Frequency Counter and Crystal Tester (Part II)

RE: Frequency Counter and Crystal Tester (Part II)

D_Hersey — Thu, 28 Apr 2016 00:09:47 GMT

[mention:fb1238db64d142ca970213a1c696a43d:e9ed411860ed4f2ba0265705b8793d05] , IIRC was trying to explain the difference between comparators and op-amps to a relative noob.

https://www.fairchildsemi.com/application-notes/an/an-118.pdf

shows how to construct oscillators with an odd number >1 inverters.

If you took a quad of op-amps, perhaps using the fourth as a buffer, you could make this oscillator and see how fast it runs. Then you could pop the chip and replace it with a quad comparator.

Just sayin'

RE: Frequency Counter and Crystal Tester (Part II)

mcb1 — Tue, 26 Apr 2016 04:11:25 GMT

Nice work John.

These make a very interesting series on cheap but useful test equipment.

Mark

RE: Frequency Counter and Crystal Tester (Part II)

D_Hersey — Sun, 24 Apr 2016 06:22:49 GMT

Typically they used a special, orange, thumbnail-sized 1uF cap there, IIRC it was a mylar-film with a very high voltage rating, like 1/4KV, on the input stage.

For LF work, at some point, ya' wanna switch over to period counting.

You should use a squarer circuit, maybe, on the input side. This is a Schmidt-trigger made from discretes. One way might be to follow with another inverter stage, say one with a (gasp!) P-chan. If you get about a 2M (hysteresis) resistor and hook it up between the output and the input of the primary stage, you will get positive feedback, which will square-up your input signal. This (discrete Q Schmidt-trigger) is a question that can be googled.

RE: Frequency Counter and Crystal Tester (Part II)

DAB — Sat, 23 Apr 2016 19:02:06 GMT

Great post John.

You can probably play around with the capacitor on the input to see if it gets down to lower frequencies.

You could go with DC coupling, but there are risks there as well.

DAB

RE: Frequency Counter and Crystal Tester (Part II)

D_Hersey — Sat, 23 Apr 2016 13:25:21 GMT

Truth, John, can be eternal.

What's All This V-F Converter Stuff, Anyhow (Part 1) | Analog content from Electronic Design

RE: Frequency Counter and Crystal Tester (Part II)

jw0752 — Sat, 23 Apr 2016 05:01:04 GMT

Big break through tonight. I discovered that the cause of the failure to zero was coming from interference from the Crystal Oscillator when a crystal isn't installed. On a chance idea I put a 1 nF capacitor in the empty crystal socket. The counter immediately set to zero. I then ran limit tests as I had before. Where my lower limit before was 500 Hz I was now able to read all the way down to 1 Hz with only a 100 mV PP square wave signal level as an input. Here is a picture of the unit reading 1 Hz at 100 mV. Now I feel better about the out come of the build.

[View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-523f33b0-5eb2-4d61-9311-5f28ab16c78c/6082.contentimage_5F00_181215.jpg:620:465]

RE: Frequency Counter and Crystal Tester (Part II)

D_Hersey — Sat, 23 Apr 2016 02:35:12 GMT

These were big when the PIC hit.

http://www.vhfcomm.co.uk/Counter.pdf