Some measurements below…. Still no real solution for this problem (EDIT: see update at bottom of this post).
Equipment used: Taranis + Turnigy 9X transmitters, 2x RF explorers, 2x Deluxe OLRS 1W transmitters (box, not module). Screenshots and measurements are done with one taranis+deluxe+rf explorer combo, but similar noise visible on second pack (some measurements on second pack here and here).
TX itself is silent enough, without external OLRS module – no noise (except expected hopping in 2.4G band), all other bands (1-1.3GHz and so on) are clean.
Noise is visible in flying – turning on OLRS transmitter cuts max distance with 2.4G video in half.
So, simple tests in living room – RF explorer with stock Nagoya antenna (collapsed), 2360-2540MHz range, iterations=10, Average and Max Peak marked. Samples for “average” ar collected for at least 10 seconds. TX with Deluxe OLRS + Diamond SRH-771 is 60cm away, all antennas are vertical. OLRS setup:
TX not turned on, noise floor in living room (some wifi AP’s around…):
So, noise floor around -103dBm, with some peaks (probably WiFi). TX turned on, no mods yet:
Noise floor (average) shifts up by few dB, but there are peaks all over ISM band (and around it, too).
Signal/power (PPM/GND/VCC) wires to module are twisted over ferrite ring (4 turns) and made as short as possible. Not much difference:
Deluxe disassembled, filter on output replaced with genuine LFCN-490 (maybe earlier one was fine too, but just to be sure), deluxe case grounded (scratched silkscreen, soldered aditional wire, mounted it to case). Slight difference, but again – not much, as most of noise is coming from antenna:
Antenna was mounted via short pigtail, to keep it angled compared to TX. This time mounted it directly to SMA from Deluxe. Again, no difference for average:
So far, no solution found. Let’s try workarounds: reduced hopping channel count on OLRS:
Single channel setup should give clear picture what’s happening…
So, measurements with all earlier mods (antenna again on pigtail) but with single channel:
Clearly an improvement over 12ch hopping setup! But 1ch is not reliable, let’s try 2ch hopping setup (one more channel added, everything else is the same, frequencies shifted slightly):
Probably something usable?
Compared to 12ch setup it’s def. cleaner, but still noisy.
Things to do:
- add second LFCN-490 immediately after first one (not so easy task to do on deluxe, no side-strips for GND and short track until SMA, this mod will require SMA unsoldering, shortening middle pin, re-soldering again)
- 3 and 4 hopping channels setup
Cleaning up noise would be solution, limiting to few channels – poor workaround….
Update: dual filter mod looks much better:
(heat up SMA, pull it up (probably unsolder stock filter first, to be safe), pull up signal track from PCB, solder both filterst together, solder SMA back, add wires to keep second filter grounded)
Looks much cleaner now, 2ch hopping mode:
12ch (compare to other screens above), without PCB-ground mod:
And gif which shows RF explorer output – TX off, TX on (12ch hopping), TX off. Looks much better to me…
So… Practical tests ahead, but in short – single LFCN-490 does not filter good enough. Mini-circuits filter costs ~4$ shipped, but this would be “one-way action” – if you want to remove it, you would need to wire missing PCB part…
So, probably better to just get low pass filter with SMA connector, like this one (they are available cheaper, i just put random link from google search) and do not trust sellers who tell that “you do not need one” ;-)