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- cntr/README, ecn/INDEX, ecn/ecn0006.txt: moved discussion of the input circuit from README to ECN0006 - cntr/cntr.sch: changed pointer from README to ECN0006 - ecn/ecn0006.txt: added more measurements, explanations, and an analysis of the situation
133 lines
4.9 KiB
Plaintext
133 lines
4.9 KiB
Plaintext
CNTR version 2 input circuit
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Problem description
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-------------------
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The input circuit only works up to about 1 or 2 MHz. The problem is that
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we discharge too slowly though the base of Q1, which in turn keeps the
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transistor turned on too long.
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Attempted solutions
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-------------------
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The following alternative designs have been tried:
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- Alternative 1: set R2 to zero, add a 47 Ohm termination resistor in
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parallel with VR4, and place a 1 kOhm resistor between VR4 and Q1.
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Works up to about 2 MHz, but accepts a lot of HF noise and is very
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sensitive to the signal amplitude.
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- Alternative 2: increase R2 to 100 Ohm and add a 100 Ohm resistor
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between the input (P5) and ground. This works up to 3 MHz, but only
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for a very limited amplitude range.
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- Alternative 3: set R2 to zero, add a 100 Ohm resistor in parallel
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with VR4, and add a 100 Ohm resistor between VR4 and Q1.
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Experimental results
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--------------------
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Lab test were performed on all version 2 variants and also on a version
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1 device. The counters were connected with a ~1.95 m RG-174 cable to a
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Picotest G5100A function generator. The version 1 counter was also
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tested with an unshielded 0.1" ribbon cable of 2.2 m.
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The signal consisted of square wave bursts with a 50% duty cycle and
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~ 5 ns raise/fall time.
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Design Frequency Source amplitude Probe input am- V range
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(nominal) (nominal) pli. (measured) acceptable
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------------- ---------- ---------------- --------------- ----------
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Version 1 3 MHz 2.3 - 5.5 V * 2.35 - 5.65 V Y/Y
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(RG-174) 2 MHz 2.1 - 5.5 V * 2.15 - 5.7 V Y/Y
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1 MHz 1.8 - 5.5 V * 1.85 - 5.7 V Y/Y
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Version 1 3 MHz 1.9 - 5.5 V * 2.2 - 6.5 V + Y/(Y)
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(ribbon) 2 MHz 1.9 - 5.5 V * 1.9 - 6 V + Y/(Y)
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1 MHz 1.8 - 5.5 V * 1.9 - 5.7 V + Y/(Y)
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Version 2 3 MHz 0.8 - 1.2 V 0.8 - 1.0 V Y/N
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2 MHz 0.8 - 1.6 V 0.8 - 1.0 V Y/N
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1 MHz 0.8 - 5.1 V 0.8 - 2.8 V Y/Y
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Version 2, 3 MHz 1.7 - 2.8 V 0.85 - 1.4 V N/N
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alternative 1 2 MHz 1.6 - 3.5 V 0.80 - 1.75 V Y/Y
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1 MHz 1.5 - 7.2 V 0.75 - 3.6 V Y/Y
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Version 2, 3 MHz 1.2 - 2.0 V 0.77 - 1.1 V Y/N
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alternative 2 2 MHz 1.2 - 2.6 V 0.80 - 1.4 V Y/N
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1 MHz 1.1 - 7.3 V 0.75 - 3.9 V Y/Y
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Version 2, 3 MHz 1.1 - 1.7 V 0.74 - 1.0 V Y/N
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alternative 3 2 MHz 1.1 - 2.4 V 0.74 - 1.3 V Y/N
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1 MHz 1.1 - 7.3 V 0.74 - 3.8 V Y/Y
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* = range limited by maximum input voltage
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+ = considerable overshoot, reaching about 6.7 V
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The following drawing illustrates the setup:
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Source ----- 50 R ----- Probe -----[1.8 m]----- Cntr
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^ (internal) ^
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Source, nominal Probe input, measured
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In each test the frequency was set and then the nominal source voltage
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was adjusted in increments of 100 mV to find the range at which ten
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consecutive bursts of 50000 cycles each were all received correctly.
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The source has an output impedance of 50 Ohm, so voltage at the probe
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input (indicated in the table) is roughly half the nominal source
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voltage in the first alternative design, which has a fixed impedance.
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With version 1, which has a high-impedance input, source and probe
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voltage are roughly the same.
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The amplitude range of version 2 was considered acceptable if the
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minimum source amplitude was less than 1.65 V and the maximum probe
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input amplitude was greater than 1.65 V.
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Version 1 amplitudes were considered acceptable if the minimum source
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amplitude was less than or equal to 2.5 V and the maximum source
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amplitude was at least 5.0 V. The ribbon had a better amplitude range
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than the coax cable but produced about 20% overshoot. (Only about
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10-15% can be considered safe at TTL levels.)
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Analysis
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--------
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None of the attempts at rearranging the resistors produced a
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significantly better input circuit. Perhaps a reduction of the
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capacitance of VR4 or could have helped, but this was not tried.
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I "clean" solution would require a fast comparator. This would also
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allow the implementation of a settable threshold voltage, e.g, for
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compatibility with 1.8 V logic.
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The version 1 board performs extremely well at 3.3 V and 5 V logic
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levels, particularly when using a coax cable. For shorter distances,
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also a ribbon cable should be adequate.
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Conclusion
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----------
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Revert the input circuit to version 1, with the following changes:
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- change R2 from useless 100 kOhm to 1 kOhm or less. Consider
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adding a second switchable resistor that can be put in parallel.
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- use the same TVS VR4 as for VR1 through VR3, to reduce the BOM
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count
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- use a 0.1" connector with three contacts instead of two, so that
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the signal is in the middle. This will prevent accidental shorts
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and it makes it easy to build an adapter to an MMCX jack.
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