[time-nuts] schematics of frequency counter
kb8tq at n1k.org
Fri Dec 26 09:12:32 EST 2014
Don’t go to crazy on the front end. You can spend a year optimizing something like this. The objective is to see if the front end is a big problem now. It’s very easy to get to many things going on in a project. That makes it hard to complete.
All front end circuits will work better with worse with a 1 mV input than with a larger input signal. Some very common circuits have odd things (frequency doubling…) that happen as the input drops. Chains with a lot of gain can oscillate with certain combinations of input level and source impedance.
Some decisions you will eventually need to make:
Do you need a high input impedance counter input?
Most commercial counters have a >= 1 mega ohm input impedance capability. This lets you put an oscilloscope probe on the counter. It’s nice for probing around in a circuit. I have rarely used this feature. It’s *much* more convenient to take the output of the oscilloscope and feed it into the counter. That way the probe stays on the scope and you can *see* the signal you are probing as well as count it.
Do you need to deal with low frequency signals?
Things like pulse per second inputs are a TimeNut thing to look at. Most of the world does not try to count 1 Hz. Timing signals tend to be DC coupled. They often have odd duty cycles even if they are not low frequency. A DC coupled input channel implies a range of adjustable trigger levels. This can get very crazy very fast. A simple TTL compatible input that triggers at ~ 1 V and will accept 2 to 5V logic signals is an easy way to go. Is that enough?
Some decisions that commercial counter people get to make:
Do you need to deal with low level RF signals?
Do you need to deal with modulated RF signals?
Do you need to deal with microwave signals?
Do you need adjustable front end filtering to reject RF on your signals?
Do you need to tolerate 250V AC or 1KV DC on the counter input?
For now I’d think about the second set of decisions, but not worry about them. Even the two decisions in the first group are not all that important to make right now. They all have many sub decisions associated with them. One example is adding a negative power supply to allow a DC trigger at zero volts.
A very common solution: Build the counter with just logic level inputs. Keep things on the main board simple and easy to work with. Run that board with it’s own regulators. Get it running with 3.3V signals. Once that is done, build the input channel(s) on their own board(s). They will need their own regulators to keep noise down (regulators are cheap). You can optimize the input channel circuits as part of a separate project.
> On Dec 26, 2014, at 8:21 AM, Li Ang <lllaaa at gmail.com> wrote:
> Thanks for the suggestion. I will do some experiments with the front
> end :)
> 2014-12-25 4:32 GMT+08:00 Bob Camp <kb8tq at n1k.org>:
>> Very interesting !! Thanks for sharing.
>> As you can see from the Fluke schematics, the input amplifiers on counters
>> can get quite complex. I would definitely recommend playing a bit with the
>> input channels on your board. Here’s what I would do, there are many other
>> 1) Set up a high speed CMOS biased gate limiter with an OCXO. Quick
>> approach is two 10K ohm resistors for bias (one to B+ one to ground), AC
>> couple the sine wave into the junction. Junction also goes to the gate
>> 2) Assume that the signal is good. (it may not be).
>> 3) Compare the CMOS signal on one channel to your input amplifier on the
>> other channel.
>> 4) Attenuate the signal to the input amplifier and see what happens.
>> Again, there are *lots* of different ways to do the same sort of thing. I
>> would not go overboard doing this with complicated circuits. You simply
>> want a way to figure out what the input circuits are doing.
>> Have Fun!
>>> On Dec 24, 2014, at 11:19 AM, Li Ang <lllaaa at gmail.com> wrote:
>>> http://www.qsl.net/bi7lnq/freqcnt_bi7lnq_v4.pdf this is my current
>>> I'm not a hardware guy, feel free to correct my mistakes. :)
>>> http://assets.fluke.com/manuals/6690____smeng0000.pdf schematic of cnt90
>>> aka pm6690
>>> Happy holidays
>>> Li Ang
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