swharden
commented
4 years ago hi @freemo, thank you for your excellent advice!
Original Project
You are referring to: https://github.com/swharden/AVR-projects/tree/master/ATMega328%202016-09-04%20SN74LV8154
The solution itself is simple, you mentioned you were using 50 Ohm feed lines. That means you need to also add a 50 Ohm resistor from pin 1 to ground and from pin 7 to ground on your sn74lv8154 chip.
Indeed, properly terminating coax will prevent reflection of the signal. \ I'll add a note to that page to make this clear, thanks for pointing it out!
Newer Project
Note that the referecned project spawned its own repo: https://github.com/swharden/USB-Counter
... and reviewing that schematic it looks like the line terminated like you suggest:
freemo
Re: ATMega328 2016-09-04 SN74LV8154
Hi, EE here and took a quick glance at your project and it was immediately obvious why you struggled with signals above 50 MHz, and its an easy fix so i wanted to explain it here.
The solution itself is simple, you mentioned you were using 50 Ohm feed lines. That means you need to also add a 50 Ohm resistor from pin 1 to ground and from pin 7 to ground on your sn74lv8154 chip. Problem solved.
So I want to explain why though just so you can understand this. This will allow your load (the frequency counter) to present as a 50 Ohm impedance, this will mean your load will match your feedline impedance and hopefully your signal generator as well (Which should be a 50 Ohm source, which it likely is). The way you have it designed here your frequency counter presents as a high impedance load, and is thus drastically mismatched with the rest of the system. This means any signal that come in will not actually get into the chip and instead will be reflected back at the source. In turn this will mean the signal will appear very weak and wont trigger a high-low state. The effect of this gets worse with longer feedlines and with higher frequencies, this is why it worked earlier before you moved it to the case.
If you need me to clarify this further just reply with any questions, happy to help explain it.