High Sensitivity Pocket Dopplers Service Manual

Table of Contents

Contents

Page No.

1. General Information...4 1.1 1.2 1.3 1.4 1.5

Introduction...4 Servicing Policy...4 Description...4 Antistatic Handling...5 Construction...5

2. Safety Aspects...6 2.1 2.2 2.3 2.4

Safety...6 Maintenance / Inspection...6 Cautions...7 Care And Cleaning...8

3. Specifications...10 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

Safety...10 Power Supply...10 Enclosure...10 LCD Display Elements...11 Controls...11 Auto Shut Off...11 Outputs...12 Probes...12

4. Technical Description...13 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19

The Doppler Principle...13 Doppler Audio Processing...13 Fetal Heart Rate Processing, (FD2, MD2, FD1, FD3)...13 Bi-directional Signal Processing (MD2, SD2, RD2)...13 Probe Identification...14 Active Noise Reduction...14 Intelligent Auto Shut Off (FD2, SD2, MD2, FD1, FD3, RD2)...14 Calibration Pulses...14 Waveform Conversion...14 PPG...15 Data Output...15 Microcontroller...15 Standard/Smoothed Mode FHR...15 Manual Mode FHR...15 Audio Dopplex (D920, D930), Mini Dopplex (D900)...16 FD2, SD2, MD2, FD1, FD3, RD2...18 Obstetric Probe (OP2HS, OP3HS, D920, D930, FD1, FD3)...21 Vascular Probe (VP4HS, VP5HS, VP8HS, VP10HS, EZ8)...22 PPG Probe...23

5. Electrostatic Discharge (ESD) Precaution...25 5.1 5.2

What Is Static Electricity?...25 Protective Measures...25

6. Servicing Procedures - Control Unit...26 6.1 6.2 6.3 6.4

Control Unit Dismantling Procedure...26 PCB Removal...27 Changing Components...27 Control Unit Reassembly...28

7. Probe Head Replacement Procedure...32 7.1 7.2

2

7.3 7.4a 7.4b 7.5 7.6 7.7a 7.7b 7.8

Equipment Required...32 Preparation - For High Sensitivity Probes... (OP2HS,OP3HS,VP4HS,VP5HS,VP8HS,VP10HS,EZ8)...32 Dismantling Procedure - All Probes except D920, D930, FD1 & FD3...33 Dismantling Procedure - D920, D930 & FD1...33 Dismantling Procedure - D920, D930, FD1 & FD3...34 New Head Fitting and Alignment...35 Reassembly Procedure - All Probes except D920, D930, FD1 & FD3...38 Reassembly Procedure - D920, D930 & FD1...39 Reassembly Procedure - D920, D930, FD1 & FD3...41 Waterproof Inspection & Test Procedure - D920, D930, FD1 & FD3...43

8.1 8.2 8.3

...44

Equipment Required...44 Functional Tests - Vascular Probes VP4HS - VP10HS...44 Functional Test - Obstetric Probes OP2HS, OP3HS, D920, D930, FD1 & FD3...45

9. Fault Finding...46 9.1 9.2 9.3 9.4 9.5 9.6 9.7

LCD - Marks On Display...46 No Sound...46 Unit Dead...46 No Output From Sockets...46 Poor Heart Rate Tracking...46 Poor Sensitivity, Crackling...46 PPG not locking on...46

10. Spare Parts list 10.1 10.2 10.3 10.4 10.5 10.5a 10.5b 10.6

...47

Table of Contents

8. Probe Functional Test Specification

D900 Parts List...47 D920, D930 Parts List...49 MD2, SD2, FD2, FD1, FD3, RD2 Parts List...51 Obstetric and Vascular Probe Parts List...53 Fixed Waterproof Probes...54 D920, D930, FD1 HSB Series Probe Parts List...55 D920, D930, FD1, FD3 Probe Parts List...56 ISP3 Probe Parts List...57

11. Ordering Information...58 11.1

Service Returns

...58

Appendix A...59 Special Handling Procedures...59 Recommended Soldering Equipment for Rework...59

Appendix B...60 Device History Record Sheet...60

Appendix C...61 Recommended Recorder Specifications...61

Appendix D...62 Test Equipment Specifications...62

Appendix E...63 D920, D930, FD1, FD3 Probe Cable Wiring Details...63

Appendix F...64 Waveform Output Connector Details RD2 / MD2...64

Appendix G...65 Trace Examples...65

Appendix H...66 Electromagnetic Compatibility...66

3

General Information

1. General Information Although every care has been taken to ensure that the information in this manual is accurate, continuous development may result in equipment changes. The Company reserves the right to make such changes without prior notification, and resulting manual inaccuracies may occur. This manual and any changes are protected by copyright.

