1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising there
2、from, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions. Copyright 2012 SAE International All rights reserved. No part of this p
3、ublication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-497
4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/AS6456AEROSPACESTANDARDAS6456 Issued 2012-07 Aerospace Analog Fiber Optic Link RA
5、TIONALE The aerospace industry requires precise standards for avionics system design. This document identifies and defines a common set of parameters necessary to describe an analog link over an optical fiber interconnect system. This common set of parameters will allow a consistent measure of analo
6、g fiber optic links between vendors and techniques. TABLE OF CONTENTS 1. SCOPE 22. REFERENCES 23. INTRODUCTION . 24. MEASURED ANALOG PARAMETERS 24.1 Gain . 24.2 Operating Frequency Range . 24.3 Maximum Survivable Input Power 24.4 Noise Figure 34.5 Group Delay 34.6 Return Loss . 44.7 Compression Poin
7、t 44.8 Intermodulation Products 54.8.1 Second Order Intercept . 54.8.2 Third Order Intercept . 64.9 Phase Noise 75. CALCULATED ANALOG PARAMETERS 75.1 Reflection Coefficient 75.2 VSWR 85.3 SFDR. 86. OPTICAL PARAMETERS . 96.1 Center Wavelength . 96.2 Spectral Width . 96.3 Optical Power in Fiber . 96.4
8、 Optical Loss 96.5 Optical Return Loss . 107. NOTES 10SAE AS6456 Page 2 of 10 1. SCOPE This SAE Aerospace Standard (AS) defines the parameters that describe the performance of an analog link transported over an optical fiber interconnect system suitable for aerospace applications. 2. REFERENCES Ther
9、e are no referenced publications specified herein. 3. INTRODUCTION This document details parameter definitions for an analog optical interconnect system. The general block diagram is shown in Figure 1. FIGURE 1 - BLOCK DIAGRAM OF ANALOG OPTICAL INTERCONNECT SYSTEM As shown in Figure 1 the RF electri
10、cal signal is input into a RF/optical transmitter where it is modulated onto an optical signal. This modulated signal is then guided through an optical fiber. The benefit of converting into an optical signal is thatit offers low loss and low crosstalk even in harsh environments. The modulated optica
11、l signal is then received at the optical/RF receiver where the optical signal is converted back to an RF electrical signal. The desired RF electrical signal is then output. 4. MEASURED ANALOG PARAMETERS 4.1 Gain Gain is defined as the ratio of the output power to the input power in the linear operat
12、ing region of the device such that compression is not a factor. The input power is the power into the RF optical link. Similarly, the output power is the power at the output of the RF optical link. Gain is typically frequency dependent, so it should be measured and plotted versus frequency. 4.2 Oper
13、ating Frequency Range The frequency range of the operating device is the frequencies contained within the 3 dB bandwidth relative to the peak value of the gain curve. 4.3 Maximum Survivable Input Power The maximum survivable input power is the maximum power that can be injected into the device for a
14、ny duration without causing permanent damage. The RF optical link shall return to its normal operating state upon decreasing the power from the maximum survivable input power. SAE AS6456 Page 3 of 10 4.4 Noise Figure Noise figure is a figure of merit that describes how the output signal to noise rat
15、io (SNRo) compares to the input signal to noise ratio (SNRi). For all practical devices the SNRowill be less than the SNRi. Noise figure is defined as shown in Equation 1. ( L (Eq. 1) The input signal to noise ratio used in Equation 1 is defined in Equation 2. 504LL (Eq. 2) Siand Niare the input sig
16、nal power and noise power to the RF optical link, respectively. In Equation 2, k is Boltzmanns constant, T is device temperature in Kelvin, and B is bandwidth of the RF optical link. The output signal to noise ratio is shown in Equation 3. The output noise of the link is the combination of amplified
17、 noise power injected into the device and the noise power of the device. The output signal power is the input signal power multiplied by the gain. 504LL : ;(Eq. 3) Taking the ratio of Equation 2 and Equation 3 yields the noise factor given in Equation 4. ( Ls E (Eq. 4) From Equation 4 noise factor i
18、s calculated by taking one plus the ratio of noise power created by the link to the device temperature noise power. This quantity will always be greater than or equal to one. Noise factor is converted to noise figure (NF) in dB by sr:(; .4.5 Group Delay Group delay is defined as the negative rate of
