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Principles of Communications Engineering (Matched Filters (Required…
Principles of Communications Engineering
Noise In AM and FM systems
Noise Current and Voltage
Noise Voltage
Noise Current
Noise in AM Systems
Noise in FM Systems
Matched Filters
Required Characteristics
Filter has to have a band pass filter, with the frequency set to the desired transmitted signal to be received.
If the transmitted signal has small frequency components at other frequencies then we need a stop band to reject any noise present in this frequency range.
Transversal Filter
Intergrate and Dump Filter
Correlator receiver
Spread Spectrum Systems : S.S
S.S transmits data at a much larger frequency that what is actually required.
Bandwidth is increased by altering data signal using PN code
Receiver requires the same PN code to recover the transmitted data.
ADVANTAGES OF SS SYSTEMS
Low probability of interception. Finite power used to transmit message waveform is spread over a large bandwidth. This makes the message extremely hard to detect by an eavesdropper.
Good at rejecting interfering signals i.e. jamming signals.
Higher Bandwidth=Higher data rates
Multiple access: SS. systems use PN codes to spread data. As each receiver must have the PN code in order to recover the data, multiples signals can occupy the same bandwidth without interference.
Direct Spread Spectrum System (DSS)
Transmitter and Receiver consisting of BPSK modulator and de-modulators respectively
Searching
Circuit searches for the synchronisation of transmitted PN code and the replicated PN code on the receiver end
CIRCUIT DIAGRAM
Control logic constantly adjusts tie delay of the PN code.
The control logic does this till the output from the comparator rises above threshold level VRef
When the output rises above VRef, PN code generator is synchronised within half the period fo the PN code (T_ch)
(Image: Synchronisation not achieved) only a small amount of energy passes through the band pass filter.
The comparator input is therefore small and the system acknowledges that synchronisation is not achieved.
(Image: Synchronisation Achieved).
Tracking (Delay Lock Loop)
Maintains synchronisation of PN codes
(Circuit diagram)
The PN code generators create 'Early' and 'Late' PN codes which are multiplied with the DSSS signal.
NON SYNCHRONISED CONDITION:
VCO is fed with + or - voltage (if Early signal difference is greater or if Late signal difference is greater respectively)
Signal is then sped up (more positive votlage) or slowed down (more negative voltage) in order to correct the signal into synchronisation
SYNCHRONISED CONDITION:
late and early signals are synchronous with the DSSS signal, no error voltage difference is present.
VCO (Voltage controlled Oscillator) does not change and the system remains in synchronisation
Spreading by multiplication of PN code; S.S signal changes much faster than the data signal, which equates to higher bandwidth.
The S.S signal transmitted looks noisy. Therefore any eavesdropper without the PN code to demodulate the signal would only see a noisy signal
