MSK (Minimum Shift Keying) CML Microcircuits, MSK (Minimum Shift Keying) Datasheet - Page 19

• Manufacturer Part Number: MSK (Minimum Shift Keying)
• Description: Minimum Shift Keying and its Application to Wireless Data Transmission
• Manufacturer: CML Microcircuits

MSK and its Application to Wireless Data Transmission
3.2.1.5 Discriminator Low Pass Filter
The discriminator low pass filter is designed to pass frequencies up to half the bit rate. This corresponds to the highest
frequency needed to pass an NRZ data sequence 101010... As seen in spectral plots in Figure 21 and Figure 22, the
discriminator low pass filter rolls off sharply beyond its cutoff to remove high frequency noise out of the discriminator.
These plots depict a finite impulse response filter with weights of a windowed sinc( ) function. See section 3.4 for a finite
impulse response filter model.
3.2.1.6 Bit Slicer
The bit slicer is simply a comparator with a threshold half way between a one and a zero. That is it outputs 1 for inputs
greater than 0.5 and output 0 for inputs less than 0.5. Section 3.4 shows pseudo-code for a comparator. That model
provides for hysterisis; however for use as a bit slicer both the upper and lower threshold can be set to 0.5 to essentially
eliminate the hysterisis.
3.2.2 MSK Carrier Detection
The carrier detector is made up of three blocks, the bad zero crossing detector, sliding window filter and Comparator with hysterisis.
The carrier detect section monitors the information out of the zero crossing detector to determine whether the zero crossing are timed
properly for a 1200 or 2400 baud MSK signal. If the zero crossings are properly spaced a logic level is output to indicate that a valid
MSK signal is being received.
© 1997 MX COM Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
Carrier
Detect
Input
www.mxcom.com Tele: 800 638-5577 910 744-5050
Bad Zero
Crossing
Detector
Figure 23: Carrier Detect Block Diagram.
-12
-15
-18
-21
-24
-27
-30
-33
-36
-39
-42
-45
-48
-51
-54
-57
-60
-63
-66
-69
-72
-12
-15
-18
-21
-24
-27
-30
-33
-36
-39
-42
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-48
-51
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-60
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-66
-69
-72
-3
-6
-9
-3
-6
-9
3
0
3
0
0
0
Figure 21: 1200 Baud LPF.
Figure 22: 2400 Baud LPF.
1200
600
Sliding
Window
Averager
19
1200
2400
Frequency (Hz)
Frequency (Hz)
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1800
3600
2400
4800
Comparator
w/Hyteresis
3000
6000
Fax: [phone]
3600
7200
Carrier
Detect
Output
Doc. # 20830084.001
APPLICATION