Homebrew frequency counter - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

MrBungle · 13th Dec 2016, 7:48 am

It is spot on that. I think the input impedance is quite low and at higher frequencies it is dragging the input low enough not to trigger it. I've designed a constant 10M impedance follower and Schmitt trigger that is good to 120MHz with 40mV input in LTSpice so will attempt to construct that at some point.

Motivation is dwindling though

Terry_VK5TM · 15th Dec 2016, 3:12 am

Or use a FET input stage before the bipolar stage to up the input impedance?

Know what you mean re the motivation, mine keeps sliding and I have to give it a good kick to get it working again

Terry

MrBungle · 15th Dec 2016, 10:11 am

That's roughly what I've sketched out. Got a FET follower with a back to back BJT clamp in front of it, a BJT follower, coupling cap, high gain amp to cause saturation at polarity change in the cap, then another follower. Schmitt trigger is gone; couldn't get it to work consistently. Works in LTspice so far. Hopefully reality will be close. Pushed 205MHz out of it at 100mV which isn't too bad with some careful substitution of VHF transistors.

Kicked my motivation a bit finally. Sleep and coffee appears to help.

13th Dec 2016, 7:48 am	#21
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Homebrew frequency counter It is spot on that. I think the input impedance is quite low and at higher frequencies it is dragging the input low enough not to trigger it. I've designed a constant 10M impedance follower and Schmitt trigger that is good to 120MHz with 40mV input in LTSpice so will attempt to construct that at some point. Motivation is dwindling though

15th Dec 2016, 3:12 am	#22
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	Re: Homebrew frequency counter Or use a FET input stage before the bipolar stage to up the input impedance? Know what you mean re the motivation, mine keeps sliding and I have to give it a good kick to get it working again Terry

15th Dec 2016, 10:11 am	#23
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Homebrew frequency counter That's roughly what I've sketched out. Got a FET follower with a back to back BJT clamp in front of it, a BJT follower, coupling cap, high gain amp to cause saturation at polarity change in the cap, then another follower. Schmitt trigger is gone; couldn't get it to work consistently. Works in LTspice so far. Hopefully reality will be close. Pushed 205MHz out of it at 100mV which isn't too bad with some careful substitution of VHF transistors. Kicked my motivation a bit finally. Sleep and coffee appears to help.