IEEE 802.15.4 subsystem
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IEEE 802.15.4 subsystem Commit Details
Date: | 2010-11-05 18:45:12 (13 years 4 months ago) |
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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 | ||
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69 | 69 | Known issues (version 2 hardware) |
70 | 70 | --------------------------------- |
71 | 71 | |
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. | |
100 | 73 | - the MMCX connector is hard to solder because of its large thermal |
101 | 74 | capacitance and surface |
102 | 75 | - the lateral pads of the MMCX connector could be wider |
cntr/cntr.sch | ||
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1 | EESchema Schematic File Version 2 date Thu Nov 4 21:28:19 2010 | |
1 | EESchema Schematic File Version 2 date Fri Nov 5 14:44:54 2010 | |
2 | 2 | LIBS:power |
3 | 3 | LIBS:device |
4 | 4 | LIBS:conn |
... | ... | |
19 | 19 | Comment4 "" |
20 | 20 | $EndDescr |
21 | 21 | Text Notes 8250 1000 0 100 ~ 20 |
22 | Input circuit has known bugs.\nSee README for details. | |
22 | Input circuit has known bugs.\nSee ECN0006 for details. | |
23 | 23 | NoConn ~ 8400 3800 |
24 | 24 | NoConn ~ 8400 2900 |
25 | 25 | Wire Wire Line |
ecn/INDEX | ||
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5 | 5 | 0003 Edit Replace balun and filter with integrated balun |
6 | 6 | 0004 Edit Take into account layout considerations for RF |
7 | 7 | 0005 Edit Correct atusd clock voltage divider |
8 | 0006 Edit CNTR version 2 input circuit |
ecn/ecn0006.txt | ||
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1 | CNTR version 2 input circuit | |
2 | ||
3 | ||
4 | Problem description | |
5 | ------------------- | |
6 | ||
7 | The input circuit only works up to about 1 or 2 MHz. The problem is that | |
8 | we discharge too slowly though the base of Q1, which in turn keeps the | |
9 | transistor turned on too long. | |
10 | ||
11 | ||
12 | Attempted solutions | |
13 | ------------------- | |
14 | ||
15 | The 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 | ||
30 | Experimental results | |
31 | -------------------- | |
32 | ||
33 | Lab test were performed on all version 2 variants and also on a version | |
34 | 1 device. The counters were connected with a ~1.95 m RG-174 cable to a | |
35 | Picotest G5100A function generator. The version 1 counter was also | |
36 | tested with an unshielded 0.1" ribbon cable of 2.2 m. | |
37 | ||
38 | The signal consisted of square wave bursts with a 50% duty cycle and | |
39 | ~ 5 ns raise/fall time. | |
40 | ||
41 | ||
42 | Design Frequency Source amplitude Probe input am- V range | |
43 | (nominal) (nominal) pli. (measured) acceptable | |
44 | ------------- ---------- ---------------- --------------- ---------- | |
45 | Version 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 | ||
49 | Version 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 | ||
53 | Version 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 | ||
57 | Version 2, 3 MHz 1.7 - 2.8 V 0.85 - 1.4 V N/N | |
58 | alternative 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 | ||
61 | Version 2, 3 MHz 1.2 - 2.0 V 0.77 - 1.1 V Y/N | |
62 | alternative 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 | ||
65 | Version 2, 3 MHz 1.1 - 1.7 V 0.74 - 1.0 V Y/N | |
66 | alternative 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 | ||
73 | The following drawing illustrates the setup: | |
74 | ||
75 | Source ----- 50 R ----- Probe -----[1.8 m]----- Cntr | |
76 | ^ (internal) ^ | |
77 | | | | |
78 | Source, nominal Probe input, measured | |
79 | ||
80 | ||
81 | In each test the frequency was set and then the nominal source voltage | |
82 | was adjusted in increments of 100 mV to find the range at which ten | |
83 | consecutive bursts of 50000 cycles each were all received correctly. | |
84 | ||
85 | The source has an output impedance of 50 Ohm, so voltage at the probe | |
86 | input (indicated in the table) is roughly half the nominal source | |
87 | voltage in the first alternative design, which has a fixed impedance. | |
88 | ||
89 | With version 1, which has a high-impedance input, source and probe | |
90 | voltage are roughly the same. | |
91 | ||
92 | The amplitude range of version 2 was considered acceptable if the | |
93 | minimum source amplitude was less than 1.65 V and the maximum probe | |
94 | input amplitude was greater than 1.65 V. | |
95 | ||
96 | Version 1 amplitudes were considered acceptable if the minimum source | |
97 | amplitude was less than or equal to 2.5 V and the maximum source | |
98 | amplitude was at least 5.0 V. The ribbon had a better amplitude range | |
99 | than the coax cable but produced about 20% overshoot. (Only about | |
100 | 10-15% can be considered safe at TTL levels.) | |
101 | ||
102 | ||
103 | Analysis | |
104 | -------- | |
105 | ||
106 | None of the attempts at rearranging the resistors produced a | |
107 | significantly better input circuit. Perhaps a reduction of the | |
108 | capacitance of VR4 or could have helped, but this was not tried. | |
109 | ||
110 | I "clean" solution would require a fast comparator. This would also | |
111 | allow the implementation of a settable threshold voltage, e.g, for | |
112 | compatibility with 1.8 V logic. | |
113 | ||
114 | The version 1 board performs extremely well at 3.3 V and 5 V logic | |
115 | levels, particularly when using a coax cable. For shorter distances, | |
116 | also a ribbon cable should be adequate. | |
117 | ||
118 | ||
119 | Conclusion | |
120 | ---------- | |
121 | ||
122 | Revert 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. |