Frequency counter Project

[David Hess]

You might want to consider spending your time making a front end to one of these:

But I'm a bit puzzled as to what a front end would entail? Just a BNC with shielded wire to the input of the module, prescalers? Seems like that module is ready to go, but that also sounds a bit too good to be true.

Things you might add:

1. Slope and level/sensitivity controls.
2. Input coupling options.
3. Input protection.  This one is interesting if you want to add input protection to a 50 ohm input.
4. Input bandwidth limiting.
5. Input attenuation options.
6. A high impedance input so standard oscilloscope probes may be used or the frequency counter may be attached inline.

[Kleinstein]

An high impedance input that can be used inline with a standard 1 M scope input is kind of difficult, as there is usually one very little spare capacitance for compensation. So the input would need to be very high impedance. Many DSOs also already include a kind of counter function. Some of the older analog scopes have an analog signal output, that could be used for the counter.

The more likely way would be an extra output to monitor the signal after an high impedance input stage (e.g. JFET based buffer, just like in most scope inputs) of the counter.

To make it a full counter instrument the input section is kind of still missing, but it's kind of OK to have this on a second board. It might be interesting for someone with good RF knowledge to design such an input stage, as the digital part looks really promising.

[David Hess]

An high impedance input that can be used inline with a standard 1 M scope input is kind of difficult, as there is usually one very little spare capacitance for compensation.

I meant inline of a low impedance transmission line like with a BNC-T adapter.

[Kleinstein]

An high impedance input that can be used inline with a standard 1 M scope input is kind of difficult, as there is usually one very little spare capacitance for compensation.

I meant inline of a low impedance transmission line like with a BNC-T adapter.

Adding a second amplifier in parallel to the scope input would cause more trouble than good - at least it would be very tricky to make it super high impedance (e.g. < 5 pF) and not causing high frequency distortion. It might work if you accept something like reducing the overall input impedance to lets say half. So a 1:10 probe would change to a 1:20 probe.  Still imperfections of the counters input would limit the useful BW for the scope.

[David Hess]

An high impedance input that can be used inline with a standard 1 M scope input is kind of difficult, as there is usually one very little spare capacitance for compensation.

I meant inline of a low impedance transmission line like with a BNC-T adapter.

Adding a second amplifier in parallel to the scope input would cause more trouble than good - at least it would be very tricky to make it super high impedance (e.g. < 5 pF) and not causing high frequency distortion. It might work if you accept something like reducing the overall input impedance to lets say half. So a 1:10 probe would change to a 1:20 probe.  Still imperfections of the counters input would limit the useful BW for the scope.

It is not usually done to oscilloscope input and it is no different than using a high impedance oscilloscope input on a 50 ohm transmission line to do the same thing.  Distortion at higher frequencies is less of a concern because high impedance inputs would not be used anyway where that matters.  For example most high impedance counter inputs only operate to about 100 MHz and it is rare for them to operate at 200 MHz and higher.

There was a good example in another recent discussion about using chained high impedance inputs along a 50 ohm transmission line for distributing a frequency reference.  Attaching a high impedance counter or oscilloscope input is the same thing.

There is a way to do what you describe though to a low impedance transmission line or at least there used to be.  BNC-T and GR-874-T to probe tip adapters used to be available which allowed attaching a high or low impedance probe inline to a transmission line.  The attenuation of the probe lowered the input capacitance and the construction minimized the stub length.  Essentially it was a way to turn an oscilloscope probe into a signal pick-off although it would also be useful in place of a terminated probe tip adapter for probe calibration.  Tektronix even made these for their active probes.  Feedthrough sampling heads can be built this way as well.

[SiliconWizard]

You might want to consider spending your time making a front end to one of these:

http://www.ebay.com/itm/0-1MHz-2400MHz-Frequency-Counter-Tester-Measurement-For-Ham-Radio-/171964071838?hash=item2809db7f9e:g:io0AAOSw~bFWGIMq

I have one, works well for what it is.  Quality is very good.

I also bought one of those with an integrated tcxo. It compared very favorably with a lab frequency counter worth over $600.
Of course it doesn't have a sophisticated input stage and is not particularly well protected, but you can't beat the price.

[David Hess]

I also bought one of those with an integrated tcxo. It compared very favorably with a lab frequency counter worth over $600.
Of course it doesn't have a sophisticated input stage and is not particularly well protected, but you can't beat the price.

When you get up to 12 digits or low increments of time, the analog and digital circuits before the counter can bias to the readings is subtle ways which require attention in design.  Frequency stability of the reference is not sufficient by itself.

[SiliconWizard]

I also bought one of those with an integrated tcxo. It compared very favorably with a lab frequency counter worth over $600.
Of course it doesn't have a sophisticated input stage and is not particularly well protected, but you can't beat the price.

When you get up to 12 digits or low increments of time, the analog and digital circuits before the counter can bias to the readings is subtle ways which require attention in design.  Frequency stability of the reference is not sufficient by itself.

That's for sure. We were basically answering the author who wanted to build a "simple cheap microcontroller frequency counter circuit".
And unless it's meant as a design exercise, he'd be hard-pressed to do better than this and if he did, that would cost a lot more.