Hello and thank you both very much for taking the time and trouble to respond.
UPDATE:-
Since writing my original post, I have done further testing and have stripped my project right back until I was left with the transmitter module and power supply (3A DC-DC Buck Converter @ 4V).
I then took the transmitter module to one side, powered it up from the Buck Converter and with no audio connected, I put out a blank carrier and that annoying 200 - 250Hz buzzing was present and still as strong (showing around -50dB on the SDR I was using to monitor).
I also noticed lots of similar peaks spaced equally apart moving up in frequency which decreased in strength the higher up in frequency they appeared. It looked like and probably is some sort of harmonic interference....?
Further Troubleshooting Steps & Results:-
- I have been using a rubber duck style antenna since day one but to see if that was anything to do with the problem such as the RF being too close, I connected the transmitter to an externally mounted antenna but the result was exactly the same.
- I next considered that the Buck Converters might be being affected somehow so I powered the module up from a good known working adjustable bench power supply capable of delivering up to 35V and 5A. Interestingly, when I put the transmitter into transmit, it transmitted briefly (1/2 - 1 sec and then cut out) - So I'm not sure what to make of that other than the RF got into the power cables and overloaded the Transmitter Module.
Tests Yet to be Carried Out:-
So now I've narrowed things down, this is pretty much all I can think of that I can try now.
Any other test suggestions would be welcomed because I'm rapidly running out of ideas now.
I have tried two modules so far and both do the same thing so unless I've damaged them both in the same way somehow, the problem isn't with the transmitter module itself but most likely my design(s) implementing it.
These are the things I've got left to try...
- To rule out all these converters and power supplies, I am going to source a 3.7V battery pack and see what happens when I power it directly without any other electronics in the equation.
- There is no filtering between the Antenna Output on the Module and the SMA socket. With there not being too much information available on these transmitter modules, I only had the design of others to reference. The datasheets are not worth mentioning they are so poor. Many of the designs I looked at which use these Transmitters do not have any Low Pass Filtering on the RF Out so I didn't bother either. I didn't see the point because I wasn't getting any issues until now. But in light of this, I've ordered a couple LCFN-160+ SMD Low Pass Filter Packages to try and see if that stops it
Transmitter Breakout Board Layout & Schematic + Power Supply DC-DC Converter:-
So this is what I'm working with and where the problem lies - somewhere.
Transmitter Module Breakout Schematic
Transmitter Module Breakout Board Layout
The board is made from double sided FR4 with top and bottom ground planes and some via stitching tying the top to the bottom. Any unused header pins that are not connected to the transmitter module have also been tied to the ground planes to try and form an RF sheild - of sorts.
No SMA edge mount connector is shown in the diagrams because it was a last minute addition. Originally I was using a very short fly lead soldered directly to the module with an SMA on the other end but I discovered later that I could just get an SMA connector to fit snugly without shorting anything.
SUN-D 3A Dc-Dc Buck Converter
And finally, this is the DC-DC buck converter I am using. I power it from a 12V SLA battery and step it down to 3.7 - 4V for the transmitter and other modules. It uses the MP1584EN which was the lowest consumption converter I could find. My project is battery operated so low power is a must and these converters draw very little with my project.
Thanks again!