. Find the loss in dB.
dB = 10 log10(1 / 10) = −10 dB, so the loss is 10 dB.
5. A receiver gives clear sound at 2 µV, another at 8 µV. Which is more sensitive?
The one that needs only 2 µV. A smaller needed signal means higher sensitivity.
Common mistakes
- Reading the scope screen as frequency. The screen shows time; you must compute f = 1/T.
- Measuring transmitter power straight into the antenna instead of a dummy load.
- Thinking a bigger sensitivity number is better. A smaller microvolt number is better.
- Adding powers instead of dB. dB values add; milliwatt values multiply.