Features of an ECG Wireless Telemetry System

The advancement in technology has made it possible where necessary to monitor subjects from some distance using wireless telemetry. The biological parameter commonly studied this way is the electrocardiogram. The display of ECG and cardiac rates gives enough information on the loading of the cardiovascular system of the active subjects. In this article, we discuss the key components that make up an ECG Wireless Telemetry System.

The figure below shows the block diagram of a single channel telemetry system suitable for the transmission of an electrocardiogram:

In reference to Figure 1.0 above, there are two main parts:

• Telemetry Transmitter: This consists of an ECG amplifier, a sub-carrier modulator and a UHF transmitter along with dry cell batteries.
• Telemetry Receiver: It consists of a high frequency unit and a demodulator, to which an electrocardiograph can be connected to record, a cardioscope to display and a storage device to store the ECG. A heart rate meter with an alarm feature can be provided to continuously monitor the beat-to-beat heart rate of the subject.

To ensure the transmission of the ECG is free from distortions, the following requirements must be met:

• Motion artefacts and muscle potential interference should be kept at minimum.
• The battery life should be long enough so that a complete experimental procedure may be carried out.
• The subject should be able to carry on with normal activities while carrying the instruments without the slightest discomfort. Additionally the subject should be able to forget the presence of the instruments after some minutes of application.
• Whilst monitoring paced patients for ECG through telemetry, it is necessary to reduce pacemaker pulses. The amplitude of pacemaker pulses can be as large as 80 mV compared to 1-2 mV, which is typical of the ECG. The ECG amplifiers in the transmitter are slew rate (rate of change of output) limited so that the relatively narrow pacemaker pulses are reduced in amplitude significantly.

The ECG Telemetry Transmitter

A block diagram of the transmitter is illustrated below:

With reference to Figure 1.1 above, the ECG signal, picked up by three pre-gelled electrodes attached to the patient’s chest, is amplified and used to frequency modulate a 1 kHz sub-carrier that in turn frequency-modulates the UHF carrier. The resulting signal is radiated by one of the electrode leads (RL), which serves as the antenna. The input circuitry is protected against large amplitude pulses that may result in defibrillation.

The ECG input amplifier is ac coupled to the succeeding stages. The coupling capacitor eliminates the dc voltages that results from the contact potentials at the patient-electrode interface and also determines the low-frequency cut-off of the system which is usually 0.4 Hz. The sub-currier oscillator is a current-controlled multi-vibrator which provides ±320 Hz deviation from the 1 kHz center frequency for a full range (±5 mV) ECG signal. The sub-carrier filter removes the square-wave harmonic and results in a sinusoid for modulating the RF carrier. In the event of one of the electrodes falling off, the frequency of the multi-vibrator shifts by about 100 Hz. This condition when sensed in the receiver turns on an ‘Electrode inoperative’ alarm.

The carrier is generated in a crystal oscillator operating at 115 MHz. The crystal is a fifth overtone device and is connected and operated in the series resonant mode. This is followed by two frequency doubler stages. The first stage is a class-C transistor doubler and the second is a series connected step recovery diode doubler. With the output power around 2 mW; the system has an operating range of 60 m within a hospital facility.

The ECG Telemetry Receiver

The receiver employs an omnidirectional receiving antenna which is a quarter-wave monopole, mounted vertically over the ground plane of the receiver top cover. This arrangement works well to pick up the randomly polarized signals transmitted by moving subjects.

Figure 1.3 below shows a block diagram a typical ECG telemetry receiver:

In reference to the Figure 1.3 above, the receiver consists of the Radio Frequency (RF) amplifier, which provides a low noise figure, RF filtering and image-frequency rejection. Furthermore, the RF amplifier suppresses local oscillator radiation to -60 dBm to minimize the possibility of cross-coupling where several receivers are used in one central station.

The local oscillator uses a crystal (115 MHz) similar to the one in the transmitter and x4 multiplier and a tuned amplifier. The mixer uses the square law characteristics of a FET to avoid interference problems due to third-order intermodulation. The mixer is followed by an 8-pole crystal filter that establishes the receiver selectivity. This filter with a 10 kHz bandwidth provides a 60 dB of rejection for signals 13 kHz from the intermediate frequency (IF) center frequency (21.82 MHz). The IF amplifier provides the requisite gain stages and operates an AGC amplifier which reduces the mixer gain under strong signal conditions to avoid overloading at the IF stages. The IF amplifier is followed by a discriminator, a quadrature detector. The output of the discriminator is the 1 kHz sub-carrier. This output is averaged and fed back to the local oscillator for automatic frequency control (AFC). The 1 kHz sub-carrier is demodulated to convert frequency-to-voltage to recover the original ECG waveform. The ECG is passed through a low-pass filter having a cut-off frequency of 50 Hz and then given to a monitoring instrument.

The 1 kHz sub-carrier is examined to establish whether or not a satisfactory signal is being received. This is done by establishing a window of acceptability for the sub-carrier amplitude. If the amplitude is within the window, then the received signal is received valid. In the case of AM or FM interference, an operative alarm lamp lights up

Related articles:

• How Biotelemetry is Integrated with Telemedicine
• Biotelemetry Monitoring Instruments for Telemedicine
• Key Features and Applications of Telemedicine
• Biotelemetry and How it is Applied in Medical Field
• Key Features of Modern ECG Systems
• Principle Parts and Types of ECG Recorders
• The Instrumentation for Recording ECG Signals
• Key Features of ECG Amplifiers