1.1

Introduction This service manual provides the technical information required for repair and maintenance of the Huntleigh Healthcare Ltd... •

Mini Dopplex - D900

•

Audio Dopplex - D920 (with fixed waterproof 2MHz probe), D930 (with fixed waterproof 3MHz probe)

•

Fetal Dopplex - FD1 (with fixed waterproof 2MHz probe)

•

Fetal Dopplex - FD3 (with fixed waterproof 3MHz probe)

•

Fetal Dopplex II - FD2

•

Super Dopplex II - SD2

•

Multi Dopplex II - MD2

•

Rheo Dopplex

R

... hand held Dopplex range, including the range of OPHS (Obstetric High Sensitivity Probes) and VPHS (Vascular High Sensitivity Probes) interchangeable transducers.

Please note that the Super Dopplex 1, Multi Dopplex 1 and (NTD) probes are not compatible with the D900, FD2, SD2, MD2, RD2.

1.2

Servicing Policy Due to the nature of static sensitive surface mount technology, specialised equipment and training is required when working on the surface mounted components used within these products. For this reason, circuit diagrams are not included in this manual. Block diagrams and fault finding sections are included to make fault finding to leaded component level possible. Units within the warranty period must not be dismantled and should be returned to Huntleigh Healthcare Ltd for repair. Any units returned showing signs of tampering or accidental damage will not be covered under the warranty (refer to user manual for further details).

1.3

Description The Dopplex range of products includes the D920, D930, D900, FD2 (Obstetric Doppler with heart rate display) SD2 (dedicated vascular Doppler), MD2 (Bi-directional vascular Doppler), D920, D930 (Mini Dopplex with waterproof probe), FD1, FD3 (Obstetric Doppler with heart rate display and waterproof probe) and RD2 (vascular Dopplex with Photoplethysmographic sensor).

4

Mini Dopplex (D900 )

is primarily a vascular unit with a non-directional waveform output. The Mini Dopplex accepts the obstetric probes, providing audio only output.

Fetal Dopplex II (FD2)

has interchangeable probes accepting the full range of obstetric and vascular probes, although it is primarily a Fetal Dopplex with a heart rate display. Using the vascular probes, only audio signals are available and there is no rate counting facility, or waveform outputs.

Super Dopplex II (SD2)

is a dedicated vascular Doppler with all the vascular functions of the MD2, but without the RS232 waveform output.

Multi Dopplex II (MD2)

is a multi-purpose Doppler with bi-directional waveform and stereo headphone outputs. Graphical representation of velocity waveforms is given by the LCD display. The same obstetric features of the FD2 are incorporated into the MD2.

Audio Dopplex (D920 D930)

are dedicated obstetric Dopplers with fixed waterproof 2MHz (D920) and 3MHz (D930) probes.

Aqua Dopplex Plus (FD1)

is derived from the FD2, but has a hardwired waterproof 2MHz probe.

FD3

is derived from the FD2, but has a hardwired waterproof 3MHz probe.

Rheo Dopplex II (RD2)

has the vascular functions of the MD2 with the addition of a Photoplethysmographic sensor.

General Information

The FD1 and D920 have dedicated 2MHz probes that are hard wired via a retractile cable. The FD3 and D930 have a dedicated 3MHz probe that is hard wired via a retractile cable.

For further details on the controls, please refer to the user manuals.

1.4

Antistatic Handling Due to the nature of the components used within these products, special precautions must be taken to avoid damage to the CMOS and microcontroller based circuitry. Static damage may not be immediately evident but could cause premature failure. This series of units must only be dismantled and serviced within a specialised handling area (SHA) as defined by CECC00015 (published by CENELEC) to avoid damage to the assemblies.

1.5

Construction The control unit comprises a single PCB on which all circuitry and electro-mechanical components are mounted directly except for the loudspeaker which is attached by flying leads. A hard wired cable provides the connection to the probe. The PCB has surface mounted components on both sides. All leaded and electro-mechanical components are on one side. All electro-mechanical and through hole components on the control PCB are serviceable using standard tools and soldering techniques, provided that anti-static precautions are always observed. The case of the control unit is moulded in ABS polycarbonate alloy, and comprises two halves. The loudspeaker, on/off switch and display are on the front of the unit.

