[time-nuts] schematics of frequency counter

Sat Dec 27 08:22:49 EST 2014

Hi Bob,
  Here is the data and test scheme.
  It does not show much difference.

2014-12-26 22:12 GMT+08:00 Bob Camp <kb8tq at n1k.org>:

> Hi
>
> 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.
>
> Bob
>
>
>
>
> > On Dec 26, 2014, at 8:21 AM, Li Ang <lllaaa at gmail.com> wrote:
> >
> > Hi
> >    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>:
> >
> >> Hi
> >>
> >> 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
> >> approaches:
> >>
> >> 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
> >> input.
> >>
> >> 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!
> >>
> >> Bob
> >>
> >>
> >>> 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
> >> board.
> >>> 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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