bandwidth part 1 what does it mean? different things to different people
TRANSCRIPT
Bandwidth part 1
What does it mean?
Different things to different people.
The bandwidth of a signal is the range of frequencies it contains
Bandwidth = f max – f min
The bandwidth of this audio signal is about 3,500 Hz
Telephone call bandwidth = 4 kHz
CD quality music bandwidth = 20 kHz
Digital TV channel bandwidth = 8 MHz
FM radio station bandwidth = 15 kHz
So I want to transmit Radio Drummy to the people
What frequency radio waves do I transmit?
Frequency modulation
I transmit at a very high frequency
( VHF ) e.g. 91 Mhz
But modulate it slightly
Change the frequency.
The change in the frequency is my
15 kHz signal.
I transmit 91 MHz + or – 7.5 kHz
Carrier frequency
e.g. 100.8 MHz
The government owns the airwaves. You have to pay for bandwidth.
You also need a gap between stations. Why?
What bandwidth does a digital signal need?
How could we send a digital signal on the airwaves?
We can’t just send this
An electromagnetic wave is made up of oscillations.
The minimum frequency we need to send the above is this …
Minimum frequency (bandwidth) needed = ½ bits / second
In practice we need higher frequency components
As a rule of thumb …
Minimum required bandwidth = Bit transmission rate
A digital TV channel is 8 million bits /sec. What bandwidth does it need?
Could digital TV be transmitted in the VHF waveband?
Digital TV channels are transmitted at extremely high frequencies
MICROWAVES
e.g. between 30 and 31 GHz
Why?
Bandwidth part 2
Bandwidth of a signal
= the range of frequencies it contains, the width of its spectrum W
Minimum sampling rate needed (Nyquist rate)
= 2 x highest frequency contained in signal = 2 W
Bit rate = sampling rate x bits per sample = 2 W b
Bandwidth needed to transmit a digital signal
(how much of the waveband we need)
Minimum bandwidth needed = ½ bit rate
In practice we need more than this
Bandwidth needed = bit rate = B = 2Wb
The amount of noise in a signal limits the number of bits per sample b.
Why?
Noise is greater than difference between sampling levels.
Sampled bits include noise information.
Noise falls between sampling levels
To avoid sampling noise
Maximum levels = 2b = V total / V noise
e.g. if we are sampling a varying voltage of amplitude 200mV at 8 bits per sample how much noise is tolerable?
V noise = V total / 256 = 0.781 mV