Improving receiver performance for Pi-Star MMDVM hotspots

 Since on the subject of my Pi-Star MMDVM hotspots, here is a tip for improving  the receiver performance if you running multiple hotspots of even a single hotspot in a high RF  environment.

Remember these MMDVM hotspots and even the other hotspots like OpenSpot have a simple wide-band receiver which can be easily overloaded, one thing I noticed when running my MMDVM in P25 mode was that  I was getting a lot of artifacts on my transmissions from my Motorola handheld when I operated in the same room or nearby rooms to my hotspot, it was much better when I operated from outside my house or other end of the house. First I thought don't use the little attached external antenna, but even when using just the onboard cct board antenna I could still overload the receiver up close  and I also noticed that when my DMR hotspot was transmitting I was getting errors on P25 receiver, even though the two hotspots operate almost 20MHz apart, I had all my hotpots running off the same USB power adaptor and I could get different behaviour by moving them around.

I decided to try some ferrite core clipons on the USB cables,  this was because I realised I could hear the USB power adapter noise getting into my HF receiver and it varied as I moved the hotspot position around, so I added some big clip-on ferrite cores and wrapped the USB cable through the hole a few times and experimented for best and minimum  effect on HF, I cleaned up all the noise. I was not quite sure who was the main culprit, so I added the multi-turned ferrite end of the cable closest to the hotspot. 

Later and purely by accident,  I was playing on P25 and noticed I not getting the errors I was previously getting, as normally I could not run my Motorola handheld in close proximity without getting artifacts on my transmissions, I also noticed the DMR hotspot now not de-sensing the P25 hotspot. This proved the hotspot receiver was picking up RF via the USB power connector, from this I made sure all my hotspots had a either toroid or big chunky ferrite clipon (14mm ID) with several turns of USB cable and fitted as close to the hotspot power connector as possible,  Now I can operate ether either DMR handheld or P25 handheld in reasonably close proximity to the hotspot without interfering (desensing)  with the reception of opposing hotspots. I also added smaller 3.5mm clipon ferrites at the power adaptor end of the USB cables.

It was a win-win situation, I reduced noise affecting my HF receiver radiating from hotspots and USB power adaptor and improvement of receiver performance of my hotspots by reducing receiver desense from other hotspots RF entering via USB power cables and localised RF from handheld transceivers. In terms of the MMDVM boards, if operating in a high RF environment, you are better not to use the external antenna, just use the on-board cct antenna. Only use the external antenna if operating out on low noise floor environment like a farm. Also if you running multiple hotspots, try to separate them from each other, the further the better, remember even with mW transmissions, the signals are still flattening each other's receiver front end at close proximity.