From 6d4ea61ae13ca2d8bdfcd39423ae49a80ef6e6f6 Mon Sep 17 00:00:00 2001 From: Werner Almesberger Date: Fri, 5 Nov 2010 14:45:12 -0300 Subject: [PATCH] More detailed examination of the input circuit problem. - cntr/README, ecn/INDEX, ecn/ecn0006.txt: moved discussion of the input circuit from README to ECN0006 - cntr/cntr.sch: changed pointer from README to ECN0006 - ecn/ecn0006.txt: added more measurements, explanations, and an analysis of the situation --- cntr/README | 29 +---------- cntr/cntr.sch | 4 +- ecn/INDEX | 1 + ecn/ecn0006.txt | 132 ++++++++++++++++++++++++++++++++++++++++++++++++ 4 files changed, 136 insertions(+), 30 deletions(-) create mode 100644 ecn/ecn0006.txt diff --git a/cntr/README b/cntr/README index 656365c..419eb0b 100644 --- a/cntr/README +++ b/cntr/README @@ -69,34 +69,7 @@ cntr -r; sleep 1 && dfu-util -d 0x20b7:0xcb72 -D cntr.bin Known issues (version 2 hardware) --------------------------------- -- the input circuit only works up to about 1 MHz. The problem is that - we discharge too slowly though the base of Q1, which in turn keeps - the transistor turned on too long. - - An alternative design that sets R2 to zero, adds a 47 Ohm termination - resistor in parallel to VR4, and places a 1 kOhm resistor between - VR4 and Q1 works up to about 2 MHz, but accepts a lot of HF noise - and is very sensitive to the signal amplitude. - - Some test results with a ~1.8 m RG-174 cable, square wave bursts - with a 50% duty cycle and ~ 5 ns raise/fall time: - - Design Frequency Source amplitude Input amplitude - (nominal) (nominal) (measured) - ------------ ---------- ---------------- --------------- - Original 1 MHz 700 mV ~700 mV - Alternative 2 MHz 1.6 V ~800 mV - 1 MHz 1.5 V ~750 mV - - The test consisted of setting the frequency and adjusting the nominal - source voltage in increments of 100 mV for the lowest voltage at - which which ten consecutive bursts of 50000 cycles each were all - received correctly. - - The source has an output impedance of 50 Ohm, so voltage at the probe - input (indicated in the table) is roughly half the nominal source - voltage in the alternative design. - +- the input circuit does not perform well. See ECN0006 for details. - the MMCX connector is hard to solder because of its large thermal capacitance and surface - the lateral pads of the MMCX connector could be wider diff --git a/cntr/cntr.sch b/cntr/cntr.sch index 1d41f57..396ebc5 100644 --- a/cntr/cntr.sch +++ b/cntr/cntr.sch @@ -1,4 +1,4 @@ -EESchema Schematic File Version 2 date Thu Nov 4 21:28:19 2010 +EESchema Schematic File Version 2 date Fri Nov 5 14:44:54 2010 LIBS:power LIBS:device LIBS:conn @@ -19,7 +19,7 @@ Comment3 "" Comment4 "" $EndDescr Text Notes 8250 1000 0 100 ~ 20 -Input circuit has known bugs.\nSee README for details. +Input circuit has known bugs.\nSee ECN0006 for details. NoConn ~ 8400 3800 NoConn ~ 8400 2900 Wire Wire Line diff --git a/ecn/INDEX b/ecn/INDEX index 3781ccc..b8777ae 100644 --- a/ecn/INDEX +++ b/ecn/INDEX @@ -5,3 +5,4 @@ Number Status Description 0003 Edit Replace balun and filter with integrated balun 0004 Edit Take into account layout considerations for RF 0005 Edit Correct atusd clock voltage divider +0006 Edit CNTR version 2 input circuit diff --git a/ecn/ecn0006.txt b/ecn/ecn0006.txt new file mode 100644 index 0000000..13302a0 --- /dev/null +++ b/ecn/ecn0006.txt @@ -0,0 +1,132 @@ +CNTR version 2 input circuit + + +Problem description +------------------- + +The input circuit only works up to about 1 or 2 MHz. The problem is that +we discharge too slowly though the base of Q1, which in turn keeps the +transistor turned on too long. + + +Attempted solutions +------------------- + +The following alternative designs have been tried: + +- Alternative 1: set R2 to zero, add a 47 Ohm termination resistor in + parallel with VR4, and place a 1 kOhm resistor between VR4 and Q1. + Works up to about 2 MHz, but accepts a lot of HF noise and is very + sensitive to the signal amplitude. + +- Alternative 2: increase R2 to 100 Ohm and