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

board.

https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

On Dec 26, 2014, at 8:21 AM, Li Ang lllaaa@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@n1k.org:

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

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

counters

the

other

I

cnt90

https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

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

counters

the

other

I

cnt90

https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Hi

(In reply to several posts. It’s easier for me this way)

Ok, that’s good news !!! (and useful data)

Your counter performance degraded a bit when you put in 5 db and not much
when you put in 8 db.

It’s also maybe too good news. I suspect that cross talk between the
channels may be impacting your results.

Next step is to try it with two independent sources and a bit more
attenuation. When you try it with two sources, you need to attenuate first
one source and then switch the attenuators to the other source. That will
help you see if crosstalk from one channel is more of a problem than from
the other channel.

One parts hint:

Cable TV attenuators are much cheaper than their fancy 50 ohm MIniCircuits
cousins. They are also something you can pick up down at the corner
electronics store. For this sort of testing they are perfectly fine to use.
At this point in the testing the mismatch between 75 ohms and 50 ohms is
not a big deal. You will need to adapt connectors, but you probably still
will save money.

=======

Op-amps that have enough bandwidth and performance for a high input
impedance counter input are rare items. They also are not cheap. Often they
come as some sort of current feedback part with low(er) input impedance. If
you want your counter to work to 300 MHz, it should accept a 300 MHz square
wave. That might mean passing the third or even the fifth harmonic of the
square wave. An input channel with 900 or 1500 MHz bandwidth is quite a
challenge.

One very simple solution is to just grab a high speed comparator like the
one used by Fluke / Pendulum (ADCMP565). Drive it directly with your input
or clock. Make it your front end device. That’s not an ideal solution, but
it will give you the bandwidth and a reasonable input impedance. It
requires messy things like a negative supply  or a “fake” ground (so would
the op amp). It also has an ECL output that needs to be converted to match
your FPGA ( hint: use the clock inputs, they are LVPECL compatible).
Driving into the FPGA with a differential signal is probably needed to
reduce crosstalk.

No matter how you do it, input channels are not an easy thing to do
properly. Even on commercial counters, they often are easy to fool.
Designing one is only the start. Fully testing it is equally complex.

=========

Do not underrate your skills in any way. You are doing far more on this
project than any of the rest of the list members have done. We have talked
and talked forever about these chips. We talk a lot about these ideas. We
suggest lots of complex solutions to various possible problems (like the
expensive comparator I suggested above). What we almost never do is
actually build a counter. If we build something we don’t fully test it. I
have never seen any list member share their results the way you have. I
suspect that most of us (yes this includes me) are a bit to scared of the
response.

Please do not stop your work. Keep letting us know how it is going. This
is very exciting !!!

Bob

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.