Date:2010-11-05 18:45:12 (8 years 9 months ago)
Author:Werner Almesberger
Commit:6d4ea61ae13ca2d8bdfcd39423ae49a80ef6e6f6
Message: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
Files: cntr/README (1 diff)
cntr/cntr.sch (2 diffs)
ecn/INDEX (1 diff)
ecn/ecn0006.txt (1 diff)

Change Details

cntr/README
6969Known issues (version 2 hardware)
7070---------------------------------
7171
72- the input circuit only works up to about 1 MHz. The problem is that
73  we discharge too slowly though the base of Q1, which in turn keeps
74  the transistor turned on too long.
75
76  An alternative design that sets R2 to zero, adds a 47 Ohm termination
77  resistor in parallel to VR4, and places a 1 kOhm resistor between
78  VR4 and Q1 works up to about 2 MHz, but accepts a lot of HF noise
79  and is very sensitive to the signal amplitude.
80
81  Some test results with a ~1.8 m RG-174 cable, square wave bursts
82  with a 50% duty cycle and ~ 5 ns raise/fall time:
83
84  Design Frequency Source amplitude Input amplitude
85               (nominal) (nominal) (measured)
86  ------------ ---------- ---------------- ---------------
87  Original 1 MHz 700 mV ~700 mV
88  Alternative 2 MHz 1.6 V ~800 mV
89               1 MHz 1.5 V ~750 mV
90
91  The test consisted of setting the frequency and adjusting the nominal
92  source voltage in increments of 100 mV for the lowest voltage at
93  which which ten consecutive bursts of 50000 cycles each were all
94  received correctly.
95
96  The source has an output impedance of 50 Ohm, so voltage at the probe
97  input (indicated in the table) is roughly half the nominal source
98  voltage in the alternative design.
99
72- the input circuit does not perform well. See ECN0006 for details.
10073- the MMCX connector is hard to solder because of its large thermal
10174  capacitance and surface
10275- the lateral pads of the MMCX connector could be wider
cntr/cntr.sch
1EESchema Schematic File Version 2 date Thu Nov 4 21:28:19 2010
1EESchema Schematic File Version 2 date Fri Nov 5 14:44:54 2010
22LIBS:power
33LIBS:device
44LIBS:conn
...... 
1919Comment4 ""
2020$EndDescr
2121Text Notes 8250 1000 0 100 ~ 20
22Input circuit has known bugs.\nSee README for details.
22Input circuit has known bugs.\nSee ECN0006 for details.
2323NoConn ~ 8400 3800
2424NoConn ~ 8400 2900
2525Wire Wire Line
ecn/INDEX
550003 Edit Replace balun and filter with integrated balun
660004 Edit Take into account layout considerations for RF
770005 Edit Correct atusd clock voltage divider
80006 Edit CNTR version 2 input circuit
ecn/ecn0006.txt
1CNTR version 2 input circuit
2
3
4Problem description
5-------------------
6
7The input circuit only works up to about 1 or 2 MHz. The problem is that
8we discharge too slowly though the base of Q1, which in turn keeps the
9transistor turned on too long.
10
11
12Attempted solutions
13-------------------
14
15The following alternative designs have been tried:
16
17- Alternative 1: set R2 to zero, add a 47 Ohm termination resistor in
18  parallel with VR4, and place a 1 kOhm resistor between VR4 and Q1.
19  Works up to about 2 MHz, but accepts a lot of HF noise and is very
20  sensitive to the signal amplitude.
21
22- Alternative 2: increase R2 to 100 Ohm and add a 100 Ohm resistor
23  between the input (P5) and ground. This works up to 3 MHz, but only
24  for a very limited amplitude range.
25
26- Alternative 3: set R2 to zero, add a 100 Ohm resistor in parallel
27  with VR4, and add a 100 Ohm resistor between VR4 and Q1.
28
29
30Experimental results
31--------------------
32
33Lab test were performed on all version 2 variants and also on a version
341 device. The counters were connected with a ~1.95 m RG-174 cable to a
35Picotest G5100A function generator. The version 1 counter was also
