MSK (Minimum Shift Keying) CML Microcircuits, MSK (Minimum Shift Keying) Datasheet - Page 17

• 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
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I/O, Variables, and Constants
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Pseudo Code for Monostable
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3.2.1.4 Discriminator
The discriminator uses the output signal of the monostable to reconstruct the digital information. In Figure 19 and Figure
20, the operation of the discriminator is displayed. The signals in these figures are under perfect conditions without
passing through the receive filter. Under more realistic conditions the output the discriminator looks much less perfect and
hence needs to pass through the Post Discriminator Filter to look more like the transmitted bits. See the discriminator
output trace in Figure 31 to understand the need for the low pass filter.
3.2.1.4.1 1200 Baud Case
In Figure 19, the TX data in signal is included as a reference to the digital data being decoded. The TX MSK out signal is
the modulated signal that is applied to the zero crossing detector. As seen in this signal, a data bit one has 2 zero
crossings while a data bit zero has 3 zero crossings per bit time. The monostable output signal is the input to the
discriminator. (Refer to the monostable for an explanation of it’s output signal). The input signal to the discriminator is
time shifted to create two additional signals. The first is shifted by one sixth of a bit time and the second is shifted by one
third. These three signal are logically NANDed together to produce the discriminator's output signal. For a logic one the
zero crossings are spaced such that the output of the monostable is low for one sixth of a bit time before the middle of the
bit and again before the end of the bit. The logical NANDing of the three signals creates a pulse width equal to one half of
the bit period for each of the two zero crossings in a logic one. This creates a digital one with the appropriate bit timing.
For the zero bit, the zero crossings keep the monostable output high so the NAND output stays low.
3.2.1.4.2 2400 Baud Case
In Figure 20, the TX data in signal is included as a reference to the digital data being decoded. The TX MSK out signal is
the transmitted signal for which the discriminator is decoding. As seen in this signal, a data bit with the value of one has 1
zero crossings and a data bit with the value of zero has 2 zero crossings per bit time. The monostable output signal is the
input to the discriminator. The input signal to the discriminator is time shifted by one half of a bit period and is logically
NANDed with the input. The logical NANDing of the two signals creates the proper bit period. For a zero, the zero
crossings keep the monostable output high so the output of the NAND stays low.
© 1997 MX COM Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
input[i]
output[i]
T
T
timer = 0
if input[i] = 1
if timer > 0
if timer > 0
ms
ms
= N/3
= N/2
// Current input sample
// Current output sample
// 1/3 Bit time @ 1200 Baud
// 1/2 Bit time @ 2400 Baud
// initialize timer first time
then
then
else
then
www.mxcom.com Tele: 800 638-5577 910 744-5050
timer = T
output[i] = 1;
output[i] = 0;
decrement timer;
Pseudo-Code Listing 3 Monostable Model
ms
;
17
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Doc. # 20830084.001
APPLICATION