Démodulation Fsk
The Communications Edge ™
Tech-note
Author: Bob Watson
FSK: Signals and Demodulation
The most commonly used signal parameters for describing an FSK signal are shown in
Figure 2. The minimum duration of a mark or space condition is called the element length. Typical values for element length are between 5 and 22 milliseconds, but element lengths of less than 1 microsecond and greater than 1 second have been used. Bandwidth constraints in telephone channels and signal propagation considerations in HF channels generally require the element length to be greater than 0.5 millisecond. An alternate way of specifying element length is in terms of the keying speed. The keying speed in “bauds” is equal to the inverse of the element length in seconds. For example, an element length of
20 milliseconds (.02 seconds) is equivalent to a 50-baud keying speed.
Frequency measurements of the FSK signal are usually stated in terms of “shift” and cen-
DATA
Binary FSK (usually referred to simply as
FSK) is a modulation scheme typically used to send digital information between digital equipment such as teleprinters and computers. The data are transmitted by shifting the frequency of a continuous carrier in a binary manner to one or the other of two discrete frequencies. One frequency is designated as the “mark” frequency and the other as the
“space” frequency. The mark and space correspond to binary one and zero, respectively. By convention, mark corresponds to the higher radio frequency. Figure 1 shows the relationship between the data and the transmitted signal. quency and the mark or space frequencies.
The deviation is also equal, numerically, to one-half of the shift.
FSK can be transmitted coherently or noncoherently. Coherency implies that the phase of each mark or space tone has a fixed phase relationship with respect to a reference. This is similar to generating an FSK signal by switch-
1
time (a)
0
SIGNAL
Tech-note
Author: Bob Watson
FSK: Signals and Demodulation
The most commonly used signal parameters for describing an FSK signal are shown in
Figure 2. The minimum duration of a mark or space condition is called the element length. Typical values for element length are between 5 and 22 milliseconds, but element lengths of less than 1 microsecond and greater than 1 second have been used. Bandwidth constraints in telephone channels and signal propagation considerations in HF channels generally require the element length to be greater than 0.5 millisecond. An alternate way of specifying element length is in terms of the keying speed. The keying speed in “bauds” is equal to the inverse of the element length in seconds. For example, an element length of
20 milliseconds (.02 seconds) is equivalent to a 50-baud keying speed.
Frequency measurements of the FSK signal are usually stated in terms of “shift” and cen-
DATA
Binary FSK (usually referred to simply as
FSK) is a modulation scheme typically used to send digital information between digital equipment such as teleprinters and computers. The data are transmitted by shifting the frequency of a continuous carrier in a binary manner to one or the other of two discrete frequencies. One frequency is designated as the “mark” frequency and the other as the
“space” frequency. The mark and space correspond to binary one and zero, respectively. By convention, mark corresponds to the higher radio frequency. Figure 1 shows the relationship between the data and the transmitted signal. quency and the mark or space frequencies.
The deviation is also equal, numerically, to one-half of the shift.
FSK can be transmitted coherently or noncoherently. Coherency implies that the phase of each mark or space tone has a fixed phase relationship with respect to a reference. This is similar to generating an FSK signal by switch-
1
time (a)
0
SIGNAL