I am attempting to build the following amplifier
- The amps used are MAR-8A+'s from minicircuits
- C1, C2, C3, L1 - while these are available on the board they have not been placed
The filters are epcos saw filters with a Pass Band Frequency: 433MHz to 434.71MHz
I have designed and built the following PCB:
Upon building it the first few times I found the amps were blowing, I changed the resistor values and seem to have fixed this (schematic shows updated values).
Then I discovered it was oscillating, after fiddling with the physical power connections I found using pins and croc clips did not work and resorted to soldering wires on. This fixed the issue on the boards and subsequent ones up until now.
However recently they have started oscillating again at approx. 433MHz. First off it is just small oscillations, and then when the antenna is attached they multiply massively. I have noticed there is interference at 1.825 GHz and 945 MHz, this happens even when the board is not populated. As the board size is a quarter wavelength of the higher frequency I suspect this may have a lot to do with it.
I think I may also be needing the front end tuning that I have left out (due to not being able to make it accurate enough). Is there anything major I have missed? such as 50 ohm matching and termination?
Many Thanks, let me know if I have missed any important info and I will add it.