Hi for all,
There was a topic about grounding, etc.
I also found some other topics about hiss, background noise in external DAC-s and other mystic problems and I think my problem is also related.
Roku M500 (EU/Asian) is connected to a Panasonic SA-XR55 (EU) via coax. Receiving acceptable FM signal with the receiver in tuner mode is not possible, the RFI is so high. There is a modulated noise over the whole FM range. Modulated noise pattern closely related to display activity.
0. Situation as above.
1. Coax disconnected both end, M500 operates in Internet Radio mode - no change, noise still exists
2. Roku PSU disconnect - perfect FM radio signal
3. PSU re-connect, M500 set to standby - noise changing from modulated to static (no display "activity" just time and date)
4. Web interface config - standby screen set to off - noise static, a bit lower but still exists.
I made further tests to judge how the RF noise goes into the receiver, trough the antenna wire and/or trough the 230V also. I disconnected my receiver from the wall plug and made run it from an UPS. No connection between the Roku and Pana units. RF noise is the same.
Test setup for reproducing the problem:
Roku M500 switching PSU is at floor level. M500 is on a high shelf, so the 9V supply wire is full lenght vertical. SA-XR55 with factory FM single wire antenna (not shielded coax for aerial) connected. FM antenna and supply wire are within 0,5 feet. No connection between units. Receiver is in tuner mode.
I am interested if anyone can reproduce the problem maybe with the SA-XR55, maybe with other receivers also.
IMHO source of problem:
The Roku M500 digital circuits and especially the display drive generates current spikes to the 9V supply. The current spike runs trough the long and unshielded supply wire and changes that wire to a FM transmitter antenna.
1. place far or use shielded cable for aerial. But I dont want aerial, I live close to the FM transmitter.
2. no standby - when not used, disconnected. Lost standby functionality (bye, nice clock).
3. Using different receiver - I tried an old Yamaha RX-V520 and behaved a lot better. Maybe its FM tuner has better AM rejection.
These are NOT solving the problem, the RF noise is still emitted from Roku supply wire.
IMHO re-engineering solutions (needs testing, but I dont want to loose garantee):
1. In the M500 a low-ESR 1000u/16V electrolytic puffer cap paralelled an FM-shunt ceramic capacitor connected directly to the display supply connectors. This way the puffer cap locally feeds the transient current need, so the spike runs out to the PSU wiring is reduced - transmitting power also reduced.
2. A small size, low series R, at least 2A inductor connected series to the caps so the digital circuits get the power trough it.
Analogue output stage needs a separate stab IC what get its feed from the other end of the inductor (directly from the PSU).
2a. If display drive has a specific base freq or the RF emission shows a freq peek than with one more inductor there can be a T-filter tuned to that freq.
3. There can be an RF shunt cap in the PSU also and a ferrite donut on the cable.
4. Several small digital units (like modems) use small PSU with long cabling but those PSU output is small voltage AC. The RF-shunt, rectifier and the puffer cap is in the modem housing. It might not be the good solution because of 50Hz pollution, but worth a try.