19、 change of the phase shift with respect to frequency. It quantifies the variation in propagation delay for different frequencies. This parameter is important to describe the integrity of a received pulse. If the various frequency components that make up the pulse propagate at different rates the rec
20、eived pulse will be distorted. This is the case if the group delay is large. Figure 2 shows an example of ideal and non-ideal group delays. FIGURE 2 - PROPAGATION DELAY VERSUS FREQUENCY IS PLOTTED WITH THE IDEAL SCENARIO RESULTING IN AN UNDISTORTED RECEIVED PULSE SHOWN BY THE DASHED LINE. MEASURED G
21、ROUP DELAY IS THE PEAK-TO-PEAK CHANGE IN PROPAGATION. SAE AS6456 Page 4 of 10 Group delay is found by measuring the phase of the transmission coefficient and taking its derivative. If the phase is linear the derivative will be a constant, meaning all frequencies propagate at the same rate and a rece
22、ived pulse will be undistorted. 4.6 Return Loss A common quantity used to describe the amount of impedance mismatch is return loss. The return loss is found by taking the logarithm of the magnitude of the voltage reflection coefficient. The return loss is defined as shown in Equation 5. 4. L Ftr : ;
23、 (Eq. 5) When the input impedance of a device is exactly the same impedance as the feedline, the return loss will be infinite. The return loss is 0 dB if the mismatch between the feedline and device causes complete reflection of the incident signal. Return loss is typically measured / calculated usi
24、ng S-parameters. 4.7 Compression Point Output power tends to level off or saturate for high input powers in practical devices. The 1 dB compression point is the point on the gain curve where the gain is 1 dB down from the linear behavior of the lower input powers. This is shown in Figure 3. The outp
25、ut-referred 1 dB compression power (Po1) is the output power level associated with this point. The point can also be related to input power and is called the input-referred 1 dB compression point (Pi1). The gain curve is frequency dependent, thus the 1 dB compression point stated in the requirements
26、 shall be at the frequency creating maximum gain. FIGURE 3 - THE COMPRESSION POINT FOR AN ANALOG OPTICAL INTERCONNECT IS INDICATED BY THE GAIN CURVE SAE AS6456 Page 5 of 10 4.8 Intermodulation Products The signal response of a device can be described by Taylor series expansion (Equation 6) with diff
27、erent weighting coefficients denoted by co, c1, c2, etc. B : T ; L?E?5T E?6T6E?7T7E?8T8E (Eq. 6) When a signal with two or more frequencies goes through a device that has non-negligible higher order terms, intermodulation products are created. Second order terms are important when the two frequencie
28、s into the device are far apart because the second order term of the lower frequency may lie near the first order term of the higher frequency. Third order terms created are also of particular interest because some portions of these terms may lie near the first order terms of the higher frequencies
29、when the input frequencies are close. 4.8.1 Second Order Intercept Second order products are created when multiple frequencies (i.e., f1and f2) that are relatively far from each other are mixed. Second order terms are found at 2f1, 2f2, and f1f2. The second order intercept (IP2) is found by extrapol
30、ating the linear region for the second order intermodulation curve and looking at the point it intersects the first order line. Graphical representation is shown in Figure 4. FIGURE 4 - GAIN CURVES FOR FIRST AND SECOND ORDER PRODUCTS. SECOND ORDER INTERCEPT (IP2) IS SHOWN AT INTERSECTION OF FIRST AN
31、D SECOND ORDER LINES SAE AS6456 Page 6 of 10 This quantity is measured by injecting two pure tone frequencies (f1and f2) into the RF optical link. Both of these frequencies shall be within the operating frequency range of the link and one of the two frequencies shall be considerably less than the ot
32、her. The output of the RF optical link will have second order products of interest at 2f1, 2f2, and f1f2. The term f1f2will be the one close to the fundamental. The power levels of the fundamental (Pfundamental) and second order products (Psecond harmonic) are measured and the second order intercept
33、 (IP2) can be calculated using Equation 7. +2t : $ ; Lt2 : $I ; F2 :$I; (Eq. 7) 4.8.2 Third Order Intercept Theoretically the third order intercept (IP3) is found by extrapolating the linear region for the third order intermodulation curve and looking at the point it intersects the first order line.