5

Safety Aspects

2. Safety Aspects 2.1

Safety The Doppler units and their probes are designed to high standards of performance, reliability and safety. Checks should always be made after carrying out any repairs or dismantling of the equipment. Attention, consult accompanying documents / Instructions to Use Attention consult the Instructions for Use. Refer to safety section.

2.2

Maintenance / Inspection Inspection is recommended each time the product is used, paying particular attention to the probe tip, checking for cracks etc., and to the cable and connector. Any crackling or intermittent behaviour should be investigated. This product does not require periodic maintenance. For functional testing of specific product features, refer to the user manual. If you require any assistance with safety testing your Dopplex equipment, contact Huntleigh Healthcare Ltd or your supplier directly. For the U.K. refer to the Health Equipment Information Document No 95 - Code Of Practice For Acceptance Testing Of Medical Equipment. The following safety summary should be read before operating or carrying out any of the procedures described in this manual.

6

Cautions

Do not use in the sterile field unless additional barrier precautions are taken.

Do not use in the presence of flammable gases such as anaesthetic agents

Do Not : • immerse in any liquid, (except probe on D920, D930, FD1, FD3) • use solvent cleaner, • use high temperature sterilising processes (such as autoclaving), • use E-beam or gamma radiation sterilisation.

Safety Aspects

2.3

Do not dispose of batteries in fire as this can cause them to explode.

Do not attempt to recharge normal dry-cell batteries. They may leak, cause a fire or even explode.

This product contains sensitive electronics, therefore, strong radio frequency fields could possibly interfere with it. This will be indicated by unusual sounds from the loudspeaker. We recommend that the source of interference is identified and eliminated. Any equipment connected to RS232 interface must be compliant with IEC60601-1:2005.

Connect headphones only to the headphone socket.

WARNING :

This product is not designed for sterile use. For underwater use where contamination or cross contamination may occur, e.g. during labour and delivery, additional barrier precautions must be taken.

7

Safety Aspects

2.4

Care And Cleaning

General Care All Huntleigh products have been designed to withstand normal clinical use, however they can contain delicate components, for example the probe tip, which should be handled and treated with care. Periodically, and whenever the integrity of the system is in doubt, carry out a check of all functions as described in the relevant section of the IFU. If there are any defects to the housing contact Huntleigh or your distributor for repair or to order a replacement.

Please ensure that you check with your facility’s local infection control policy and medical equipment cleaning procedures.

Observe warnings and guidance on cleaning fluid labelling regarding use and personal protective equipment (PPE).

Do not use abrasive cloths or cleaners.

Do not use automatic washers or autoclaves.

Phenolic detergent based disinfectants, solutions containing cationic surfactants, ammonia based compounds or perfumes and antiseptic solutions such as Steriscol or Hibiscrub should never be used. If detergent or disinfectant wipes are used ensure that excess solution is squeezed from the wipe prior to use.

Do not allow any fluid to enter the products and do not immerse in any solution.

Always wipe off disinfectant using a cloth dampened with clean water.

Storage:

If your Dopplex unit is to be stored for a long period of time, the battery should be removed.

Coupling Gel: The use of water based gel supplied by Huntleigh Healthcare is strongly recommended. Oil based gels can damage the probe and must not be used. The use of oil based gels will invalidate your warranty.

8

Always keep the external surfaces clean and free of dirt and fluids using a clean dry cloth. 1.

Wipe any fluids from the surface of the product using a clean dry cloth.

2.

Wipe with a cloth dampened in 70% Isopropyl Alcohol.

3.

Completely dry with a clean, dry lint free cloth.

4.

If the product has been contaminated use the methods described for patient applied parts.

Safety Aspects

General Cleaning and Disinfecting

Cleaning and Disinfecting Patient Applied Parts Clean the probes before examining a patient using low risk cleaning method below. Following patient examination, clean and/or disinfect the probes by the appropriate method based upon the level of cross contamination risk, as defined below:

Risk

Definition

Procedure

Low

Normal use or low risk situations include patients having intact skin and no known infection and the transducers have not been contaminated with blood

1. Remove soiling, wipe with a mild neutral detergent and then wipe with a cloth dampened in water. 2. Completely dry with a clean lint free cloth.

