Hi,
A while ago I designed two similar in topology and rather simple devices (for learning and PCB design improvement purposes). One of those was the notorious Jim Williams pulse generator, and the other one was a time domain reflectometer used for measuring an unknown cable`s length. Both devices are based on, or at least heavily rely on the transistor avalanche breakdown, so I hope I can put both of those devices in one thread... Neither of them work, but now I have a bit more time to dive deeper. I guess I dont have to make long presentations here. The Jim Williams pulse generator is supposed to generate an extremely fast (on the order of 100s of picoseconds) rise/fall times voltage spikes. The time domain reflectometer should also generate very fast pulses just with a larger amplitude.
Jim Williams generator was based on: https://www.analog.com/media/en/technical-documentation/application-notes/an98f.pdf
Time domain reflectometer was based on:
The problem is that when I connect either of these devices to my scope and power them on (a simple pushbutton works as a switch providing power to the device from a CR2477 3.3V battery, both devices are made the same in terms of power supply), literally nothing happens. I don't see any pulses, or at least the origins of what should be a pulse, not a single mV above the noise floor. Absolutely nothing. It has to be mentioned, that my scope is Siglent sds1204x-e. I agree that 200 Mhz scope is far too slow to capture picosecond pulses, but let's be honest, if there was at least some kind of reaction from the board, I would capture something, heavily attenuated and distorted, but still something. I don't have any designators on the PCB as it was useless, granted I wanted a white solder mask, but I guess my board design isnt making any obstacles...Do I really really really need several Ghz scope to see something? Or is there a major flaw in my design somewhere? Thank you in advance.
A while ago I designed two similar in topology and rather simple devices (for learning and PCB design improvement purposes). One of those was the notorious Jim Williams pulse generator, and the other one was a time domain reflectometer used for measuring an unknown cable`s length. Both devices are based on, or at least heavily rely on the transistor avalanche breakdown, so I hope I can put both of those devices in one thread... Neither of them work, but now I have a bit more time to dive deeper. I guess I dont have to make long presentations here. The Jim Williams pulse generator is supposed to generate an extremely fast (on the order of 100s of picoseconds) rise/fall times voltage spikes. The time domain reflectometer should also generate very fast pulses just with a larger amplitude.
Jim Williams generator was based on: https://www.analog.com/media/en/technical-documentation/application-notes/an98f.pdf
Time domain reflectometer was based on:
The problem is that when I connect either of these devices to my scope and power them on (a simple pushbutton works as a switch providing power to the device from a CR2477 3.3V battery, both devices are made the same in terms of power supply), literally nothing happens. I don't see any pulses, or at least the origins of what should be a pulse, not a single mV above the noise floor. Absolutely nothing. It has to be mentioned, that my scope is Siglent sds1204x-e. I agree that 200 Mhz scope is far too slow to capture picosecond pulses, but let's be honest, if there was at least some kind of reaction from the board, I would capture something, heavily attenuated and distorted, but still something. I don't have any designators on the PCB as it was useless, granted I wanted a white solder mask, but I guess my board design isnt making any obstacles...Do I really really really need several Ghz scope to see something? Or is there a major flaw in my design somewhere? Thank you in advance.
