IN N O VAT IO N ROX Series TM S O L InGaAs U T I OAPD N S Laser Rangefinder Receivers TECHNOLOGY E X P E R I E N C E ROX™ Laser Rangefinder Receiver User Guide IN N O VAT IO N Ve r sion 1.4 S O L U T I O N S TECHNOLOGY E X P E R I E N C E IN N O VAT IO N S O L U T I O N S TECHNOLOGY E X P E R I E N C E IN N O VAT IO N S O L U T I O N S TECHNOLOGY I N N O VAT I V E PHOTONIC SYSTEMS & DE VICES E X P E R I E N C E Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 IN N O VAT IO N This page intentionally left blank ROX Series TM InGaAs APD Laser Rangefinder Receivers Contents ULTRA LOW NOISE MODEL (RVC1-JIAA) WIDE-AREA, LOW-NOISE MODEL (RVC1-NIAA) BLOCK DIAGRAM 3 5 7 Deschutes™ avalanche photodiode die ROX™ application specific integrated circuit (ASIC) Microcontroller APD bias controller EEPROM 7 7 8 8 8 TO-8 PACKAGE PINOUT ROX™ OPERATION Mode 0 Modes 1–5 ROX™ TEMPERATURE COMPENSATION ROX™ PERFORMANCE Noise Equivalent Power (NEP) Sensitivity Temperature compensation of gain Optimized sensitivity Gain Stability Dynamic range Linear Dynamic Range Saturated Dynamic Range Time-Varied Threshold RANGE WALK COMPENSATION OVERLOAD PROTECTION USER SELECTABLE OPERATING MODES MIL SPEC QUALIFICATION Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 9 10 12 12 12 13 13 13 14 14 15 16 18 18 18 18 19 20 20 21 This page intentionally left blank InGaAs APD Laser Rangefinder Receivers ROX Series TM Ultra Low Noise Model ROX ™ Series Lowest Cost, Best Performance LRF Receivers Features ▪▪ 950 – 1700 nm spectral response ▪▪ High gain, low noise APD allows superior range and low false alarm rate ▪ ▪ N o i s e e q u i va l e n t p o w e r ( N E P ) o f 2 nW for 3.8 ns pulse widths ▪▪ 70 dB dynamic range ▪▪ 0.3 m range precision over entire dynamic range ▪▪ 10 nW sensitivity reduces laser pulse energy requirements ▪ ▪ Te m p e r a t u r e - c o m p e n s a t e d a va l a n c h e g a i n , w i t h n o thermoelectric coolers allows lower system power consumption ▪ ▪ Va r i a b l e t h r e s h o l d w i t h t i m e d decay Model RVC1-JIAA: 75 µm APD Receiver ROX™ performance allows system cost advantages by reducing laser power requirements, which also reduces system size, weight, and power. The ROX series of high-sensitivity laser rangefinder receiver (LRFR) integrates Voxtel’s high-performance avalanche photodiodes (APDs), custom-designed CMOS application specific integrated circuits (ASICs), and processing circuits to provide flexible system integration and reliable performance, all in a small TO-8 package. •• Performance: Superior to standard photodiode detectors (PIN detectors) by virtue of avalanche gain (M) values up to M =2 5. •• Avalanche Noise: At half the excess noise of other commercially available APDs, the ROX LRF receiver provides superior range and false alarm performance than the competition. •• Automatic Gain Control: ROX technology automatically adjusts APD bias voltage to optimize performance in multiple use case regimes. •• Threshold Optimization: Selectable threshold with APD gain control enables optimized time over threshold and eliminates range walk error. •• High Dynamic Range: 70 dB of dynamic range enables 55 db of linear range, signal detection of 8 nW to 2.5 mW, with additional dynamic range of up to 100 mW of overload protection. •• Laser Damage Protection: With control capability of ROX technology, damage protection of up to 6 MW/cm2 of peak power is standard. •• Microprocessor-controlled Gain Compensation: Allows performance to be optimized over the -40° C to 80° C temperature range, without the use of thermoelectric cooling. •• Options: The 75 μm standard standard configuration is designed for ultra low noise. A 200 μm option is designed for large areas. Both have 100MHz of bandwidth. Standard fiber pigtail options for the 75μm receivers include 62.5/125 graded-index and 105/125 step-index multi-mode fibers. ▪▪ 85,000 hrs MTBF ▪ ▪ Q u a l i f i e d t o M I L- P R F - 3 8 5 3 5 , M I LS T D - 8 3 3 , M I L- S T D - 7 5 0 , a n d M I LSTD-202 Applications ▪▪ Laser range finding ▪▪ Free-space optical communications ▪▪ Optical time domain reflectometry ▪▪ Optical coherence tomography ▪▪ Fluorescence measurements, s p e c t r o s c o p y, c h r o m a t o g r a p h y and electrophoresis ▪ ▪ Te l e c o m m u n i c a t i o n s ▪▪ LADAR/LIDAR Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 InGaAs APD Laser Rangefinder Receivers ROX Series TM Ultra Low Noise Model RVC1-JIAA Specifications 1 0 0 M H z R O X ™ R e c e i v e r w/ 7 5 - μ m D e s c h u t e s B S I ™ R - A P D i n a T O - 8 P a c k a g e Parameter Min Signal Input Pulse Width Typical Max 3.5 Spectral Response, λ 950 Optically Active Diameter APD Operating Gain, M ns (Gaussian) 1550 1750 nm 75 μm 100 MHz 100 300 kHz 1 10 Bandwidth Low Frequency Cutoff Units Pulse Pair Resolution 25 100 ns Linear Dynamic Range 25 dB Total Dynamic Range 70 dB 0.48 V Comparator Threshold level Operational Performance Small Signal Responsivity Temporal Resolution 890 1,2,4 1 71200 206 Noise Equivalent Power 1,4 Signal Sensitivity 8900 kV/W ps RMS 1.1 2 2.4 nW 5.5 10.0 12.0 nW 6 MW/cm 2 1.8 V APD supply 20 mA 5 V APD supply 10 mA 5 mA 80 °C 3,4 Maximum Instantaneous Optical Power 4 Power Requirements Low Voltage Current Draw Threshold Level High Voltage Current Draw Threshold Level < 63 V APD supply Environmental Operational Temperature Range -40 1 M=10 gain 2 20 nW signal 3 0.1% false alarm rate 4 1550 nm spectral response Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 ROX Series TM Wide-Area, Low-Noise Model InGaAs APD Laser Rangefinder Receivers RVC1-NIAA Specifications 1 0 0 M H z R O X ™ R e c e i v e r w/ 2 0 0 - μ m D e s c h u t e s B S I ™ R - A P D i n a T O - 8 P a c k a g e Parameter Min Typical Signal Input Pulse Width Max Units 3.5 Spectral Response, λ 950 ns (Gaussian) 1550 1750 nm Optically Active Diameter 200 μm Bandwidth 100 MHz 100 300 kHz 1 10 Low Frequency Cutoff APD Operating Gain, M Pulse Pair Resolution 25 100 ns Linear Dynamic Range 25 dB Total Dynamic Range 70 dB 0.48 V Comparator Threshold level Operational Performance Small Signal Responsivity 890 8900 Temporal Resolution1,2,4 Noise Equivalent Power Signal Sensitivity 1 71200 kV/W 206 1,4 3,4 Maximum Instantaneous Optical Power ps RMS 1.8 3.2 3.8 nW 8.8 16 19 nW 6 MW/cm 4 2 Power Requirements Low Voltage Current Draw Threshold Level 1.8 V APD supply 20 mA 5 V APD supply 10 mA High Voltage current Draw Threshold Level <63 V APD supply 5 mA 80 °C Environmental Operational Temperature Range 1 M=10 gain -40 1 2 20 nW signal 0.8 3 0.1% false alarm rate 0.6 4 1550 nm spectral response 0.4 Spectral responsivity and quantum efficiency of 200 µm APD at 298K. 0.2 0.0 900 1100 1300 1500 Wavelength [nm] Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 1700 This page intentionally left blank InGaAs APD Laser Rangefinder Receivers ROX Series TM BLOCK DIAGRAM The ROX™ LRFR’s includes six primary components: (1) Voxtel’s Deschutes™ NIR APD, (2) a custom-designed ROX™ amplification and pulse processing ASIC, (3) a bias supply and conditioning circuit, (4) a microcontroller and (5) an EEPROM; all are mounted on a circuit board integrated in (6) a hermetic TO-8 package. Figure 1: ROX™ Receiver Block Diagram Deschutes™ avalanche photodiode die The ROX™ receiver integrates Voxtel’s Deschutes™ VFC1 Series of InGaAs APD. The Deschutes™ APDs are sensitive over the 950 nm to 1700 nm spectral range and have stable avalanche gain up to M=25. The excess noise of the Deschutes APDs is characterized by McIntyre parameterization of k < 0.2. The datasheets for the 75 μm and 200 μm diameter die can be viewed at www.voxtel-inc.com. ROX™ application specific integrated circuit (ASIC) The Voxtel ROX™ ASIC performs signal amplification, conditioning, pulse detection, pulse generation and differential output. The user supplied VCMOS1 bias (+1.8 VDC) is used to power the ASIC. The ROX™ ASIC includes a two-stage resistive transimpedance amplifier (TIA) with a 100 MHz bandwidth. The ASIC is designed to convert the current output of the APD into an amplified voltage signal that can be detected by the pulse detection circuits. The ROX™ ASIC is designed to accommodate a wide range of input signal levels, so it is configured with a leading edge pulse discriminator, with a threshold reference level, Vth, provided by the user. The Vth threshold bias includes a RC circuit, which allows the threshold to have temporal decay, preventing false triggering due to unwanted returns during the initial period of the pulse transmission. Upon detecting a signal, the pulse detection circuit generates a differential output pulse (Sig(+) and Sig(-)) with a duration that is proportional to the pulse amplitude. The timing and duration of the pulse allow high range accuracy to be achieved. The ASIC also includes a temperature sensor which is used by the ROX™ processing circuits for temperature compensation of the APD reverse bias and gain optimization for maximum sensitivity. Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 7 ROX Series TM InGaAs APD Laser Rangefinder Receivers An analog signal monitoring point (SigMon) is used at the factory for alignment and diagnostics, and can be useful for system troubleshooting. The buffered output signal is generally not used for operational range processing. However, it may be used as an analog signal in applications that do not require the full dynamic range and sensitivity of the primary signal chain. Figure 2: ROX™ ASIC Block Diagram Microcontroller A Microchip PIC12F series microcontroller is used inside the TO-8 package to perform state machine control including temperature sensing and gain compensation. The controller’s specifications can be viewed at http://www.microchip. com. The microcontroller is configured at the factory with 5 user-selectable programs. The default program provides automatic temperature compensation of the APD’s reverse bias and gain optimization. The microcontroller can be programmed with custom startup sequences and temperature compensation schemes. APD bias controller The APD bias controller accepts a user supplied bias (VAPD) and generates a conditioned bias voltage for the APD, using signals from the microcontroller to achieve the desired avalanche gain at the most recent temperature measurement. Note: If the temperature of the APD has drifted since the last APD temperature calibration, gain variation may result. EEPROM The EEPROM stores the lookup table (LUT) that is generated at the factory to provide temperature compensation of APD bias and to optimize the APD gain. The LUT is used by the microcontroller to determine the APD bias voltage. Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 8 ROX Series TM InGaAs APD Laser Rangefinder Receivers TO-8 PACKAGE The ROX™ receiver cap consists of a fused silica flat window (Schott D273T) with a wideband NIR anti-reflection coating on both sides (Figure 3). Inside the package, the APD is mounted directly onto the ASIC, minimizing capacitance and improving reliability. The reflectance curve for the AR performance of the window is shown in Figure 4. Figure 3: TO-8 Package Mechanical Drawing Figure 4: TO-8 Window Reflectance Curve (0 deg incidence) Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 9 InGaAs APD Laser Rangefinder Receivers ROX Series TM PINOUT The TO-8 package has 12-pins, which include: six user required inputs, a differential signal output pair, two optional LRFR monitor points (bias and buffered signal), and two pins for factory calibration and servicing. Pin Name Type 1 SigMon Monitor output Nom. Value Use Requirements This pin provides a buffered and reduced amplitude version of the signal. This monitor output is an aid during set-up and alignment of the laser and optics, and may be used for diagnostics, and operationally, for built-in-test (BIT). The output does not maintain the receiver’s full bandwidth, dynamic range, or sensitivity. This signal has a DC offset of approximately 100 mV, and is designed to drive a 50 Ω load impedance. If this is unused this pin may be left floating. (optional) 2 V CMOS2 Power Supply Input +5 VDC This pin provides power to the microcontroller, EEPROM, APD bias controller, and related electronics. The APD will not receive a bias voltage until both the VAPD (Pin 10) and V CMOS2 signals are applied. <1% ripple 3 V CMOS1 Power Supply Input +1.8 VDC This pin provides power to the ASIC. <1% ripple 4 START User input +5 V T TL Pulse Rising edge initiates operation and the pulse width determines the program mode of the receiver. 