These laboratory-based tutorials using MultiSim simulation package are intended to re-inforce topics covered in lectures.













BTEC-2                AM DEMODULATION








BTEC 6                 PHASE SHIFT KEYING (PSK)




These laboratory-based tutorials using MultiSim simulation package are intended to re-inforce topics covered in lectures.


They will not be assessed but you should keep a good record and file to support your learning. You will work on these in timetabled laboratory sessions, but you may also work on them as part of your student centred learning. You should aim to work through as many as you can.


In these tutorials you will review and investigate some basic modulation and demodulation techniques and processes.


Where appropriate equations will be given: you should also refer to your notes for further detail, derivations and examples.








The schematic diagram above shows an amplitude modulator in which a DC voltage is added to a message signal, and this is then multiplied by a carrier.


The DC voltage, VDC, is set by R1.

The signal generator at 1 kHz represents the message signal, m(t) = Vmcosωmt.

The signal generator at 10 kHz represents the carrier, cosωct.


The output modulated signal may be represented by:




                     vS(t) = (VDC  +  m(t)) cosωct


                              = VDC cosωct + m(t)cosωct


Carrier component


Upper and Lower Sidebands

(USB and LSB)





Alternatively, with m(t) = Vmcosωmt,


                     vS(t) = (VDC  + Vmcosωmt) cosωct


                     vS(t) = VDC   cosωct + Vmcosωmt cosωct


A  trigonometric identity is:


            cos A  cos B = ½ cos (A+B) + ½ cos (A - B)





Carrier component at fc Hz.


USB at (fc + fm) Hz.


LSB at (fc - fm) Hz.







Observe the input and output signals on the oscilloscopes and the spectrum of the DSBAM signal on the spectrum analyser. Use a spectrum analyser to observe the input signals. Relate what you see to the outline theory presented above and your notes.



Modulation depth in AM is defined as m = , hence changing VDC is one way of changing the modulation depth. Change the DC offset and observe the effect on the waveforms and the output spectrum.


Keeping Vm constant, set the modulation depth to m < 1, m = 1, m > 1 and m = infinity. For each setting of modulation depth, m, observe and record the DSB waveform and spectrum, including the voltage amplitude and power in each component, noting how they relate to modulation depth. Note also, when m > 1, how the phase of the DSB envelope alternates between 0 and 180 degrees.


Refer to your class notes and handout notes and compare your results with what you would expect from theory.


Set the modulation depth to m = 0.3. Determine the ratio of power in the USB to the total power in the AM signal by calculation and measurement.






COMM2-2 comprises 3 parts.


1.      An amplitude modulator covered in COMMS2-1. This is to generate the DSB signals.


2.      An envelope or non-coherent detector (demodulator).


3.      A synchronous or coherent demodulator.



Observe the output signals for the envelope detector. Vary the  modulation depth of the AM signal. Over what nominal range of modulation depth does the envelope detector perform the demodulation function we require? Explain the operation of the envelope detector in terms of ‘large signals’ at the input.


The synchronous demodulator comprises a multiplier with a local oscillator, and a low pass filter, with a cut-off frequency of about 1 kHz. The local oscillator (LO) may in general be written as:


                        LO = cos((ωc + Δω)t + φc)


where Δω represents a frequency offset in the local oscillator and,

φc represents a phase offset in the local oscillator.


A coherent local oscillator, and hence coherent demodulation, requires that Δω and φc are both equal to zero. Hence for an ideal coherent LO,


                        LO = cosωct


Observe the demodulated output for a range of modulation depths. It is useful to do this when still observing the output from the envelope detector. Note in this case the synchronous demodulator will demodulate the AM input irrespective of the modulation depth, whereas the envelope detector does not.



For a DSB input given by:

and a local oscillator given by LO = cosωct, derive an equation for the demodulated output signal from the synchronous demodulator, and compare your results with this.


Now try adding a frequency offset and a phase offset in the local oscillator, (by double clicking on the LO signal generator) and observe the effect on the output.


Derive a further equation using a DSB input given above as    and a local oscillator given by

LO = cos((ωc + Δω)t + φc).


Use this equation to explain your results.










This model is essentially the same as COMMS2-1, amplitude modulation, but the sine wave analogue message is replaced by a square wave digital message. Thus rather than an analogue message m(t), we have a digital message d(t) consisting of ‘ 1, 0, 1, 0, 1, 0, 1, 0 …….’ Etc.


By varying the DC voltage offset, a range of amplitude shift keying (ASK) and phase reversal keying (PRK) digital modulation can be produced. PRK is a specific form of phase shift keying, PSK. Obviously then, there are strong links between analogue AM and digital ASK and PRK modulation techniques.


Switch on all the instruments and run the model. Observe how the waveforms and spectrum change for different settings of the DC offset. Identify the conditions to produce ASK as distinct from PRK.


Note that the digital message signal, (consisting as it does in this simulation of a 1, 0, 1, 0 … sequence) appears as a square wave which has only odd harmonics in its spectrum. Notice how the ASK and PRK spectrum consists of USB and LSB with only odd harmonics.


COMMS2-4 will repeat this simulation but with a more realistic ‘random’ digital message produced by a pseudo random sequence generator.





This comprises two main parts. In the first part a simulated FM signal generator is used and the waveforms and spectra of the FM signal may be observed.  In the second part a voltage-to-frequency converter (V/F) is used as the frequency modulator.


Consider the first part. The FM signal generator is set to frequency modulate a 10 kHz carrier with a 1 kHz message signal.

The separate 1 kHz signal generator is NOT linked to the FM generator and is there, only, to give a reference 1 kHz. If the FM generator is changed to give a 2 kHz message frequency for example, the reference signal generator will need changing.










Refer to class and handout notes to show that an FM signal may be represented by:




VC is the amplitude of the carrier.

β is the modulation index.

Jn(β) are Bessel coefficients obtained from tables or ‘graphs’.


Switch on the instruments and observe the waveforms and spectrum. Notice how the frequency of the modulated signal varies in relation to the amplitude of the message signal. You don’t need to change the amplitude. Note that the amplitude of the modulated signal is constant, only the frequency changes.


Now observe the spectrum of the frequency modulated signal. Initially the modulation index, β, should be set to 2.4. The spectrum should appear something like that shown below.