Amplitude Modulation and Demodulation Lab#3

Amplitude Modulation and Demodulation

Lab#3

Objective To observe amplitude modulation/demodulation in the time and frequency domain.

The 9410 AM / DSB / SSB Generator

Controls and Connectors – Carrier Level – changes the level of the carrier when using AM. To produce DSB-SC and SSB change the carrier level to min. RF Tuning – sets the carrier frequency RF Gain –adjusts the level of the RF signal in AM / DSB section AUDIO INPUT – 10k connector to insert a modulating audio signal of maximum level 0.5 Vpp. AM / DSB – 50  output of the RF signal modulated AM or DSB-SC according to the carrier level.

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What is Inside the Black Box?

From this section you can see your signal is just biased up or down with a DC level from the Carrier level pot. Check it out on your scope. Watch the output as you vary the pot. It is this dc offset that changes the double balanced modulator from suppressing the carrier to putting out a carrier.

Output

Carrier Input Signal Input

Gain Adjust

Bias

V-

The above section consists mainly of the LM1496 balanced modulator-demodulator chip from National Semiconductor. Support circuitry not shown. Your RF tuning knob is controlling a Voltage Controlled Oscillator (VCO see page 185 Lathi) not shown in this diagram. It provides the “Carrier Input” to the MC1496. The “Signal Input” is your input tone after the above section has changed its dc bias level. When a DC bias is applied to the signal input the double balanced modulator is no longer perfectly balanced so we also get a carrier component on our output. When you turn the carrier level down to zero the double balanced modulator then puts out only a DSB-SC signal. This signal is then sent to the RF Amplifier. For more information on modulators see Lathi pages 155 to 161.

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RF Amplifier

+ -

RF Output 50 ohms

RF Gain

RF In

Here your RF gain pots changes the DC bias level on the dual gate FET and that changes the gain of the RF signal coming from the mixer section. This signal is then fed into a class B amplifier to provide the power output.

The 9411 AM / DSB Receiver

Controls and Connectors RF Tuning – allows the receiver to be tuned to the input signal, just like your radio at home. AGC (Automatic Gain Control). Detectors – You can select the method of signal detection you want to use: ENV – envelope detection, good only for DSB-TC SYNC – synchronous detector for DSB-TC COSTAS – used for DSB-SC detection RF INPUT – 50  input, use this one when coming from the AM generator.

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OSC OUTPUT – the local oscillator output. AUDIO OUTPUT – the demodulated signal output

What’s inside the black box Here is the big picture. Filter the signal (image rejection), give it some gain, mix it down to the IF frequency (selectivity), filter it again and then detect (demodulate) the signal or information you have sent. The Automatic Gain Control (AGC) just helps to keep the output constant by reducing the effects of fading and multipath interference. Front End Here we see the first input filters. Notice that the tuned circuits are controlled by a pair of ganged variable capacitors. The first circuit filters out input signal for the region we are interested in (450 kHz to 1600 kHz). The second capacitor is set to create a frequency that is always 455 kHz (above or below) the station we want to tune in. i.e. It sets the local oscillator that will be mixed with our input signal to give us the intermediate frequency (IF).

RF Input 50 ohms

RF Input 50 kohms

RF Tuning 540 - 1600 kHz

Local Osc.

To LM3820 RF Inputs Radio RF amplification, generation of the local oscillator, IF gain, and mixing are done on a single chip, (the LM3820). A spec sheet and circuit schematic have not been located for this chip.

LM3820 N/A Detectors We have two type of AM (DSB-TC and/or DSB-SC) detectors. The classic and very easy to manufacture envelop detector (non-coherent demodulation) and the Phase Locked Loop (PLL) detector (synchronous demodulation).

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Envelope detection:

From the IF

1N914 0.001

1.2 k 4.7k

0.1 0.01

To the Audio Amp

Synchronous: IF in

PLL Mixer

90 Phase Shifter VCO

5 kHz

Audio Out

Detector Mixer

5 Hz

DSB Costas – used only for DSB-SC

Also see pages 187-188 LATHI

Modulation index for AM The dimensionless constant  is the modulation index or the percentage modulation when it is expressed numerically as a percentage. To avoid envelope distortion due to over modulation, the modulation index  must be kept below unity or 100%.See Lathi page 162-164 for the derivation of . The following methods are used to measure modulation index . Time domain mode:

Where m is the modulation index. A is the peak to peak amplitude of the output signal B is the minimum peak to peak amplitude.

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Time domain XY mode (trapezoidal method) In the Horizontal controls hit the Main/Delayed button

Frequency domain:

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