I.
FILTER ADJUSTMENTS
The Band pass and the low pass filter will be used extensivley in this experiment since most
detection schemes require pre-detection (band-pass) filter and a post detection (low-pass) filter.
There filters are readily available in the simulink block sets.
The filters used here are the digital FIR filters, the proper parameters as shown in figure …
The type of filter can be chosen from the pop down menu of “filter type” and the properties of the
order of the filter and the cut-off frequency is also specified. Options are also available to change
the window type from hamming, hanning, rectangular, Blackman, triangular etc.
As a method to double-check the proper tuning of the parameters, a signal can be filtered and the
parameters tuned to adjust the parameters of the filter. The simulation set up as shown in figure
… is set up for tuning the filters.
The input signal is a sine wave of frequency 1kHz and the filter used is a Digital FIR filter with a
upper cut off frequency – 0.1 (1kHz) .
The output of the filter can be seen as shown in figure …
It can be observed that by changing the upper cut-off frequency of the filter the filter
characteristics can be changed.
Similar procedure can be adopted for the band-pass filter. The simulation circuit is shown in
figure .. and the lower and upper cut of frequencies of the band are taken as 0.3 and 0.7 for a
input sine wave at 5kHz frequency. It can be noted that for filter characteristics outside this band
frequencies the filter characteristics are different.
The block parameters of the digital FIR filter design is shown in figure below.
The output of this is observed to be two spectrums at 5kHz. The peak of this can be observed at
5kHz and such that the cut off values are at 0.707 times the peak value at the cut-off frequencies.
II TRANSMITTER
Before we can receive an AM or DSB signal, we first need a method of generating such signals.
Borrowing these from the previous laboratory.
A Transmitter 1
The transmitter is generated as shown in figure .. The VCO generates the carrier
frequency. Here in another sine wave generator is used for that purpose. The whole modulation
signal is generated in the process. The thing to be noted here is that one can generate all forms of
AM and DSB signals can be generated using a dc power supply voltage. We shall call this system
the “transmitter”. Remember in building this system you need to use the other summer and
multiplier in the detection system.
The output spectrum is shown below for the input sine wave.
B TRANSMITTER 2
The transmitter can also be generated using another method which is slightly different
from the previous one. The function generator produces the carrier frequency fc, while the VCO
produces the modulation. You can generate DSB or AM by adjusting the dc level. For DSB adjust
dc until you null out the carrier (approximately –1.2 to –1.6 Volts).
III COHERENT DETECTION
In coherent detection, a reference signal of the same frequency AND phase as the carrier
signal is used. The signal derived from the “transmitter” in previous section will be used directly
here.
A. Detection with out Noise: The circuit has been implemented as shown in figure… and it can
be verified that one can receive DSB-SC, AM with 100% modulation and AM with 50%
modulation. For AM you can use either of the transmitter. The oscilloscope can be used to
examine the signal s it is processed through the detector. The reference is sinusoidal and a
direct copy of the transmitter’s carrier signal.
Various levels of modulation can be obtained using the modulation index a after the first sine
wave generator.
Note: Using a sine wave generated generates The VCO frequencies. The reference is basically a
sine wave replica of the original carrier wave form.
The input wave form is shown in figure .. The scope2 actually measures the output of the
multiplier and the waveform is as shown in figure..
This waveform when coherent detected using a reference carrier gives a replica of the input
waveform. The final output is shown in figure…
The final output is shown in figure.
It is also worth nothing the difference the lowpass filter used in the circuit makes. The output of
such a state is shown in figure…
The effect of using a different reference was also observed such as a square wave reference with
the same frequency and a phase shifted version of the reference sinusoidal signal. It was observed
that there is no significant change in the output, mainly because the frequency is the reference and
which was constant in all the cases.
B DETECTION WITH NOISE
The summer represents the communications channel where the transmitter’s signal is
perturbed by additive noise. The BPF serves as a pre-filter to band-limit the noise. The bandwidth
of the BPF is wide enough to pass the transmitter’s signal.
Figure shows the simulation layout for detection with noise.
The transmitter is the same as the one used in the previous parts. The modulated input is added
with a band limited white noise and is band pass filtered with a 5kHz center frequency. The
output of this is coherent detected and the final output is obtained.
The signal at various stages are shown in the following figures:-
The modulated input looks somewhat as shown in figure…
The output of the spectrum analyzer looks some what like this :-
The output reconstructed signal after detecting looks like what is shown in figure …
A band limited white noise is used as a random noise generator. It generates a band limited white
noise whose parameters are shown in figure
IV INCOHERENT (Envelope) Detection
The envelope detector was used to detect the modulated signal. Notice that no transmitter
reference is required for the same. The output for 100% Am and Am wave is shown in figure ..
The output waveform for different values of modulation is observed and is plotted as shown in
figure.Yes the BPF is cricitcal enough to detect and receive 50% modulation AM. And DSB-SC.
The circuit for implementation of the envelope detector is also shown in figure .. I can be seen
that the basic transmitter block of the previous section is still used as a source of generation of
generalized AM / DSB modulation with different depths of modulation.
The output spectrum is shown in figure ..
V COHERENT AM DETECTION WITHOUT A TRANSMITTER REFERENCE