IEEE 802.15.4 subsystem
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Source at commit 1fb05c221dcff657e49cc766e8ff7baa1d3434fc created 13 years 2 months ago. By Werner Almesberger, misctxrx.c (wait_for_interrupt): let "ignore" control printing more tightly | |
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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. |
133 |