Biomedical Engineering Reference
In-Depth Information
Figure 5.15
A transtelephonic ECG monitor provides a simple, effective solution for monitor-
ing pacemaker patients from the comfort of their home. To transmit real-time ECGs, the patient
simply dials the receiving center and places the telephone handset on the transmitter's cradle.
These monitors typically use a center frequency of 1965 Hz with a deviation of 100 Hz/mV to con-
vey a 0.5 to 100 Hz ECG signal detected through three chest electrodes that are dampened with
water.
linearity, you may design a simpler VCO using two op-amps or a 555 timer IC to produce
a square or pulsed output. Straightforward FM demodulation should not be used for these
waveforms. Rather, a pulse-width or pulse-position demodulator should be used. The
Matlab demodulation command (demod.m) can be told to use these demodulation methods
simply by replacing 'fm' for 'pwm' or 'ppm' in its argument line.
If your application requires remote acquisition with transmission of the modulated tone
via voice link, don't do it with a pulsed waveform, since these are distorted signi
cantly
by telephone lines and do not properly modulate conventional voice transmitters. If using
a VCO with sine-wave output (e.g., the XR-2206, Intersil's ICL8038, or Maxim's MAX038)
is still out of the question, you can convert a frequency-modulated pulse train into a quasi-
sinusoidal tone using the circuit [Allen, 1981] of Figure 5.16. A 4018 Johnson counter is
wired to sequence resistors R1-R5 to produce a 10-step staircase waveform that approxi-
mates the shape of a sine wave. The sharp edges of the staircase are removed using a simple
RC
low-pass
fi
filter formed by R6 and C4 before they are attenuated by R7 and ac-coupled
by C5 so they can be fed to the microphone input of a telephone or voice transmitter. Since
the square-to-sine converter divides the input frequency by 10, the VCO should be made
to output at a frequency range that is 10 times higher than the sinusoidal tone audio range
required.
fi
Search WWH ::
Custom Search