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Digital FM Exciter Digital Signal Processing for FM Philipp Schmid
April 24, 2012
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Nautel Digital FM Exciter Models
All are direct-to-channel exciters: no intermediate frequency (no added noise) high speed interpolating DAC (up to 1 GSPS) frequency agility Models: M50 exciter (V and FM1/2 systems) NV exciter Without HD PowerBoost: NAPE79 With HD PowerBoost: NAPE86 VS (FM only, QDUC) VSHD based on NAPE86 DAC AD9779
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Discrete Digital Signals
Maximum signal ½ sample rate => Nyquist Rate
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Discrete Signal Aliasing
2 kHz and 12 kHz produce identical sample points. Before sampling analog signal must be filtered to satisfy Nyquist requirement
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Discrete Signal Aliasing
anti aliasing filter (DAC output) negative spectrum positive spectrum
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Understanding IQ <= recovers s(t) from IQ
Any signal with amplitude and phase modulation can be expressed as an in-phase I and a quadrature Q component Simply: IQ shifts signal from carrier to 0 Hz using ejwt uses positive and negative spectrum complex valued signal
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Digital Filtering Finite Impulse Response (FIR)
Infinite Impulse Response (IIR)
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Digital Channel Modulation
2π=> channel accumulator channel tuning word + Fc = Fs * tuning word modulus <= complex frequency synthesis
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Digital Channel Modulation
Mixing Complex Frequencies cos(wc) sin(wc) I * + - * * Q * + (I+j Q) * (cos(wc) + j sin(wc)) = I cos(wc) – Q sin(wc) + j (Q cos(wc) + I sin(wc))
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Digital FM Exciter Block Diagram
upconversion AES DSP FPGA ASRC LUT RF IQ 1.2 Msps MPX FM IQ 80 Msps high speed DAC pre- correction channel mod Analog ↑16 CIC ADC MPX ADC fwd capture + eq ↑4 rev capture ↑5/3 RF Sample IQ 744 ksps Exgine PWB E2X Ethernet ADC IQ 80 Msps spectrum plot channel demod ↓4 ↓16 CIC OFDM modulator
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Adaptive Pre-correction
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Adaptive Pre-correction
Time domain scope forward (desired) signal reverse (sampled) signal Baseband signal, power envelope Amplifier driven into saturation
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Adaptive Pre-correction
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Adaptive Pre-correction
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data carriers QPSK ref carriers BPSK
IBOC Signaling data carriers QPSK ref carriers BPSK
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IBOC System Components
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Thank You
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