add a 100 Ohm resistor + between the input (P5) and ground. This works up to 3 MHz, but only + for a very limited amplitude range. + +- Alternative 3: set R2 to zero, add a 100 Ohm resistor in parallel + with VR4, and add a 100 Ohm resistor between VR4 and Q1. + + +Experimental results +-------------------- + +Lab test were performed on all version 2 variants and also on a version +1 device. The counters were connected with a ~1.95 m RG-174 cable to a +Picotest G5100A function generator. The version 1 counter was also +tested with an unshielded 0.1" ribbon cable of 2.2 m. + +The signal consisted of square wave bursts with a 50% duty cycle and +~ 5 ns raise/fall time. + + +Design Frequency Source amplitude Probe input am- V range + (nominal) (nominal) pli. (measured) acceptable +------------- ---------- ---------------- --------------- ---------- +Version 1 3 MHz 2.3 - 5.5 V * 2.35 - 5.65 V Y/Y +(RG-174) 2 MHz 2.1 - 5.5 V * 2.15 - 5.7 V Y/Y + 1 MHz 1.8 - 5.5 V * 1.85 - 5.7 V Y/Y + +Version 1 3 MHz 1.9 - 5.5 V * 2.2 - 6.5 V + Y/(Y) +(ribbon) 2 MHz 1.9 - 5.5 V * 1.9 - 6 V + Y/(Y) + 1 MHz 1.8 - 5.5 V * 1.9 - 5.7 V + Y/(Y) + +Version 2 3 MHz 0.8 - 1.2 V 0.8 - 1.0 V Y/N + 2 MHz 0.8 - 1.6 V 0.8 - 1.0 V Y/N + 1 MHz 0.8 - 5.1 V 0.8 - 2.8 V Y/Y + +Version 2, 3 MHz 1.7 - 2.8 V 0.85 - 1.4 V N/N +alternative 1 2 MHz 1.6 - 3.5 V 0.80 - 1.75 V Y/Y + 1 MHz 1.5 - 7.2 V 0.75 - 3.6 V Y/Y + +Version 2, 3 MHz 1.2 - 2.0 V 0.77 - 1.1 V Y/N +alternative 2 2 MHz 1.2 - 2.6 V 0.80 - 1.4 V Y/N + 1 MHz 1.1 - 7.3 V 0.75 - 3.9 V Y/Y + +Version 2, 3 MHz 1.1 - 1.7 V 0.74 - 1.0 V Y/N +alternative 3 2 MHz 1.1 - 2.4 V 0.74 - 1.3 V Y/N + 1 MHz 1.1 - 7.3 V 0.74 - 3.8 V Y/Y + +* = range limited by maximum input voltage ++ = considerable overshoot, reaching about 6.7 V + + +The following drawing illustrates the setup: + +Source ----- 50 R ----- Probe -----[1.8 m]----- Cntr + ^ (internal) ^ + | | + Source, nominal Probe input, measured + + +In each test the frequency was set and then the nominal source voltage +was adjusted in increments of 100 mV to find the range at which ten +consecutive bursts of 50000 cycles each were all received correctly. + +The source has an output impedance of 50 Ohm, so voltage at the probe +input (indicated in the table) is roughly half the nominal source +voltage in the first alternative design, which has a fixed impedance. + +With version 1, which has a high-impedance input, source and probe +voltage are roughly the same. + +The amplitude range of version 2 was considered acceptable if the +minimum source amplitude was less than 1.65 V and the maximum probe +input amplitude was greater than 1.65 V. + +Version 1 amplitudes were considered acceptable if the minimum source +amplitude was less than or equal to 2.5 V and the maximum source +amplitude was at least 5.0 V. The ribbon had a better amplitude range +than the coax cable but produced about 20% overshoot. (Only about +10-15% can be considered safe at TTL levels.) + + +Analysis +-------- + +None of the attempts at rearranging the resistors produced a +significantly better input circuit. Perhaps a reduction of the +capacitance of VR4 or could have helped, but this was not tried. + +I "clean" solution would require a fast comparator. This would also +allow the implementation of a settable threshold voltage, e.g, for +compatibility with 1.8 V logic. + +The version 1 board performs extremely well at 3.3 V and 5 V logic +levels, particularly when using a coax cable. For shorter distances, +also a ribbon cable should be adequate. + + +Conclusion +---------- + +Revert the input circuit to version 1, with the following changes: + +- change R2 from useless 100 kOhm to 1 kOhm or less. Consider + adding a second switchable resistor that can be put in parallel. + +- use the same TVS VR4 as for VR1 through VR3, to reduce the BOM + count + +- use a 0.1" connector with three contacts instead of two, so that + the signal is in the middle. This will prevent accidental shorts + and it makes it easy to build an adapter to an MMCX jack.