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

cntr/README

@@ -66,8 +66,36 @@ To update the firmware, run
 cntr -r && sleep 1 && dfu-util -d 0x20b7:0xcb72 -D cntr.bin
 
 
-Known issues (hardware)
+Known issues (version 2 hardware)
------------------------
+---------------------------------
+
+- the input circuit only works up to about 1 MHz. The problem is that
+  we discharge too slowly though the base of Q1, which in turn keeps
+  the transistor turned on too long.
+
+  An alternative design that sets R2 to zero, adds a 47 Ohm termination
+  resistor in parallel to VR4, and places a 1 kOhm resistor between
+  VR4 and Q1 works up to about 2 MHz, but accepts a lot of HF noise
+  and is very sensitive to the signal amplitude.
+
+  Some test results with a ~1.8 m RG-174 cable, square wave bursts
+  with a 50% duty cycle and ~ 5 ns raise/fall time:
+
+  Design       Frequency  Source amplitude Input amplitude
+               (nominal)  (nominal)        (measured)
+  ------------ ---------- ---------------- ---------------
+  Original     1 MHz        700 mV         ~700 mV
+  Alternative  2 MHz      1.6 V            ~800 mV
+               1 MHz      1.5 V            ~750 mV
+
+  The test consisted of setting the frequency and adjusting the nominal
+  source voltage in increments of 100 mV for the lowest voltage at
+  which which ten consecutive bursts of 50000 cycles each were all
+  received correctly.
+
+  The source has an output impedance of 50 Ohm, so voltage at the probe
+  input (indicated in the table) is roughly half the nominal source
+  voltage in the alternative design.
 
 - the MMCX connector is hard to solder because of its large thermal
   capacitance and surface

cntr/cntr.sch

@@ -1,17 +1,16 @@
-EESchema Schematic File Version 2  date Mon Oct 25 18:33:18 2010
+EESchema Schematic File Version 2  date Thu Nov  4 21:28:19 2010
 LIBS:power
 LIBS:device
 LIBS:conn
 LIBS:c8051f320
 LIBS:mini_usb_b
 LIBS:usb_a_plug
-LIBS:cntr-cache
 EELAYER 24  0
 EELAYER END
 $Descr A4 11700 8267
 Sheet 1 1
 Title "Arbitrary-precision Counter (3 MHz)"
-Date "25 oct 2010"
+Date "5 nov 2010"
 Rev "2"
 Comp "Werner Almesberger"
 Comment1 ""
@@ -19,6 +18,8 @@ Comment2 ""
 Comment3 ""
 Comment4 ""
 $EndDescr
+Text Notes 8250 1000 0    100  ~ 20
+Input circuit has known bugs.\nSee README for details.
 NoConn ~ 8400 3800
 NoConn ~ 8400 2900
 Wire Wire Line