1.7 Analogue Modulation Techniques Flashcards
Does FM have distinct sidebands?
No. FM has infinite sidebands since the modulation can change to any frequency (in theory), however as you move further away from the carrier frequency, the power levels become negligable.
What is the FM Modulation Index?
Expresses the maximum frequency deviation as a multiple of the maximum frequency.
What is the difference between wideband and narrowband FM signals?
Narrowband FM - Beta between 0.5 and 1
Wideband FM - Beta greater than 1
FM Transmittion Efficiency:
How does it relate to the Modulation Index?
Where is the power distributed as the system is more efficient?
FM signals are more efficient as the Modulation Index is increased.
Power in the signal is spread out into the sidebands.
What is the tradeoff with FM efficiency?
Wideband FM signals are more efficient but the require more bandwidth.
How vulnerable to noise are FM signals?
Highly resistant as information is carriedin frequency, not amplitude.
Describe the FM Capture Effect
In a receiver only the the stronger of two signals at, or near, the same frequency or channel will be demodulated.
How does the frequency spectrum of a PM signal compare to an FM signal?
Nearly identicle.
What are the noise resistnace characteristics of PM?
Similar to FM.
What is the advantage of PM?
Uses less bandwidth because it uses a single frequency.
What is the PM Modulation Index?
The amount of phase deviation used in the signal.
Describe a Non-Coherent Receiver
Used for AM demodulation.
Rectifier diode removes negative components.
RC circuit acts as a low pass filter and removes high requency carrier wave, leaving behind the baseband.
Capacitor removes the DC offset. Message is ready.
Describe a Coherent receiver
Used for AM Demodulation.
Sidebands are split in half and spread in both directions.
High frequency (carrier) filtered.
Remaining sidebands are constructively added together as the message.
Describe FM Demodulation
FM creates a pseudo-AM signal which is then demodulated using AM Demodulation techniques.
What is a Superheterodyne Receiver?
A local Oscillator mixes to an intermediate Frequency (IF) and then acts like a fixed-frequency receiver.