5 Sig(-) Signal output 0 to +1.8 V This pin is the negative of the LRFR’s differential digital output. When the amplified pulse echo signal exceeds the user supplied [V th pin (PIN 8)] threshold reference, this output transitions to a “Low” state. An internal 500-Ω resistor is in series with this output. Assumes high impedance load 6 Sig(+) Signal output This pin is the compliment to Sig(-) (PIN 5); this pin normally is in the “Low” state and transitions to “High” when the pulse echo is detected. As internal 500Ω resistor is in series with this output. Assumes high impedance load Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 10 InGaAs APD Laser Rangefinder Receivers ROX Series TM Nom. Pin Name Type 7 μDATA Factory input 5 V T TL This pin is the data line for the microcontroller’s I 2 C port and related hardware. It is used only in the factory for testing and calibration, or during LRFR servicing. In final applications leave this pin floating as it is used by the microcontroller inside the ROX™ receiver to communicate with the other internal I 2 C devices. 8 V th User input 0.4 to 1 V This pin receives the user supplied threshold voltage reference used by the pulse detection circuit for threshold pulse detection. The level is generally chosen to maximize pulse detection efficiency (PDE) and minimize the False Alarm Rate (FAR). 9 BiasMon Factory test point 10 VAPD User input +60 VDC This pin provides the high voltage that is used by the APD Bias Controller to generate the conditioned APD bias voltage. 11 A gnd User input GND This pin is used to supply the ground for both the analog and digital circuitry inside the receiver. Note that the case is not internally connected to this ground pin. The case may be grounded externally. 12 μCLK Factory input 5V T TL This pin is the clock line for the I 2 C port of the microcontroller. It is generally used only at the factory for test and calibration, but may be used for custom LRFR programming, diagnostics, and operationally for BIT. In deployed applications leave this pin floating as it is used by the microcontroller inside the ROX™ receiver to communicate with the other internal I 2 C devices. Value Use Requirements Do not exceed 1.8 V This pin is the APD current monitor test point. It is used in the factory for test and calibration, and is not required for operational use. It can be used for diagnostics or built-in-test (BIT) purposes to verify APD output and determine the APD’s gain. In the final application this pin shold be left floating. Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 11 <1% ripple I 2 C logic InGaAs APD Laser Rangefinder Receivers ROX Series TM ROX™ OPERATION Mode 0 With application of VCMOS1, VCMOS2 and VAPD the microcontroller starts its clock, enters run state, measures the temperature via a circuit on the ROX™ ASIC chip, sets the APD gain to its default value of one, and enters sleep state. These power supplies may be applied to the chip in any order without damaging the receiver. Modes 1–5 Upon receiving the START signal, the microcontroller digitizes the temperature value using its internal 10-bit analogto-digital converter (ADC). This temperature measurement is used to address lookup tables (LUTs) stored in the EEPROM. The contents of this LUT are used to set the appropriate APD bias voltage at that temperature. There are multiple LUTs stored in the EEPROM, one for each biasing scheme. The microcontroller uses a 10-bit digital-to-analog converter (DAC) to set an input voltage to the APD bias controller chip (ADL 5317). The APD bias controller amplifies the input voltage by a factor of 30 and applies the voltage to the APD provided that the external high voltage (+60 V) is present. After receiving the START pulse, the APD bias is established within 15 ms. Once the APD bias voltage is set, the microcontroller again enters “sleep” state, wherein all digital switching, including the internal clock, are stopped to reduce digital noise coupling to the analog signal chain. With the microcontroller in sleep state, the receiver will operate in the current mode, until the next START pulse is received. Upon receiving a pulse echo, the optical signal is converted to an electrical current and amplified by the APD. The APD’s output current signal is amplified in the ASIC’s TIA, which band limits the signal and converts it to a voltage pulse. The amplified voltage pulse is then processed by the ASIC’s pulse detection circuit, using a user provided threshold reference level. When the threshold reference level is exceeded, the comparator triggers, and pulse generation circuits are used to output a differential