Absolute Time Recovery
Contents
1. Agilent oe U1051A Time to Digital Converter c de ee 0790 o na s Application Note Absolute Time Recovery ee A i A A A A A A A A A A A ree whee Agilent Technologies Notices Agilent Technologies Inc 2012 No part of this manual may be reproduced in any form or by any means including electronic storage and retrieval or translation into a foreign language without prior agreement and written consent from Agilent Technologies Inc as governed by United States and international copyright laws Manual Part Number U1092 90055 Edition Edition 1 0 24 July 2012 Printed in USA Agilent Technologies Inc Sales and Technical Support To contact Agilent for sales and technical support refer to the support links on the following Agilent web resources www agilent com find U1051A product specific information and support software and documentation updates www agilent com find cPCI www agilent com find Assist worldwide contact information for repair and service Warranty The material contained in this document is provided as is and is subject to being changed without notice in future editions Further to the maximum extent permitted by applicable law Agilent disclaims all warranties either express or implied with regard to this manual and any information contained herein including but not limited to the implied warranties of merchantability and fitness fo2. ww agilent com www agilent com find modular www agilent com find embedded For more information on Agilent Technologies products applications or services please contact your local Agilent office For additional listings go to www agilent com find assist Americas Canada 877 894 4414 Brazil 11 4197 3500 Mexico 01800 5064 800 United States 800 829 4444 Asia Pacific Australia 1 800 629 485 China 800 810 0189 Hong Kong 800 938 693 India 1 800 112 929 Japan 0120 421 345 Korea 080 769 0800 Malaysia 1 800 888 848 Singapore 1 800 375 8100 Taiwan 0800 047 866 Thailand 1 800 226 008 Europe amp Middle East Austria 43 0 1360 277 1571 Belgium 32 0 2 404 93 40 Denmark 4570131515 Finland 358 0 10 855 2100 France 0825 010 700 0 125 minute Germany 49 0 7031 464 6333 Ireland 1890 924 204 Israel 972 39288 504 544 Italy 39 02 92 60 8484 Netherlands 31 0 20 547 2111 Spain 34 91 631 3300 Sweden 0200 88 22 55 Switzerland 0800 80 53 53 United Kingdom 44 0 118 9276201
3. read out of the module they will form an array of time tag events in order of their input event time Additionally when an event is stored into memory a flag is set for that entry denoting which channel 1 6 is associated with the event The key limitation of the TDC module is that it is essentially a counter that resets to zero every 10 48576 ms thus all events measured by the module will have a time tag event between 0 and 10 48576 This can cause confusion and problems for longer duration measurements For example say that you are measuring a 3 ms clock period on channel 1 of the TDC module and that you have started the TDC module with a hardware trigger synchronized to your 3 ms clock If you read the first 4 entries of data on channel 1 you would obtain the following 3 000 ms 6 000 ms 9 000 ms 2 51424 ms to 50 ps precision The first three values make sense but the fourth does not What has happened is that the TDC module has reset or rolled back to 0 between the third and fourth measurement This issue poses two problems First recovering the absolute time of the fourth and subsequent events will require accounting for this 10 48576 ms roll Second if events are infrequent gt 10 48576 ms when we read out the event times there will be an ambiguity of whether the event occurred in the next counter roll or some subsequent counter roll Figure 2 below shows how the TDC module resets to 0 every 10 48576 ms To address the amb
4. the first CH6 event to properly anchor the Raw Absolute Time which matches the exact time at Event S The Raw Absolute Time is merely the exact time of each CH6 time event Next we need to account for the time difference from other channel measured events to the previous CH6 absolute time This is done with the following 2 formulas If CHx_EventTime gt MostRecentCH6Eventlime then we are on the same ramp cycle Then Abs Event Time RawAbsTime CHx_EventTime MostRecentCH6EventTime Event T above meets this criterion lts event time is greater than the most recent CH6 event time since it is higher on the cycle Therefore to determine Event T s absolute time we take the Raw Absolute Time calculated earlier and