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APPENDIX
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EXAMPLE 1: MODEM
A modem application involves the transmission of digital
data over a short length of transmission line at 1200bps. The
modem will use FSK modulation and will send four
frequencies,
DC (Off)
900 Hz (End of Message)
1200Hz (Mark, Logic 1)
2200 Hz (Space, Logic 0)
Because of the short distance of transmission, no delay
equalizer will be necessary. Only a simple transmitter and
receiver are needed.
A 3
kHz bandwidth will be adequate.
Can this be done with the 2920?
MODEM BLOCK DIAGRAM
(As Supplied by the Customer)
RECEIVER
~I
BPF
~I
LIMITER
~L.-
_ _ _ ---'
TRANSMITTER
DIGITAL
FSK
IN
OUT
This is not sufficient becuase it does not identify 2920
building block functions.
Further questions yielded the following diagram and block
delinition.
A-6
The modem FSK modulator will convert a serial digital
input into a continuous analog FSK output which can be
transmitted over a telephone line. Since four frequencies will
be used - DC (off), 900 Hz, 1200 Hz, and 2200 Hz - two digital
inputs will be required.
The modem receiver converts the FSK signal received
from the telephone line into a serial digital bit stream. It con·
verts the received frequency into a DC level, and then uses
this level to derive a digital output corresponding to the bit
which was sent by the transmitter.
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