The ideas behind amplitude modulation.
Ten minutes of reading. Play with the modulator, then every waveform on the trainer will look familiar.
Why modulate at all?
A quarter wave antenna for a 3 kHz voice signal would be about 25 km long. At 1 MHz it is 75 m, and at 455 kHz the trainer works on a bench.
Every station shares the same air. Each one moves its message to its own carrier frequency, so a receiver can tune to just one of them.
The IT-4101 has a 455 kHz local oscillator (the standard AM radio IF) and a 1 MHz oscillator (inside the AM broadcast band).
Three signals and one product
The message does not change the carrier frequency. It changes the carrier height. The outline of the peaks is called the envelope and it has the shape of the message. Try it:
Modulation index from the scope
On the oscilloscope you cannot read m directly, but you can read the largest and the smallest peak to peak height of the envelope:
Manual example: Vmax = 10 V and Vmin = 6 V give m = 4 / 16 = 0.25.
What the spectrum shows
Multiplying two cosines makes a sum and a difference frequency. So a single tone AM signal has exactly three lines: the carrier at fc, the lower side frequency at fc − fm and the upper side frequency at fc + fm. The original 3 kHz tone is no longer in the spectrum.
Bandwidth of AM and DSB-SC is twice the highest message frequency. 1 MHz carrier and 1 kHz tone: lines at 999 and 1001 kHz, bandwidth 2 kHz.
Moving the carrier from 900 MHz to 1800 MHz moves the signal but does not widen it. Changing the amplitude does not widen it either.
The spectrum analyzer shows power in dBm. A line 6 dB lower carries a quarter of the power: at m = 1 each side line is 6 dB below the carrier.
DSB-SC and the balanced modulator
The trainer's balanced modulator is a multiplier. Inside it two matched halves each produce the carrier plus a sideband term. The carrier terms are equal and opposite, so they cancel and only the product leaves: double sideband, suppressed carrier.
- On the scope the envelope looks like |message| and touches zero; the carrier phase reverses at each zero.
- With a 3 kHz tone and the 455 kHz carrier the lines are at 452 and 458 kHz, with no line at 455 kHz.
- The Carrier NULL trimmer sets the balance. Turn it away from the null and the carrier comes back: that is AM with carrier.
- At m = 1, two thirds of AM power sits in the carrier, which carries no information. DSB-SC saves it, but the receiver needs a local carrier (Lab 6).
SSB with the ceramic filter
Both sidebands carry the same message, so one is enough. The trainer's ceramic band pass filter is centred near 450 kHz and is only a few kHz wide. Feed it the 455 kHz DSB-SC signal and the lower sideband at 452 kHz passes while the upper sideband at 458 kHz is pushed down by about 34 dB. The result is single sideband: half the bandwidth, and on the scope a constant envelope.
The trainer: what connects to what
Before each part: Interface Selector on Audio Generator Output, speaker OFF, antenna OFF, then power ON and set the audio to about 3 kHz and 700 mV p-p on CH1.
Using the instruments
Oscilloscope
- Clip the probe tip on the test point and the ground clip on any GND socket. Without the ground clip you see 50 Hz hum.
- Audio O/P on CH1 at 200 us/div and 200 mV/div shows about six cycles of 3 kHz.
- Trigger on CH1 (the audio). The 455 kHz carrier on CH2 is 150 times faster, so it looks like a filled band inside the envelope.
- Autoset picks the scales for you; Measure shows Vpp and frequency; Cursors read Vmax and Vmin.
Spectrum analyzer
- Plug the analyzer input on the modulator output.
- Centre 455 kHz, span 30 kHz (or the 455 kHz preset). Markers M1 to M4 list the strongest lines.
- For the 1 MHz parts use the 1 MHz preset.
Multimeter and counter
- The meter reads RMS: 700 mV p-p of sine is 0.247 V.
- A handheld meter reads AC only up to about 100 kHz, so it cannot measure the RF carrier. Use the scope.
- The counter on the 455 kHz output reads 455.00 kHz.
What you should see
| Where | Expected |
|---|---|
| Audio O/P (CH1) | 3.00 kHz, 700 mV p-p |
| Balanced Modulator O/P (5.5.1) | DSB-SC, about 500 mV p-p; lines at 452 and 458 kHz, carrier about 32 dB lower |
| 1MHz Balanced Modulator O/P (5.5.2) | lines at 997 and 1003 kHz |
| Power Amplifier O/P (5.5.3) | 150 mV p-p at 0.7 V audio; 200 mV p-p at about 1.0 V audio |
| Ceramic filter O/P (SSB) | 452 kHz strong, 458 kHz about 34 dB lower, flat envelope |
Check yourself
Q1A modulated signal swings between 10 V (max) and 5 V (min). What is m?
m = (10 − 5) / (10 + 5) = 1/3 ≈ 0.33.
Q2What range of m allows proper recovery with an envelope detector?
0 < m ≤ 1. Above 1 the envelope is distorted (over-modulation).
Q3The message bandwidth is B. What is the AM bandwidth?
2B: one sideband on each side of the carrier.
Q4The carrier changes from 900 MHz to 1800 MHz. What happens to the bandwidth?
Nothing. The bandwidth depends only on the message.
Q5Why can the carrier not be lower than the message frequency?
The lower sideband would fold below zero and overlap the upper one; the envelope would no longer follow the message, and the antenna would be impractically large.