Date:2010-11-05 02:33:04 (9 years 1 month ago)
Author:Werner Almesberger
Commit:8cb17bb1b3f2f8772eb5305ae0251785a64f0a0a
Message:Document that the input circuit does not reach the design frequency (3 MHz) yet.

- cntr/cntr.sch: added warning that input circuit isn't good enough yet
- cntr/README: document input circuit problem, with measurement results
Files: cntr/README (1 diff)
cntr/cntr.sch (2 diffs)

Change Details

cntr/README
6666cntr -r && sleep 1 && dfu-util -d 0x20b7:0xcb72 -D cntr.bin
6767
6868
69Known issues (hardware)
69Known issues (version 2 hardware)
70---------------------------------
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.
7099
71100- the MMCX connector is hard to solder because of its large thermal
72101  capacitance and surface
cntr/cntr.sch
1EESchema Schematic File Version 2 date Mon Oct 25 18:33:18 2010
1EESchema Schematic File Version 2 date Thu Nov 4 21:28:19 2010
22LIBS:power
33LIBS:device
44LIBS:conn
55LIBS:c8051f320
66LIBS:mini_usb_b
77LIBS:usb_a_plug
8LIBS:cntr-cache
98EELAYER 24 0
109EELAYER END
1110$Descr A4 11700 8267
1211Sheet 1 1
1312Title "Arbitrary-precision Counter (3 MHz)"
14Date "25 oct 2010"
13Date "5 nov 2010"
1514Rev "2"
1615Comp "Werner Almesberger"
1716Comment1 ""
...... 
1918Comment3 ""
2019Comment4 ""
2120$EndDescr
21Text Notes 8250 1000 0 100 ~ 20
22Input circuit has known bugs.\nSee README for details.
2223NoConn ~ 8400 3800
2324NoConn ~ 8400 2900
2425Wire Wire Line

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