36tested with an unshielded 0.1" ribbon cable of 2.2 m.
37
38The signal consisted of square wave bursts with a 50% duty cycle and
39~ 5 ns raise/fall time.
40
41
42Design Frequency Source amplitude Probe input am- V range
43               (nominal) (nominal) pli. (measured) acceptable
44------------- ---------- ---------------- --------------- ----------
45Version 1 3 MHz 2.3 - 5.5 V * 2.35 - 5.65 V Y/Y
46(RG-174) 2 MHz 2.1 - 5.5 V * 2.15 - 5.7 V Y/Y
47               1 MHz 1.8 - 5.5 V * 1.85 - 5.7 V Y/Y
48
49Version 1 3 MHz 1.9 - 5.5 V * 2.2 - 6.5 V + Y/(Y)
50(ribbon) 2 MHz 1.9 - 5.5 V * 1.9 - 6 V + Y/(Y)
51               1 MHz 1.8 - 5.5 V * 1.9 - 5.7 V + Y/(Y)
52
53Version 2 3 MHz 0.8 - 1.2 V 0.8 - 1.0 V Y/N
54               2 MHz 0.8 - 1.6 V 0.8 - 1.0 V Y/N
55               1 MHz 0.8 - 5.1 V 0.8 - 2.8 V Y/Y
56
57Version 2, 3 MHz 1.7 - 2.8 V 0.85 - 1.4 V N/N
58alternative 1 2 MHz 1.6 - 3.5 V 0.80 - 1.75 V Y/Y
59               1 MHz 1.5 - 7.2 V 0.75 - 3.6 V Y/Y
60
61Version 2, 3 MHz 1.2 - 2.0 V 0.77 - 1.1 V Y/N
62alternative 2 2 MHz 1.2 - 2.6 V 0.80 - 1.4 V Y/N
63               1 MHz 1.1 - 7.3 V 0.75 - 3.9 V Y/Y
64
65Version 2, 3 MHz 1.1 - 1.7 V 0.74 - 1.0 V Y/N
66alternative 3 2 MHz 1.1 - 2.4 V 0.74 - 1.3 V Y/N
67               1 MHz 1.1 - 7.3 V 0.74 - 3.8 V Y/Y
68
69* = range limited by maximum input voltage
70+ = considerable overshoot, reaching about 6.7 V
71
72
73The following drawing illustrates the setup:
74
75Source ----- 50 R ----- Probe -----[1.8 m]----- Cntr
76         ^ (internal) ^
77         | |
78     Source, nominal Probe input, measured
79
80
81In each test the frequency was set and then the nominal source voltage
82was adjusted in increments of 100 mV to find the range at which ten
83consecutive bursts of 50000 cycles each were all received correctly.
84
85The source has an output impedance of 50 Ohm, so voltage at the probe
86input (indicated in the table) is roughly half the nominal source
87voltage in the first alternative design, which has a fixed impedance.
88
89With version 1, which has a high-impedance input, source and probe
90voltage are roughly the same.
91
92The amplitude range of version 2 was considered acceptable if the
93minimum source amplitude was less than 1.65 V and the maximum probe
94input amplitude was greater than 1.65 V.
95
96Version 1 amplitudes were considered acceptable if the minimum source
97amplitude was less than or equal to 2.5 V and the maximum source
98amplitude was at least 5.0 V. The ribbon had a better amplitude range
99than the coax cable but produced about 20% overshoot. (Only about
10010-15% can be considered safe at TTL levels.)
101
102
103Analysis
104--------
105
106None of the attempts at rearranging the resistors produced a
107significantly better input circuit. Perhaps a reduction of the
108capacitance of VR4 or could have helped, but this was not tried.
109
110I "clean" solution would require a fast comparator. This would also
111allow the implementation of a settable threshold voltage, e.g, for
112compatibility with 1.8 V logic.
113
114The version 1 board performs extremely well at 3.3 V and 5 V logic
115levels, particularly when using a coax cable. For shorter distances,
116also a ribbon cable should be adequate.
117
118
119Conclusion
120----------
121
122Revert the input circuit to version 1, with the following changes:
123
124- change R2 from useless 100 kOhm to 1 kOhm or less. Consider
125  adding a second switchable resistor that can be put in parallel.
126
127- use the same TVS VR4 as for VR1 through VR3, to reduce the BOM
128  count
129
130- use a 0.1" connector with three contacts instead of two, so that
131  the signal is in the middle. This will prevent accidental shorts
132  and it makes it easy to build an adapter to an MMCX jack.

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