Medium

The patient has a known infection, skin is not intact, the part is heavily soiled, or the patient has given birth in a water bath.

1. Follow low risk procedure then wipe with a cloth dampened in Sodium Hypochlorite (1,000ppm). 2. After two minutes wipe with a cloth dampened in water and then dry with a clean lint free cloth.

High

This procedure should only be used when the part has been contaminated by blood.

1. Follow low risk procedure then wipe with a cloth dampened in Sodium Hypochlorite (10,000ppm). 2. After two minutes wipe with a cloth dampened in water and then dry with a clean lint free cloth.

Warning: Sodium Hypochlorite @ 10,000 ppm for disinfecting should only be used in situations described in the High Risk definition. Unnecessary use of this concentrated solution for Low and Medium risk situations may result in damage to the product. Do not allow Sodium Hypochlorite solutions to come into contact with metal parts.

The use of disinfectant materials other than those listed is the responsibility of the user for their efficacy and compatibility with the device.

9

Specifications

3. Specifications 3.1

3.2

Safety Type of protection against electric shock.

Internally powered equipment.

Degree of protection against electric shock

Type B applied part

Mode of operation.

Continuous

Degree of protection against water ingress

Hand Unit: IP20 Probes D920, D930 FD1, FD3 : IPX7 All other probes (tip only) : IPX1

Degree of Safety in Presence of Flammable Gases:

Equipment not suitable for use in the presence of a FLAMMABLE ANAESTHETIC MIXTURE WITH AIR, OXYGEN OR NITROUS OXIDE

Power Supply Battery Type

3.3

10

9 Volt alkaline IEC, 6LR61 or IEC6LP3146.

Enclosure

Case Material

Injection moulded ABS Polycarbonate Alloy.

Dimensions (mm) (Height x Width x Depth)

140 x 74 x 27

Weight

295 gms (including probe and battery)

LCD Display Elements

Symbol

Description Indicates Standard FHR Mode Selected (FD1, FD3, FD2, MD2 only).

Indicates Smoothed FHR Mode Selected (FD1, FD3, FD2, MD2 only).

Specifications

3.4

Indicates Manual FHR Mode Selected (FD1, FD3, FD2, MD2 only). Represents Bi-directional blood flow velocity. RD2, in PPG mode indicates size of signal from sensor. (MD2, SD2 and RD2 only). Indicates RS232 communication enabled. This flashes when communication is in progress. (FD2, MD2P only). Audio/Mini- indicates power on. FD2/SD2/RD2/MD2- indicates power on with probe not connected. Indicates battery low and requires replacement.

RT indicates Venous Refilling Time (RD2 only).

VP indicates Venous Pump Power (RD2 only).

Shows when blood flow waveforms are inverted (RD2 only).

3.5

3.6

Controls On/Off Button

Front panel push-on / push-off

Mode Buttons

Refer to Instructions for Use

Auto Shut Off FD2, SD2, MD2, FD1, FD3, RD2

3 minute no signal 10 minute unconditional

D900, D920, D930

Fixed 5 minute

11

Specifications

3.7

Outputs

Audio Output Max. Audio Output (Loudspeaker)

500mW rms into 8

Headphones

Max. output Power: 25 mW rms (32Ω) Connector: 3.5mm stereo jack socket Max. applied voltage: +9Vdc

D920, D930, D900, FD2, FD1, FD3

3.5mm stereo socket on top panel (mono output).

MD2, RD2, SD2

3.5mm stereo socket on top panel (stereo output).

internal speaker

RS232 Interface FD2, MD2, RD2

Data format: RS232C Connector: 8pin subminiature DIN socket Max. applied voltage: +5Vdc

Waveform Socket Non-directional (Mini Dopplex)

3.5mm mono socket. Fixed f/V 0.5V/kHz.

Bi-directional (MD2, RD2)

Sub-miniature 8 pin DIN socket, 3.5V Full scale. Variable f/V depending on gain setting - refer to user manual.

Calibration Signal

3.8

12

Mini Dopplex

0kHz, 1kHz, 2kHz, 0kHz stepped pulse.

MD2, RD2

Stepped pulse sequence at 0.05% and 0.1% of probe frequency. Zero baseline at start and end of sequence (available on x1 gain setting only).

Probes D900, FD2, SD2, MD2, RD2 (in addition to PPG)

OP2HS, OP3HS Obstetric Probes VP4HS, VP5HS, VP8HS, VP10HS, EZ8 Vascular Probes

D920, FD1

Fixed 2MHz obstetric waterproof probes.

