1. Digital FM Exciter Digital Signal Processing for FM Philipp Schmid
April 24, 2012

2. 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

3. Discrete Digital Signals
Maximum signal ½ sample rate => Nyquist Rate

4. Discrete Signal Aliasing
2 kHz and 12 kHz produce identical sample points.
Before sampling analog signal must be filtered to satisfy Nyquist requirement

5. Discrete Signal Aliasing
anti aliasing filter
(DAC output)
negative spectrum
positive spectrum

6. 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

7. Digital Filtering Finite Impulse Response (FIR)
Infinite Impulse Response (IIR)

8. Digital Channel Modulation
2π=>
channel
accumulator
channel
tuning word +
Fc = Fs * tuning word
modulus
<= complex frequency synthesis

9. 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))

10. 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

11. Adaptive Pre-correction

12. Adaptive Pre-correction
Time domain scope
forward (desired) signal
reverse (sampled) signal
Baseband signal, power envelope
Amplifier driven into saturation

13. Adaptive Pre-correction

14. Adaptive Pre-correction

15. data carriers QPSK ref carriers BPSK
IBOC Signaling
data carriers QPSK
ref carriers BPSK

16. IBOC System Components

17. Thank You