signal with duration proportional to the pulse amplitude. A breif description of each program mode follows: Mode Description 1 Set M=1, maximum damage threshold, 15ms execution time 2 Set M=10, 15ms execution time 3 Set M=20, 15ms execution time 4 Set VAPD =NEP MIN ,minimum sensitivity, 15ms execution time 5 Factory use Note: The mode setup requires 15ms to complete and resume operation Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 12 InGaAs APD Laser Rangefinder Receivers ROX Series TM ROX™ TEMPERATURE COMPENSATION In order to avoid the power draw, cost, and complexity of a thermoelectric cooler (TEC), the ROX™ receivers use a temperature dependent bias compensation scheme. Temperature compensation is necessitated by the fact that APD breakdown voltage varies with temperature (about 33 mV/°C for the Deschutes™ model APD used in the ROX™). For a fixed bias, this causes the APD’s gain to drop as the temperature increases. To stabilize gain while in use, two of the LUTs are programmed in the factory with the bias voltages required at each temperature to maintain APD gain of M=1, M=10 or M=20. The third LUT is programmed to optimize sensitivity at each temperature. In this scheme, the gain is slightly reduced at high temperatures, to mitigate the deleterious effects of APD dark current, and the gain is allowed to increase at low temperatures. The implementation of the temperature compensation scheme in the ROX™ receivers requires that the temperature must be measured and bias voltage adjusted periodically to ensure an accurate gain setting. This is especially true in situations in which the temperature of the receiver is changing rapidly. ROX™ PERFORMANCE Noise Equivalent Power (NEP) Noise equivalent power (NEP) is the input referred optical power that would result in a mean output signal equal in magnitude to the output noise of the receiver. It is a standard measure of APD receiver sensitivity. NEP can be written either as a spectral density in W/rt-Hz or as a broad-band figure in units of Watts. An estimate of the broadband NEP can be found from a spectral intensity theorem for APD shot noise (calculated at the output of the APD and approximated as spectrally white) and the TIA’s input-referred circuit noise; the two dominant noise sources are combined as the root-sum-of-squares, and then transformed to the input of the APD using the product of the APD’s unity-gain spectral responsivity and its avalanche gain: 2 NEP= √(2∙q∙M ∙F(M,k)∙Idp ∙∆fRTIA+IRTIA2 )/(R(λ)∙M) q = elementary charge in Coulombs M = APD gain (unitless) F(M,k) = excess noise factor (unitless) Idp = APD primary (before being multiplied by the gain, M) dark current in Amperes ΔfRTIA = -3dB bandwidth of the amplifier in Hz IRTIA = total input referred noise of the amplifier in Amperes R(λ) = APD spectral responsivity at unity gain in A/W Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 13 InGaAs APD Laser Rangefinder Receivers ROX Series TM Sensitivity The optical sensitivity of a receiver is defined in terms of the weakest optical signal strength at which the desired probability of pulse detection can be achieved simultaneously with the desired probability of triggering the comparator on noise. The metrics used in determining the sensitivity of a receiver are (RCA): 1. The acceptable false alarm rate: FAR 2. The desired probability of detection: Pd 3. The bandwidth of the receiver: BW 4. The input referred noise spectral intensity of the TIA: SI,TIA (A2/Hz) 5. The noise spectral intensity of the APD current: SI,APD (A2/Hz) Psens=√(-2∙ln((√3∙FAR)/BW)∙BW∙(SI,TIA+SI,APD ) )/(M∙R(λ) ) 1. For the ROX™ receiver, sensitivity is typically five times larger than the NEP assuming a FAR of 0.1%. Temperature compensation of gain APD-based receivers often use TECs to temperature-stabilize the receiver. However, TECs consume considerable power and cannot easily accommodate a LRF’s wide operating temperature range. Instead, the ROX™ uses internal bias compensation to stabilize APD gain and optimize sensitivity. The ROX™ receiver can perform temperature compensation by adjusting the APD bias to maintain a constant gain over the range of operational temperatures. The reverse bias necessary to maintain each gain are factory programmed into LUTs stored in the EEPROM (Figure 5). After