D930, FD3

Fixed 3MHz obstetric waterproof probes.

4.1

The Doppler Principle The Dopplex Range all use a technique based on the Doppler principle for non-invasively monitoring movement within the body. The Doppler principle states that if a signal is transmitted at a fixed frequency and is reflected by a moving body, the frequency of the received signal will be shifted. An increase in frequency results if the reflector is moving towards the transmitter/receiver, and a decrease results if moving away from the transmitter/receiver. The amount of frequency shift is proportional to the velocity of the reflector relative to the transmitter/receiver. In the Dopplex range, a fixed frequency ultrasonic signal is transmitted from the probe into the body. This is reflected from, for example, moving blood cells. The signal is reflected from these cells and is received by the probe. Due to the movement of the blood cells, a frequency shift results, which is proportional to the blood flow velocity. The Doppler shift is also affected by the angle between the probe and the direction of flow. The Doppler shift frequency is greatest when the flow is directly towards, or away from, the probe.

4.2

Technical Description

4. Technical Description

Doppler Audio Processing The pocket Dopplex probe contains a transmitter and receiver. The probe sends out a continuous ultrasonic signal (carrier), generated by the piezo-ceramic transmitter crystal, in the frequency range 2 to 10 MHz (depending on probe). This signal is scattered by blood cells or any other "interface" such as skin, muscle layers, organs, walls of vessels etc. A small proportion of the scattered signal will be reflected back and detected by the receiver. By demodulating the received signal (removing the high frequency carrier) the Doppler shifted component (i.e. the difference between the transmitted and received signals) can be produced. With typical target velocities found in the human body, this Doppler shift signal falls within the audio frequency range. It can therefore simply be amplified and heard through a loudspeaker. It is important to remember that the sound you hear is an artificial sound, the frequency (pitch) of which is proportional to the velocity of the moving target. It is not the real sound made by blood rushing through an artery or vein, or movement of the fetal heart.

4.3

Fetal Heart Rate Processing, (FD2, MD2, FD1, FD3) In addition to providing this Doppler sound, the circuitry in the FHR signal conditioning section generates an amplitude envelope of the Doppler audio signal. Using auto-correlation, this signal is further processed by the microcontroller to calculate FHR.

4.4

Bi-directional Signal Processing (MD2, SD2, RD2) To achieve bi-directional flow indication, the Doppler signal must be further processed to separate forward and reverse components. Components of the Doppler signal produced by positive frequency shift represent flow towards the probe, referred to as forward flow. Components of the Doppler signal produced by negative frequency shift represent flow away from the probe, referred to as reverse flow. The circuitry achieves this separation in the vascular signal processing section producing two frequency envelopes using zero crosser techniques. This signal is presented at the waveform output (MD2, RD2). The microcontroller also receives these bi-directional signals and displays them on the LCD bargraph to give a visual indication of blood flow velocity and direction.

13

Technical Description

4.5

Probe Identification Probe identification is utilised on the D900, FD2, SD2, MD2 and RD2 Doppler units. This is to allow the control unit to identify which probe has been fitted. Fitting an obstetric probe will disable the waveform output(s). The FD2/SD2/MD2/RD2, with an obstetric probe connected, will display the probe frequency whenever the probe is changed. The FD2/SD2/MD2/RD2, with a vascular probe connected, will give a continuous indication of probe frequency. In the SD2, MD2 and RD2 the frequency to voltage conversion factor is changed according to the frequency of the connected probe and the gain setting, effectively rescaling the output.

4.6

Active Noise Reduction Active noise reduction is used to reduce the amount of "hiss" that is present when the probe is not being used. When a signal is below a set level, the cut off point of the low pass filter is lowered effectively reducing the noise level. When a large signal is detected, the bandwidth is increased to allow the full range of the signal to be heard at the loudspeaker.

4.7

Intelligent Auto Shut Off (FD2, SD2, MD2, FD1, FD3, RD2) To increase battery life within the FD2, SD2, MD2, FD1, FD3 and RD2 units, the microcontroller will turn the unit off after 3 minutes of no signal. The unit will also switch off after 10 minutes regardless of signal presence. This function is disabled when it is connected to a Printa which is printing in obstetric or vascular mode (not applicable to SD2, FD1 or FD3).

