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.
- 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.