34、 The graphical representation is shown in Figure 5. FIGURE 5 - GAIN CURVES FOR FIRST AND THIRD ORDER PRODUCTS. THIRD ORDER INTERCEPT (IP3) IS SHOWN AT INTERSECTION OF FIRST AND THIRD ORDER LINES SAE AS6456 Page 7 of 10 4.9 Phase Noise Phase or frequency noise is a quantity used to describe the stabi
35、lity in the frequency of the oscillator. It is typically specified as the power level relative to the carrier (dBc) at a given frequency away (Figure 6). For this system a pure tone frequency within the operating frequency range of the link is injected and the output power is sampled over time. FIGU
36、RE 6 - PHASE NOISE IS SHOWN AND MEASURED WITH RESPECT TO THE CARRIER FREQUENCY 5. CALCULATED ANALOG PARAMETERS 5.1 Reflection Coefficient The reflection coefficient at a given reference plane is a measure of the impedance mismatch with respect to a reference impedance (typically 50 ). The reference
37、plane is usually defined at the input or output of the device under test as shown in Figure 7. FIGURE 7 - REFERENCE PLANE OF REFLECTION COEFFICIENT AT TRANSMISSION LINE/DUT INTERFRFACE SAE AS6456 Page 8 of 10 Reflection coefficient is given by Equation 9. L? (Eq. 9) Reflection coefficient typically
38、contains both an imaginary and real part. It is calculated from the measured return loss (4.6) by using Equation 10. Lsr7 : ;. ,(Eq. 10) 5.2 VSWR Voltage Standing Wave Ratio (VSWR) is the ratio of the maximum voltage to the minimum voltage of a standing wave. The VSWR is related to and can be calcul
39、ated using the reflection coefficient as given by Equation 11. 8594 L L5 5 ? (Eq. 11) VSWR is 1 for a matched feedline to load (no reflection) and infinite for a completely unmatched line (total reflection). 5.3 SFDR A common figure of merit is the spurious free dynamic range (SFDR) which describes
40、the output power a device can supply before the third order intermodulation term is larger than the noise floor of the system. Figure 8 shows this graphically. FIGURE 8 - SPURIOUS FREE DYNAMIC RANGE IS SHOWN AS MAXIMUM INPUT POWER THAT CAN BE SUPPLIED TO THE DEVICE BEFORE THE THIRD ORDER TERMS EMERG
41、E FROM THE NOISE FLOOR AT THE OUTPUT SAE AS6456 Page 9 of 10 The SFDR is calculated using the third order intercept (IP3) and noise floor (FLOOR) as shown in Equation 12. 5( L67 +2u : $I ; F(.114:$I; ? (Eq. 12) SFDR and IP3(dB) can be referenced to an input power by Equation 13. +2u : $I ; L+2u : $I
42、 ; F)#+0:$; (Eq. 13) FLOOR(dB) consists of the factors making up the noise floor which are noise figure (NF) and the theoretical minimum noise floor (thermal noise). (.114 : $ ; L0( : $ ; EG6$ : $ ; (Eq. 14) In Equation 14, k is Boltzmanns constant, T is temperature in Kelvin, and B is bandwidth of
43、the RF optical link. 6. OPTICAL PARAMETERS 6.1 Center Wavelength The center wavelength is defined as the middle of the optical bandpass of the channel. 6.2 Spectral Width The spectral width is defined as the wavelength extent of the optical source. This is calculated between points 20 dB down from t
44、he peak optical power. 6.3 Optical Power in Fiber The minimum optical power in fiber is the quantity that describes the power level in the fiber that is required in order to convert back into an electrical signal at the output of the optical link and successfully meet the desired performance. 6.4 Op
45、tical Loss Optical loss is the optical power loss during optical transmission between the output of the RF/optical transmitter and the input of the optical/RF receiver. Optical loss for a given wavelength can be determined by comparing the optical input power to the optical output power of the optic
46、al fiber. This optical loss is higher than the optical loss for a continuous optical fiber between the transmitter andreceiver. An optical attenuator can be placed in the test setup as shown in Figure 9 to verify all RF requirements can still be met. FIGURE 9 - TEST SETUP TO EMULATE OPTICAL LOSS PER
47、FORMANCE REQUIREMENT SAE AS6456 Page 10 of 10 6.5 Optical Return Loss Optical Return Loss is the optical power reflected back to the RF/optical transmitter during the optical transmission between the output of the RF/optical transmitter and the input of the optical/RF receiver. 7. NOTES 7.1 A change
48、 bar (l) located in the left margin is for the convenience of the user in locating areas where technical revisions, not editorial changes, have been made to the previous issue of this document. An (R) symbol to the left of the document title indicates a complete revision of the document, including technical revisions. Change bars and (R) are not used in original publications, nor in documents
