RF interferences?

[Pierre]

I have seen some posts about RFI and EMI behaviour of ZAP Pulse and Ucd modules here. Correct me if I am wrong, but have only found graphs or data that show the emitted spectrum when the module is off and on but with no signal.
I would like to know if anyone (preferably the designers) has tested any of these modules with a dummy load and sine input, with a commercial FM receiver near (several cm or even a couple of meters). Does the tuner receive any audible perturbation as the level is increased up to full power?

Thanks!

 

[Tekko]

No, class D amps work around 200-400KHz while radio is 1000+KHz at lowest (am) and fm about 88MHz as lowest. The waveform in a class D amp is similar to fm but squarewave instead on sinewave so a radio wont detect anything.

 

[Pierre]

Yes, the main fundamental of switching frequency is far lower than the (around) 100MHz FM band, but armonics and parasitics oscillations can be significant at tens of MHz and can (and DO) harm reception.
Some of my older prototypes interferred with FM, indeed.

 

[Pafi]

Sorry, Tekko, you are wrong, Class D amps can interfere with both AM, and FM broadcast signals. Every harmonics get (amplitude) modulated by signal (quite distorted way), and self-oscillating amps changes even its freq, so every stations can be disturbed on and around n*f0 frequency. At very high freq the square wave's spectral density is very low, but here a very short wire can act as antenne, so radiation become relatively efficient. Even if there was no PWM signal over 88 MHz, the receiver can be disturbed on 10,7MHz (intercarrier, or what is the name of it).

So correct EMI compatibility requires attention, it is not automatical.

One won't hear any noticable sound of original music on FM radio, only ugly noise, and not on every stations.

Pierre!

Unfortunately I don't have experience on these modules, but UcD intended to produce extremely low radiation, so in a metal box it can be perfect. But wait for Bruno, he will tell! (You can ask this question on the specific topics too.)

 

[HDTVman]

The square wave switching of this type of device will generate signals at several times the fundumental frequency and some of them can be very powerful signals. Try using a Spectrum Analyzer or scope with FFT function to test for harmonic content. This can be very educational.

Keep in mind that AM and FM broadcast are not the only radio frequencies used and interferance with other services is possible and can be costly if the FCC or the governing body in you location gets involved because of interference.

The key here is shielding and low pass filtering of the inputs (power and signal) and output. Don't forget about by-pass caps and careful circuit design. Multilayer circuit boards with the signal layers inside and power and ground layers on the outside can help a lot too.

 

[Pierre]

My old prototypes tended to interfere FM at high power levels, not at idle. But I am talking about the bare amplifiers, because once I enclosed them in a metal rack case, there was no problem.
That's why I would like to know if Hypex and LCAudio modules produce the same problems when not shielded by the enclosure.
I think both of them are good enough and well proven in a wide variety of situations, so if they DO harm reception, then I am happier because the problem is not so worrying

I would like to hear Bruno and Lars opinion experience on this...

 

[Pierre]

No responses?
I would love to hear from the manufacturers or users of both great modules, only to see if they receive interference at FM band when the module is ON, only for my peace of mind ;-)

 

[Jocko Homo]

I can assure that they will all have some energy in the FM band. And low VHF TV. Some more so than others...............

I believe that J-P has posted some results for one of his creations, as well as a competitor. The competitor was much worse.

Jocko

 

[Pierre]

I am not very familiar with RFI measurements, allowable levels, and what levels are harmful or annoying depending on the band, etc.

That's why I asked for practical experiences from users or the designers in usual situations, like having the amplifier near a FM tuner, etc, with these popular modules, mainly ZapPulse and UcD.

Thanks anyway!

 

[Jocko Homo]

If you don't have fancy test equipment.........or you just are not in the mood to put up with the noise from all the fans in them...............you can do what I do:

Get a really cheap TV (or tuner), hook up a whip antenna, and put it near the stuff you want to measure. You won't get a calibrated result, but if you are less than 1 m or so away, and it shows up as interference, you can bet it won't pass.

If it does radiate, you will then have to futz with what the source is. DC supply cables, speaker leads are the most likely. Anything close to the output coil can act as an antenna. AC mains could be a source, especially if a SMPS is used.

Jocko

 

[HDTVman]

Jocko's advice is very good here. A FM receiver is not a very good device to test for RFI because it's not very sensitive to the noise that would come the amp you are designing. A TV would be a much better choice. Try looking at several channels also. Try a Short Wave Radio. That would be a good detector for the kind of noise that would be generated in your amp.

BZ

 

[Lars Clausen]

Most commercially available Class D amplifiers are CE or UL approved. (Which goes also for the two models mentioned earlier in this thread). That should give you some assurance, that interference level is low enough to not cause any interference problems, as long the directions from the manufacturer are observed.