4.8

Calibration Pulses The calibration pulses for the D900 are of 1KHz and 2kHz irrespective of probe frequency. The MD2 and RD2 will output pulses of 0.05% and 0.1% of the frequency of the connected probe. The MD2 and RD2 have bi-directional calibration pulses to scale both forward and reverse channels. The calibration facility is disabled when using gain settings other than x1.

4.9

Waveform Conversion The D900 has a fixed waveform conversion factor of 0.5V/kHz, whereas the SD2, MD2 and RD2 have a conversion factor determined by the probe fitted and the gain setting. The use of a variable conversion factor results in the waveform output presenting a voltage that is essentially constant regardless of the frequency of probe fitted for the same velocity signal.

14

The RD2 incorporates a photoplethysmographic sensor which uses a sensitive infra-red sensor to assess, non-invasively, venous incompetency of the lower limb, in response to a calf muscle exercise. The electronics for the PPG is enclosed within a pod to which the PPG sensor is attached by a lightweight straight cable.

4.11 Data Output The data output feature on the FD2 and MD2 allows fetal heart rate data to be sent to the Dopplex Printa in real time. The MD2 and RD2 units send fetal heart rate data via the data socket to the Dopplex Printa in real time OR vascular data to the Dopplex Reporter/Dopplex Printa. The FD2, MD2 and RD2 will flash the data symbol (

Technical Description

4.10 PPG

)during information transfer.

The MD2 and RD2 will output bi directional waveform information in an analogue format via the data socket.

4.12 Microcontrollers The D900, D920, D930 units use an 8 bit microcontroller to carry out "house keeping" duties, such as driving the LCD display and auto shut-off timer. The FD2, SD2, MD2, FD1, FD3 and RD2 have an 8 bit microcontroller that also performs FHR processing (including auto-correlation), controls the data output, probe decoding and waveform generation.

4.13 Standard/Smoothed Mode FHR The FD2 and MD2 can display FHR in Standard or Smoothed mode. Smoothed mode FHR uses 8 beat averaging while Standard mode FHR uses 4 beats averaging.

4.14 Manual Mode FHR The manual mode on the FD2 and MD2 can be used when the heart sound can be heard, but is too weak or noisy for the microcontroller to calculate the heart rate automatically. In this mode, the Dopplex unit operates as a stop watch calibrated in BPM (beats per minute). The stop watch is activated by depressing the "Start/Stop" button, 10 heart beats should then be counted and the button released. The display will then show the average heart rate over the last 10 beats.

15

Block Diagram

ANR Probe Connector

Technical Description

4.15 Audio Dopplex (D920, D930), Mini Dopplex (D900)

Soft Limiter

Audio Amplifier Speaker

Supply

Ground Zero * Crossing Detector

Probe * Identification

Waveform * Output

Reset

Microcontroller Battery Low Monitor

Battery

Display

PSU

On/Off Switch

Split rail Generator

* Mini Dopplex Only

Power Supply

16

The power supply regulates the battery voltage and provides 5V dc to the microcontroller and all circuitry apart from the audio power amplifier which is supplied directly from the battery. The power supply is activated by the on/off switch. The supply is firstly turned on by the on/off switch and then the microcontroller takes over and keeps the power supply on, turning the supply off when the auto shut off time has elapsed or when the on/off switch is depressed again.

Resets the microcontroller whenever the unit is first switched on.

Probe Identification (D900) The probe identification feature is used to disable the waveform output whenever an obstetric probe is being used. The probe identification detects the DC level on the output from the probe thus determining the type of probe fitted.

Battery Monitor The battery monitor circuit enables the microcontroller to assess the battery voltage by comparing this to a reference voltage.

Technical Description

Power On Reset

Active Noise Reduction (ANR) Active noise reduction is a technique used to reduce the amount of noise or "hiss" depending on signal level. When a large signal has been detected the cut off point of the filter is moved so that there is less reduction in high frequency content of the signal. When the signal level is low, or no longer present the filter is switched in to reduce the high frequency hiss.

Soft Limiter The soft limiter reduces the level of any harsh overload signals that may occur, for example, when the probe is moved.

Audio Power Amplifier The audio power amplifier drives the loudspeaker or headphones. The input level is set by the volume control. The output signal is fed to the loudspeaker, via the switched headphone socket. On start up there is a delay in the supply of power to the audio amplifier. This allows the circuit to stabilise.

Zero Crossing Detector (D900) A zero crossing detector converts the Doppler signal to a corresponding pulse train. This is then fed to the microcontroller which outputs a pulse position modulated signal which is low pass filtered to produce the waveform output voltage.

