ECE 4710: Lecture #25 1 Frequency Shift Keying  Frequency Shift Keying = FSK  Two Major FSK Categories  Continuous Phase »Phase between bit transitions is continuous  no abrupt change  Discontinuous Phase »Phase between bit transitions is discontinuous  abrupt change

ECE 4710: Lecture #25 2 Discontinuous Phase FSK  Discontinuous Phase FSK = DP FSK  Generated by switching Tx output between two different oscillators with different frequencies (1/0) and phases (not synchronized)

ECE 4710: Lecture #25 3 Discontinuous Phase FSK  Instantaneous phase + frequency change at bit transitions cause high-frequency amplitude variations in output signal  No longer constant envelope as depicted here  High-frequency variations increase BW of signal  BW is made worse if non-linear Class C amplifiers are used  Linear amplifiers needed for reasonable BWs »Class A or B amplifiers »Poor DC to RF efficiencies  typically 40-65%  DP FSK is normally not used b/c of these issues

ECE 4710: Lecture #25 4 Continuous Phase FSK  Continuous Phase FSK = CP FSK  Generated by feeding binary signal into frequency modulator  Bandpass Signal   Complex Envelope Representation

ECE 4710: Lecture #25 5  Polar NRZ line code  Baseband m(t) is discontinuous between bits    ( t ) is continuous b/c integration of m(t) is continuous  Polar signal produces binary FSK (BFSK)  Constant envelope is preserved »Signal BW is much better than DP FSK »Non-linear PAs used »Class C amplifiers with 80-90% DC to RF efficiencies »Important for wireless applications that rely on battery power (cell phone, PDA, etc.) Continuous Phase FSK

ECE 4710: Lecture #25 6 BFSK  Polar NRZ line code  BFSK signal  Multi-level line code  M-FSK signal  BFSK widely used for early computer modems  bps  Still used for low speed data and signals in landline telephony »Caller ID, call forwarding, etc.  Dial-up computer modems (not DSL or cable)  28.8 kbps V.34 Modem  QAM (studied next!)  56 kbps V.90 Modem  PCM (Section 3-3 for more details)

ECE 4710: Lecture #25 7 Communication Channels  Full Duplex (FDX) Transmission  Tx and Rx for simultaneous two-way communication  Cell phone, computer modem, etc.  Half-Duplex (HDX) Transmission  Two-way communication but Tx and Rx not simultaneous  Listen before talk  Walkie Talkie or CB Radio  Simplex (SX) Transmission  One way communication only  Text pager

ECE 4710: Lecture #25 8 BFSK Modems  Modem = Mod + Demod  FDX Transmission  Tx + Rx in one device  BFSK for low data rates »300 bps  Bell 103 standard »1200 bps  Bell 202 standard (caller ID)

ECE 4710: Lecture #25 9 BFSK Modems  Bell 103 Modem (300 bps)  Telephone line has VF range from 300 – 3,300 Hz  Most line codes (unipolar, polar) have significant energy at frequencies < 300 Hz »Must use line code to modulate carrier for transmission over phone line  Two separate frequency bands used »One for Tx + One for Rx = FDX »Carrier #1 & #2 centered around 1.2 kHz & 2.1 kHz »Peak-to-peak f deviation about each carrier  2   f = 200 Hz

ECE 4710: Lecture #25 10  PSD of BFSK signal?  Difficult to evaluate  just like analog FM spectrum  Non-linear relationship between g(t) and m(t) Bell 103 BFSK Modem 2   f = 200 Hz

ECE 4710: Lecture #25 11 Bell 103 BFSK Modem  Evaluate BFSK PSD for worst-case widest BW signal  Deterministic square wave corresponding to alternating pattern  Frequent data transitions  high signal BW since line code pulse width is small

ECE 4710: Lecture #25 12 Bell 103 BFSK Modem  Digital Modulation Index   PSD is line spectrum since m(t) is periodic  Complex mathematical solution (see Eqs and 5-86)  Calculate numerically (computer code) for different R and  F  As h  then BFSK approaches wideband FM spectrum  spectrum f c ±  F  Worst case deterministic waveform  worst-case PSD

ECE 4710: Lecture #25 13 Bell 103 BFSK PSD Bell 103 Modem Parameters Originate Mode

ECE 4710: Lecture #25 14 Other BFSK PSD’s

ECE 4710: Lecture #25 15 Other BFSK PSD’s PSD’s for Tx & Rx would overlap and interfere for Bell 103 Standard Since f Tx  1 kHz and f Rx  2 kHz

ECE 4710: Lecture #25 16  Use Carson’s Rule (just like analog FM)   F = peak frequency deviation  B is bandwidth of baseband digital signal m(t)  If B = FNBW of rectangular m(t) then B = R »  If RC filter is used to shape rectangular m(t) then »  Narrowband BFSK   Wideband BFSK  BFSK Signal BW

ECE 4710: Lecture #25 17  What about PSD for random data m(t) ?  Difficult to evaluate  Statistical techniques and numerical computation Note that h = 0.7  0.67 for Bell 103 Modem Standard Note that h = 1 we have  delta function at f c R BFSK PSD

ECE 4710: Lecture #25 18 BFSK Detection  Non-coherent detection  Measure frequency of incoming signal » f c +  F  “1” f c -  F  “0” »Zero crossing detector + digital counter + logical comparator

ECE 4710: Lecture #25 19 BFSK Detection  Coherent detection Let f 1 = “1” [ m(t)  +1] Let f 2 = “0” [ m(t)  -1] For “1” then cos(2  f 1 t ) path yields and cos(2  f 2 t ) path yields which is rejected by LPF so