add in the difference from the current event T minus the last CH6 event time Next if the event time for the channel is less than the most recent CH6 event time then we use this formula If CHx EventTime lt MostRecentCH6EventTime then we are on the next ramp cycle and we need to factor in the 10 48576 ms thus Abs Event Time RawAbsTime 10 48576 ms MostRecentCH6EventTime CHx_EventTime Event U above meets this criterion It has a smaller value in time relatively speaking than the Event time S so we know that a counter roll has occurred Therefore we need to take the Raw Absolute Time at Event S and add in the full counter time of 10 48576 ms minus the most recent CH6 event time and then we need to add in t
5. e Figure 5 below we have another scattering of events CH6 Count 5 CH6 Count 6 Cnt 10 48576ms fi CH1 CH2 CH6 CH1 CH2 EventQ EventR EventS EventT Event U 10 48576ms x N 10 48576ms x N 1 Figure 5 Example measurement events Q U As we read events Q through U out of memory they will be read out in this order Ch 1 Event Q 4 123 ms Ch 2 Event R 5 123 ms Ch 6 Event S 8 454 ms Ch 1 Event T 10 123 ms Ch 2 Event U 0 345 ms Here we see two channel events Q and R occurring on one given counter cycle followed by a CH6 clock event on the same cycle then another CH1 event T on the Formula 1 Formula 2a Formula 2b same cycle Finally event U occurs on the following counter cycle Just examining the data you can see that a counter roll has occurred because Event U has a smaller value than event T Note also that up until Event S we had a stored CH6 count of 5 events and after event S we now have a count of 6 CH6 events Recovering the absolute time of all events is now a two part process First we need to determine a Raw Absolute Time which equates to the most recent CH6 event time To do this we can use the following formula Raw Absolute Time CH6Count x 10 48576 CH6EventTime So for events Q and R above the CH6 count is 5 We would also have the first CH6 event time from our earlier data thus we can calculate the Raw Absolute time For event T we would have a CH6 count of 6 along with the event time of
6. es it easier to deal with reconstructing the time axis so this acquisition method has this advantage The disadvantage of this mode is that if the number of events that come in within the time window of 50 ms exceeds the maximum memory of the memory bank then events will be lost Let s look at our example above and the data that was collected COM Event 1 0 ms abs time 10 ms CH1 Event A 5 458 ms COM Event 2 0 ms abs time 20 ms CH2 Event B 3 123 ms COM Event 3 0 ms abs time 30 ms COM Event 4 0 ms abs time 40 ms COM Event 5 0 ms abs time 50 ms Here we see mostly a set of COM Common events that are read Each COM input resets the TDC time to 0 thus we only need to count the number of COM events that have occurred and multiply it by our 10 ms to achieve our starting reference time or Raw Absolute Time Raw Absolute Time COM_Count x 10 000 ms So for our example Event A has occurred after the 10 ms start point so we need to only add in the measured event time for Event A 5 458 ms and add it to 10 ms for an absolute time of 15 458 ms for Event A Again the TDC time is reset to 0 for every COM input and our COM input rate is 10 ms or less than the max value of 10 48576 ms thus Formula 4 Abs Event Time Raw Absolute Time CHx_EventTime The Modular Tangram The four sided geometric symbol that appears in Agilent modular product literature is called a tangram The goal of this seven piece puzzle is
7. he measured channel event time This would provide the absolute time at Event U Acquisition Method 2 Number of common events bank switch Formula 3 This method of TDC acquisition calls for the user to apply the 100 Hz clock signal 10 ms into the Common Input of the TDC module instead of one of CH6 The TDC module is then configured to bank switch on N cycles of the COM input This is simply an alternate operating mode of the TDC module Refer to Figure 6 below for this mode Bank Switch Bank Switch Cnt 10 48576ms COM Events Cnt 10 0000ms 4 i i i i i 000 Ent 0 CH2 CH1 tso TS n T F T T F T1 N 1 N 2 N 3 N 4 N 5 N 1 N 2 N 3 N 4 N 5 10ms 20ms 30ms 40ms 50ms 60ms 70ms 80ms 90ms 100ms Figure 6 Acquisition method 2 Notice that now the first pulse of the 100 Hz clock signal will begin TDC acquisition at t 0 Every time a COM input fires 10 ms the TDC time is reset back to 0 This eliminates having to deal with the counter roll phenomenon because we are never allowing the TDC to go beyond 10 ms It also dramatically simplifies recovering the absolute time In this example N is set to 5 which means that every 5th clock cycle of the 100 Hz clock or 50 ms in total the TDC module will perform a memory bank switch and allow the data from the previous bank to be read out Thus memory bank switches occur every 5th COM input 50 ms while the TDC module time is reset on every COM input 10 ms This mak
