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TELEMETRY


  • ·     Telemetry: Telemetry is an automated communications process by which measurements and other data are collected at source and transmitted to receiving equipment for monitoring.

  • Modulation Techniques:

 1)     Amplitude Modulation: the amplitude of the carrier signal is modulated according to the source  signal while keeping the frequency constant.

   
           

2)     Frequency Modulation: the frequency of the carrier signal is modulated according to the source signal while keeping the amplitude constant.
 
 
           


3)     Pulse width Modulation: the pulse width of the carrier signal is modulated according to the source signal while keeping the amplitude & frequency constant.


       


        

·       Types of Telemetry Systems:

1)     Single Channel Telemetry
a.      ECG Telemetry System
b.     Temperature Telemetry System
c.      Respiration Telemetry System
2)     Multichannel Telemetry
a.      ECG and Respiration Telemetry System
b.     Obstetrical Telemetry System


Single Channel ECG Telemetry System:

  •   Figure shows the block diagram of a single channel telemetry system for the transmission of an Electrocardiogram.
  • There are two main parts:
1)        The Telemetry Transmitter which consists of an ECG amplifier, a sub-carrier oscillator and a UHF transmitter along with dry cell batteries.
2)        Telemetry Receiver consists of a high frequency unit and a demodulator, a cardioscope to display and a magnetic tape recorder to store the ECG. A heart rate meter with an alarm facility can be provided to continuously monitor the beat-to-beat heart rate of the patient.
  • The ECG signal, picked up by three pre-gelled electrodes attached to the patient’s chest,
is amplified
  • This amplified ECG signal is used to frequency modulate a 1 kHz sub-carrier that will frequency-modulates the UHF carrier.
  • The resulting signal is radiated by the antenna.
  • The receiver uses receiving antenna which pick up the ECG signals transmitted from patients.
  • The receiver comprises an RF amplifier, which provides a low noise figure, RF filtering and
demodulator which separates patient’s ECG from carrier signal.
  • This ECG signal is then displayed on cardioscope and can be stored on magnetic tap recorder or Graphic Recorder.
       

Multi-Channel Telemetry System (ECG & Respiration):

  • An FM modulated radiotelemetry transmitter for detecting and transmitting ECG and respiration activity simultaneously on a single carrier frequency in the FM broadcast band is used.
        

  • Respiration is detected by the impedance pneumography principle by using the same pair of electrodes that are used for the ECG.
  • A 10 kHz sinusoidal constant current is injected through electrodes E1 and E2
       attached across the subject’s thoracic cavity.
  • The carrier signal is generated by a phase shift oscillator.
  • The amplitude varying respiration signal is amplified by an amplifier A1. An amplifier filter A3 recovers the respiration signal by using rectifiers and filter.
  • The ECG signal detected by electrodes E1 and E2 is amplified in A1 along with the respiratory signal. It is passed through a low-pass Butterworth filter stage A2 which passes the ECG signal but blocks respiratory signal.
  • The amplified ECG signal is then summed up with the preprocessed respiration signal in A4.
  •  The output of A4 is supplied to an astable multi-vibrator which acts as a voltage- controlled sub-carrier oscillator.
  • The output of the sub-carrier oscillator is then fed to a RF oscillator for transmission.


 Implantable Telemetry System (Blood Pressure & Blood flow):

  •   Implantable telemetry systems allow the measurement of multiple physiological variables over long periods of time without any attachment of wires, sensors or anaesthesia to the patient.
  • Single or multi-channel systems have been used successfully to monitor ECG, EEG, blood pressure, blood flow, temperature etc.
            


  • Figure shows the block diagram of an implantable telemetry system for blood pressure and blood flow measurement.
  •  Ultrasonic blood flow meter is used in which blood velocity information is converted to an electrical signal by means of two ultrasonic transducers which are mounted in a rigid cuff surrounding the vessel.
  • High frequency power for the flow transducer is generated by the 6 MHz oscillator. The 6 MHz AM receiver converts the incoming ultrasonic signal to an audio frequency signal by synchronous detection.
  • Pressure is detected by a miniature intravascular transducer by placing it directly in the blood stream.
  • Data recovery is accomplished by an internal 100 MHz, FM transmitter and an external FM receiver.
  • A demodulator, external to the body, separate blood flow & blood pressure signal from carrier signal.
  •  Blood flow & blood pressure value is then displayed on display or can be recorded on recorder.










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