IEEE 802.15.4 subsystem
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IEEE 802.15.4 subsystem Git Source Tree
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| Source at commit a5ab9bbf0f0749f37c144d7fe4586dbd67c72093 created 6 years 9 months ago. By Werner Almesberger, atusb/: use page layout similar to eeschema's traditional default | |
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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 | |