receiving the START signal, within 15 ms of the signal’s falling edge, the bias voltage is adjusted to the new value and the receiver returns to its active state. As the temperature of the receiver changes it is necessary to re-adjust the APD bias voltage to maintain the desired gain setting. This is achieved by resending the START signal for the desired/current program mode. Figure 5: Gain verses voltage curves for various temperatures Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 14 InGaAs APD Laser Rangefinder Receivers ROX Series TM Optimized sensitivity The ROX™ receiver can also be operated in a minimum sensitivity mode (Program mode #4). APDs have internal current gain (M) due to impact ionization (avalanche multiplication). Current gain is used in detector systems to boost the photocurrent from a weak optical signal above the noise floor of the amplifier. However, the gain the APD provides also contributes its own shot noise component that grows with increasing avalanche gain; as does the shot noise contribution from the APD’s dark current. So for any given level of amplifier (Johnson) noise, there is a limit to how much avalanche gain is useful. The optimum APD gain at any temperature can be found by determining the minimum NEP for each curve in Figure 7. Figure 6 shows the result of numerical modeling illustrating how the 75 μm NEP can be optimized across the operational temperature range by adjusting the gain of the APD appropriately. Figure 6: Calculated comparison of 75 μm NEP for three biasing schemes; constant gain (M=10), minimum NEP, and constant bias voltage. Figure 7: Calculated estimate of 75 μm NEP vs. APD Gain of the ROX™ Receiver Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 15 ROX Series TM InGaAs APD Laser Rangefinder Receivers Gain Stability The ROX™ receiver has been designed to provide exceptional gain stability over the entire temperature operating range without the need for thermal control of the APD. Figure 5 shows the Deschutes™ APD M-V curve at various temperatures. The slope of the curves is shown in Figure 8. A 10-bit digital-to-analog converter is used to set the bias voltage of the APD, allowing the bias controller to maintain the bias to better than 146 mV. This results in a maximum gain deviation of ±0.4 at a typical operational gain of M=10. Figure 8: Slope of the curves in Figure 5. Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 16 ROX Series TM InGaAs APD Laser Rangefinder Receivers Figure 9 illustrates the sensitivity of the APD gain to variations in temperature. For example, at a gain of M=20, dM/ dT is approximately 1 over the entire operational temperature range. This means that if the temperature drifts by one degree you will get a 1/20 or 5% drift in the APD gain. If the application requires APD gain variation less than 1%, then the ROX™ bias setting will require an update for temperature drift greater than 0.2°C. Figure 9: Gain Variance over Temperature Gain accuracy of ROX™ is limited by the 10-bit DAC used for bias control. Figure 10 displays the ROX™’s ability to repeatedly measure the temperature and set the gain. This results in a maximum APD bias voltage variation of 146 mV. For example, to repeatedly set a fixed gain with variation less than 1% , the gain must be set to M<30. Figure 10: Maximum Gain Error Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 17 InGaAs APD Laser Rangefinder Receivers ROX Series TM Dynamic range The ROX™ LRFR can accommodate optical power levels varying over 70 dB. Voxtel designed the LRFR to handle over 20 mW of input laser power and recover within 100 ns so that it can detect a pulse at the minimum specified signal level. Linear Dynamic Range The ROX™ LRFR has 25 dB of linear dynamic range in optical power (Figure 11), limited by the ASIC’s amplification circuits. Saturated Dynamic Range The receiver can handle at least an additional 45 dB of optical power above the ROX™ ASIC’s saturation level without false triggering, or otherwise increasing the FAR, while maintaining NEP-limited detection performance in the next pulse echo. The simulations of Figure 12 show the ROX™ response to two pulses separated by 160 ns. In the top plot, showing the output of the APD responding to two pulses, the first ranging from 70 nW to 70 mW and a