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.