8. iguity issue of ensuring that we have an event within every counter roll cycle it is necessary to provide a periodic clock event that is less than the 10 48 ms counter roll period We recommend using a 100 Hz clock signal into one of the TDC input channels The 100 Hz clock signal equates to a 10 ms period which will ensure that an event is measured on every clock cycle Cnt 10 48576ms Pood 10 48576ms x1 10 48576ms x2 10 48576ms x3 10 48576ms x N Figure 2 TDC counter cycle For purposes of our discussion we will assume the user has provided a 100 Hz 10 ms clock signal into the Channel 6 input of the TDC module In Figure 3 below channel 6 events are shown in green Notice that since the 10 ms clock event is less than and not fundamentally related to the clock roll interval that the CH6 events occur earlier and earlier in each subsequent counter roll The time period between CH6 events we Acquisition Method 1 Memory based bank switch know will be 10 ms This now becomes our absolute time clock counter event Eventually as shown on the right of Figure 3 below there will be times when two CH6 events are measured in the same counter roll cycle This won t pose a problem in recovering absolute time CH 6 Events Cnt 10 48576ms f Pood 10 48576ms x1 10 48576ms x2 10 48576ms x3 10 48576ms x N Figure 3 Channel 6 Events Next we will examine how to correct for absolute time for any and all e
9. ol is marked on the instrument CN This symbol indicates the time SJ period during which no hazardous or toxic substance elements are expected to leak or deteriorate during normal use Forty years is the expected useful life ofthe product This symbol indicates the instrument is sensitive to Ates ESD can damage the highly sensitive electrostatic discharge ESD components in your instrument ESD damage is most likely to occur as the module is being installed or when cables are connected or disconnected Protect the circuits from ESD damage by wearing a grounding strap that provides a high resistance path to ground Alternatively ground yourself to discharge any built up static charge by touching the outer shell of any grounded instrument chassis before touching the port connectors This symbol denotes a hot surface AN The side cover of the module will be hot after use and should be allowed to cool for several minutes Abstract Background This document describes the methodology to utilize the Agilent Acqiris U1051A Time to Digital Converter TDC module in applications requiring absolute time measurements beyond the range of the module The TDC module Is a precise event counter with 50 ps timing precision The module however has a limitation in that the counter circuitry in the module resets every 10 48 milliseconds or more precisely 10 48576 ms This poses a problem in applications requiring longer capture time
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11. s beyond this time window This document however describes a method to allow users to take advantage of the modules speed precision and channel count to extend the capabilities to virtually any event time measurement application For a detailed description of the theory of operation of the U1051A TDC module refer to the User s Manual for the module which may be obtained at http www agilent com tind U1051A along with other documentation and resources for this module Figure 1 below shows a simplified block diagram of the TDC module The module incorporates 6 input channels and a Common channel used for external trigger start operation TC890 Ch z Ch2 E 5 5 D E 5 Sy a E E E Ch4 2 a 5 BankA Sram m cha dl 8 E 2MHits Che ESEE S Common E BankB Sram z amp 2MHits IO Aux E IO Aux n Figure 1 U1051A TC890 Simplified block diagram Measured events from the channels are stored into a dual bank memory structure and are read out upon a memory bank switch event There are multiple methods of bank switching the memory prior to system readout This document will outline two methodologies that can be utilized to perform a bank switch operation 1 Memory based Setting 2 Number of Common Events The key thing to note about the TDC module is that all events are stored in memory in the order that they are received Thus when events are time tagged and subsequently The Limitation The Solution
