SIGNAL SOURCES FOR SIMULATION, TESTING, AND CALIBRATION - Design and Development of Medical Electronic Instrumentation - page 274

frequency of the signal is larger than the carrier frequency when the signal m ( t ) is positive
and is smaller when m ( t ) is negative.
The circuit of Figure 6.18 is used to demodulate the FM signal from the sound card. The
signal coming from the sound card is ac-coupled by C17 and amplitude-limited by IC1D.
Then a NE565C PLL IC demodulates the FM signal. The PLL tracks the incoming carrier
signal and internally estimates the signal based on the frequency of its internal VCO set by
R2, R4, and C2. The “error” between the actual carrier frequency and the estimate is the data
signal when the PLL is locked. A suitable FM frequency deviation for this circuit is 40%,

which allows the bandwidth of the arbitrary signal to be reproduced to be approximately
18.5% of the carrier frequency. Table 6.6 gives the signal reproduction characteristics for some
of the standard frequencies used in FM tape recorders, an application that uses the same FM
modulation/demodulation schemes. The loop output of the PLL IC is fed into a unity-gain
di er is used to elimi-

nate DC and high-frequency carrier components present at the output of the PLL.
A Maxim MAX280 switched-capacitor

ff erential ampli fi er (IC1C). The common-mode rejection of this ampli fi

filter IC is used to remove residual carrier-fre-
quency signal components from the waveform desired. This IC is a

fi

fi fifth-order all-pole

low-pass filter with no dc error, making it an excellent choice for processing low-frequency

signals. The

fi

fi filter IC uses an external resistor (R9) and capacitor (C10) to isolate the

fourth-order filter implemented within the IC from the dc signal path. The external resis-

tor and capacitor are used as part of the

fi

fi filter's feedback loop and also form one pole for

the overall fi filter circuit. The values of these components are chosen such that

1
9

.6
)(

2
C10)

f cutoff 2

π (R

where R9 should be around 20 k .

Now, for the Matlab code example given above, the demodulated signal bandwidth is
expected to be 18.5%

Ω

f c 18.5% 1687 Hz 312 Hz, which is where the 3-dB cuto ff

frequency for the low-pass fi filter should be placed. Selecting the closest standard-value

components, R9 18.2 k Ω and C10 0.047 µ F, the 3-dB cuto ff will be 301 Hz.

The chip's internal four-pole switched capacitor fi filter is driven by an internal clock that

determines the fi filter's cutoff ff frequency. For a maximally fl flat amplitude response, the clock

should be 100 times the cutoff frequency ratio of

100 : 1. The internal oscillator runs at a nominal frequency of 140 kHz that can be modi

ff frequency desired. The fi filter has a cutoff ff

ed
by connecting an external capacitor (C11) between pin 5 and ground. The clock frequency
is given by

fi

33 p 3 F 3

pF
C11

f fclock
140 140 kHz

TABLE 6.6 Signal Characteristics for Various Standard FM Tape Recorder
Frequencies (kHz) a

Carrier Deviation Limits

(for 40% FM Deviation)

Carrier Modulating Frequency

Frequency Plus Deviation Minus Deviation Bandwidth

1.687 2.362 1.012 dc-0.312

3.375 4.725 2.835 dc-0.625

6.750 9.450 4.050 dc-1.250

13.500 18.900 8.100 dc-2.500

27.000 37.800 16.200 dc-5.000

a The response bandlimits (dB) for a 100-Hz frequency response are 1%.