second pulse 30 dB lower. The middle plot shows the output of the amplifier circuits, at the input to the comparator. The bottom plot shows the output from the comparator. The pulse detection circuits show successful discrimination of the two pulses over 70 dB. Figure 11: Dynamic Range Measurements Time-Varied Threshold The ROX receiver achieves time varying threshold functionality through an RC filter circuit on the ASIC. When the external value of Vth is changed from one value (Vth,hi) to another (Vth,lo ), the internal threshold (Vth,int) changes according to the RC time constant of 2.6 μs (102 kΩ resistor and a 25.5 pF capacitor). This time constant was chosen to approximate the typical 1/R2 signal attenuation as a function of range. Vth,int (t)=Vth,lo-(Vth,hi-Vth,lo )∙e-t⁄R∙C 2. Where Vth,hi is the initial threshold value, and Vth,lo is the final desired threshold value at long ranges. This feature reduces the susceptibility of triggering from foreground pulse returns. Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 18 InGaAs APD Laser Rangefinder Receivers ROX Series TM RANGE WALK COMPENSATION Discriminators generate precise logic pulses in response to input signals exceeding a given threshold. There are two main types of discriminators, the leading edge discriminator and the constant fraction discriminator (CFD). The leading edge discriminator is the simpler of the two types. Given an input pulse, the leading edge discriminator produces an output pulse at the time when the input pulse crosses the threshold voltage reference. This, however, causes a problem for precise timing. If the amplitude is changed, but the rise time of the input pulse remains the same, a sort of “time walk” occurs. That is, an input pulse with smaller amplitude but with the same rise time as a larger pulse will cross the threshold at a later time. Thus, the timing of the output pulse is shifted by this change in amplitude. Figure 12: Illustration of range walk. The time of the comparator trigger can vary with the pulse amplitude. By also recording the falling edge of the signal, the time over threshold can be used to compensate for range walk. Voxtel has experience with leading edge, CFD, and zero-crossing detectors (ZCD), and to maximize performance over a wide dynamic range has configured the ROX™ with a leading edge (LE) discriminator with time over threshold (TOT) correction of time walk. The LE/TOT pulse detector has robust range resolution performance over a wide dynamic range of optical signals. Upon receiving the input optical pulse, the ASIC’s comparator generates a signal when the leading edge of the pulse crosses the input threshold voltage value (Vth.int). The signal has a duration 5 ns longer than the pulses’s falling edge threshold crossing. Using pulse TOT calibration, range walk can be reduced. An example of a a simple scale and offset has been used to obtain a time precision of 341 ps. This is shown in Figure 13. Further precision can be accomplished using piecewise calibration with a LUT. Figure 13: ROX™ showing time walk, and the results of using a simple linear correction with the time-above-threshold to compensate for time walk. Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 19 ROX Series TM InGaAs APD Laser Rangefinder Receivers OVERLOAD PROTECTION The APD can withstand up to 6 MW/cm2 of laser peak power density with the gain set to M=1 (Program Modes 0 or 1). USER SELECTABLE OPERATING MODES The ROX™ LRFR is programmed to operate in any one of five operating modes. Each mode is selected by applying the START signal for the duration of time listed in the “START Pulse Width” column of the table below. Mode START Pulse Width (μs) Program Description 1 125 ± 10 Set APD gain to 1 2 150 ± 10 Set APD gain to 10 3 180 ± 10 Set APD gain to 20 4 200 ± 10 Minimum sensitivity 5 1000 ± 10 Test and calibration procedure (factory use only) The details of each program mode listed in Table 1: ROX™ program overview. Table 1: ROX™ program overview Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 20 ROX Series TM InGaAs APD Laser Rangefinder Receivers MIL SPEC QUALIFICATION The Voxtel ROX receivers have been tested to the following requirements: Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel-inc.com, T 971.223.5646, F 503.296.2862 21