12. se we are looking at the first set of event data that was retrieved from the module and these initial events occurred right at the beginning of acquisition on the first counter cycle In this example we will look at 5 measured events A through E as shown on the diagram above The measured data might look like this Ch 1 Event A 4 123 ms Ch 2 Event B 5 123 ms Ch 6 Event C 8 454 ms Ch 1 Event D 10 123 ms Ch 2 Event E 0 345 ms Here we see that we have two measurement channel events 1 and 2 followed by a CH6 100 Hz clock event followed by two additional channel events 1 and 2 and the last event has occurred on the next counter roll cycle This acquisition was performed using a software trigger to start acquisition so that the start time of acquisition t 0 is independent of the CH6 clock Events A through D all occur on the first counter cycle and between events D and E the TDC module has rolled to the next counter cycle Next we will examine how to process measurement data to recover the absolute event time at any arbitrary point in the collected data In this example we re looking at arbitrary events collected by the TDC module For this process to work the user needs to collect and keep track of the following information 1 The initial event time of the first CH6 clock entry 2 The number of CH6 events that have occurred With these two pieces of information we can stitch together an accurate time axis for the data In th
13. to create shapes from simple to complex As with a tangram the possibilities may seem infinite as you begin to create a new test system With a set of clearly defined elements hardware software Agilent can help you create the system you need from simple to complex Agilent Advantage Services is committed to your success throughout your equipment s lifetime www agilent com find advantageservices DISCOVER the Alternatives Agilent Email Updates keep you informed on the latest product support and application information Agilent MODULAR Products ims www agilent com find emailupdates Agilent Channel Partners provide sales and solutions VA support For details see www agilent com find channelpartners Agilent Electronic Measureme ified ISO 9001 2008 certified For details see www agilent com quality a Quality Management System FA www pxisa org Wi AXile www axistandard org PICMG and the PICMG logo CompactPCI and the CompactPCI logo AdvancedTCA and the AdvancedTCA logo are US registered trademarks of the PCI Industrial Computers Manufacturers Group PCle and PCI EXPRESS are registered trademarks and or service marks of PC SIG Microsoft Windows Visual Studio Visual C Visual C and Visual Basic are either registered trademark or trademarks of Microsoft Corporation Product descriptions in this document are subject to change without notice Agilent Technologies Inc 2012
14. vents collected regardless of when they occurred To aid implementation of software control of the TDC module example programs are provided by Agilent covering several different programming environments These example programs cover the basic control of the module including module setup and reading of data These examples and a Programmer s Manual are available from Agilent com website These basic examples can be easily augmented with software to perform the absolute time recovery mentioned in this application note This method of TDC acquisition calls for the user to set the amount of memory used in the acquisition When the set number of events is written into memory then the TDC will switch memory banks to allow users to read out the data of the prior memory bank and begin acquiring data into the second bank This capability allows the TDC module to continuously and seamlessly acquire data events indefinitely Note Continuous and gap free operation will be maintained as long as events can be written out of the memory bank prior to the TDC module bank switching back to this memory Refer to the TDC module User Guide for rate transfer information In Figure 4 below we will look at an example measurement and how we can implement a correction routine to deal with the counter rolls Cnt 10 48576ms f CH1 CH2 CH6 CH1 CH2 EventA EventB EventC Event D Event E 10 48576ms x 1 Figure 4 Example measurement events A E In this ca
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