CNTR version 2 input circuit


Problem description
-------------------

The input circuit only works up to about 1 or 2 MHz. The problem is that
we discharge too slowly though the base of Q1, which in turn keeps the
transistor turned on too long.


Attempted solutions
-------------------

The following alternative designs have been tried:

- Alternative 1: set R2 to zero, add a 47 Ohm termination resistor in
  parallel with VR4, and place 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.

- Alternative 2: increase R2 to 100 Ohm and add a 100 Ohm resistor
  between the input (P5) and ground. This works up to 3 MHz, but only
  for a very limited amplitude range.

- Alternative 3: set R2 to zero, add a 100 Ohm resistor in parallel
  with VR4, and add a 100 Ohm resistor between VR4 and Q1.


Experimental results
--------------------

Lab test were performed on all version 2 variants and also on a version
1 device. The counters were connected with a ~1.95 m RG-174 cable to a
Picotest G5100A function generator. The version 1 counter was also
tested with an unshielded 0.1" ribbon cable of 2.2 m.

The signal consisted of square wave bursts with a 50% duty cycle and
~ 5 ns raise/fall time.


Design         Frequency   Source amplitude  Probe input am-  V range
               (nominal)   (nominal)         pli. (measured)  acceptable
-------------  ----------  ----------------  ---------------  ----------
Version 1      3 MHz       2.3 -  5.5 V *    2.35 - 5.65 V    Y/Y
(RG-174)       2 MHz       2.1 -  5.5 V *    2.15 - 5.7  V    Y/Y
               1 MHz       1.8 -  5.5 V *    1.85 - 5.7  V    Y/Y

Version 1      3 MHz       1.9 -  5.5 V *    2.2  - 6.5  V +  Y/(Y)
(ribbon)       2 MHz       1.9 -  5.5 V *    1.9  - 6    V +  Y/(Y)
               1 MHz       1.8 -  5.5 V *    1.9  - 5.7  V +  Y/(Y)

Version 2      3 MHz       0.8 -  1.2 V      0.8  - 1.0  V    Y/N
               2 MHz       0.8 -  1.6 V      0.8  - 1.0  V    Y/N
               1 MHz       0.8 -  5.1 V      0.8  - 2.8  V    Y/Y

Version 2,     3 MHz       1.7 -  2.8 V      0.85 - 1.4  V    N/N
alternative 1  2 MHz       1.6 -  3.5 V      0.80 - 1.75 V    Y/Y
               1 MHz       1.5 -  7.2 V      0.75 - 3.6  V    Y/Y

Version 2,     3 MHz       1.2 -  2.0 V      0.77 - 1.1  V    Y/N
alternative 2  2 MHz       1.2 -  2.6 V      0.80 - 1.4  V    Y/N
               1 MHz       1.1 -  7.3 V      0.75 - 3.9  V    Y/Y

Version 2,     3 MHz       1.1 -  1.7 V      0.74 - 1.0  V    Y/N
alternative 3  2 MHz       1.1 -  2.4 V      0.74 - 1.3  V    Y/N
               1 MHz       1.1 -  7.3 V      0.74 - 3.8  V    Y/Y

* = range limited by maximum input voltage
+ = considerable overshoot, reaching about 6.7 V


The following drawing illustrates the setup:

Source ----- 50 R ----- Probe -----[1.8 m]----- Cntr
         ^   (internal)   ^
         |                |
     Source, nominal  Probe input, measured


In each test the frequency was set and then the nominal source voltage
was adjusted in increments of 100 mV to find the range at 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 first alternative design, which has a fixed impedance.

With version 1, which has a high-impedance input, source and probe
voltage are roughly the same.

The amplitude range of version 2 was considered acceptable if the
minimum source amplitude was less than 1.65 V and the maximum probe
input amplitude was greater than 1.65 V.

Version 1 amplitudes were considered acceptable if the minimum source
amplitude was less than or equal to 2.5 V and the maximum source
amplitude was at least 5.0 V. The ribbon had a better amplitude range
than the coax cable but produced about 20% overshoot. (Only about
10-15% can be considered safe at TTL levels.)


Analysis
--------

None of the attempts at rearranging the resistors produced a
significantly better input circuit. Perhaps a reduction of the
capacitance of VR4 or could have helped, but this was not tried.

I "clean" solution would require a fast comparator. This would also
allow the implementation of a settable threshold voltage, e.g, for
compatibility with 1.8 V logic.

The version 1 board performs extremely well at 3.3 V and 5 V logic
levels, particularly when using a coax cable. For shorter distances,
also a ribbon cable should be adequate.


Conclusion
----------

Revert the input circuit to version 1, with the following changes:

- change R2 from useless 100 kOhm to 1 kOhm or less. Consider
  adding a second switchable resistor that can be put in parallel.

- use the same TVS VR4 as for VR1 through VR3, to reduce the BOM
  count

- use a 0.1" connector with three contacts instead of two, so that
  the signal is in the middle. This will prevent accidental shorts
  and it makes it easy to build an adapter to an MMCX jack.
