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 <[email protected]>: > 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 <[email protected]> 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 <[email protected]>: > > > >> 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 <[email protected]> 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 > >>> _______________________________________________ > >>> time-nuts mailing list -- [email protected] > >>> To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >>> and follow the instructions there. > >> > >> _______________________________________________ > >> time-nuts mailing list -- [email protected] > >> To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >
FE5650_0db.tim
Description: Binary data
FE5650_-5db.tim
Description: Binary data
FE5650_-8db.tim
Description: Binary data
_______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