Split Rail Generator The split rail generator is necessary to bias the amplifiers correctly to allow linear operation. The split rail generator supplies a +2.0V dc reference so that the maximum output swing from the amplifiers can be achieved.

17

Block Diagram All Pass Network ANR Sense

Soft Limiter

ANR

Stereo* Power Amplifier

Flow* Separation

All Pass* Network

Soft* Limiter

ANR*

Probe Connector

Technical Description

4.16 FD2, SD2, MD2, FD1, FD3, RD2

FHR^^ Signal Cond.

Tare^

Automatic Gain ^^ Control

Zero* Crossers

Gain^

Supply Probe Identification Ground Mode/ Function Buttons

Master Microcontroller

Battery Monitor

Data+ Output

Display

Battery

PSU

Split Rail Generator

Reset

Analogue* Waveform Outputs

On/Off Switch * MD2, SD2, RD2 Only + Not fitted on SD2, FD1 or FD3 ^ Fitted on RD2 ^^ Not fitted on RD2, SD2 or FD2

18

The power supply regulates the battery voltage and provides +5Vdc to the microcontroller and all circuitry apart from the audio power amplifier which is supplied directly from the battery. The power supply is activated by the on/off switch. The supply is firstly turned on by the on/off switch and then the microcontroller takes over and keeps the power supply on, turning the supply off when the auto shut off time has elapsed or the on/off switch is depressed again.

Power On Reset This resets the microcontroller whenever the unit is switched on.

Probe Identification

Technical Description

Power Supply

The probe identification feature is used to automatically select the operating mode, depending on the probe type and in the SD2, MD2 and RD2, to automatically scale the waveform output. The probe identification circuit detects the DC level on the output from the probe thus determining the type of probe fitted.

Battery Monitor The battery monitor circuit enables the microcontroller to assess the battery voltage by comparing this to a reference voltage.

Active Noise Reduction (ANR) Active noise reduction is a technique used to reduce the amount of noise or "hiss" depending on signal level. When a large signal has been detected the cut off point of the filter is moved so that there is less reduction in high frequency content of the signal. When the signal level is low, or no longer present the filter is switched in to reduce the high frequency hiss.

Active Noise Reduction Sensing The active noise reduction system relies on the control signal from the sensing circuitry. The filter response is dependent on the control signal level.

Soft Limiter The soft limiter reduces the level of any harsh overload signals that may occur, for example, when the probe is moved.

Audio Power Amplifier The audio power amplifier drives the loudspeaker or headphones. The input level is set by the volume control. The output signal is fed to the loudspeaker, via the switched headphone socket. On start up there is a delay in the supply of power to the audio amplifier. This allows the circuit to stabilise.

19

Technical Description

Zero Crossing Detector (SD2, MD2, RD2) Two zero crossing detectors convert the Doppler signals to corresponding pulse trains. The pulse trains are then fed to the microcontroller which converts them to pulse width modulated signals which are then filtered to produce the waveform output voltages.

Split Rail Generator The split rail generator is necessary to bias the amplifiers correctly to allow linear operation. The split rail generator supplies a +2.0V dc reference so that the maximum output swing from the amplifiers can be achieved.

Data Output The data control IC is an RS232 interface device enabling the microcontroller to communicate with the Dopplex Printa or a computer. The data/waveform socket presents the data from the microcontroller which represents either bi-directional flow waveforms or fetal heart rate, depending on operating mode.

All Pass Network The All Pass network is used to apply 90° phase shift to one quadrature probe output signal with respect to the other.

Flow Separation The two phase shifted signals are added and subtracted to produce the forward and reverse flow channels.

Stereo Audio Power Amplifier (SD2, MD2, RD2) The audio power amplifier provides stereo outputs to the headphone socket. These are combined in the internal loudspeaker.

FHR Signal Conditioning The FHR signal conditioning circuit produces an amplitude envelope from which the microcontroller calculates the fetal heart rate when using an obstetric probe.

Gain control The gain of the first amplifier of the FHR signal conditioning circuit is increased or decreased by the microcontroller, to regulate the signal level avoiding overloading the following stages.

Analogue Waveform Outputs

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Pulses representing the frequency envelopes of the Doppler signals are output from the microprocessor to two integrating low-pass filters. The two filter outputs are then fed to the analogue waveform/data output socket.