Jingling key test and noise

Vibrating keys causing Doppler modulation in the near field if the Tx antenna. Are the receivers FM?

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs

Perhaps there is a microphonic capacitor in the Tx. Many ceramic capacitors are microphonic.

Leon

Well...I just don't understand why it didn't happen at VHF radios. Maybe it's frequency dependent? Or perhaps I really should check types of capacitors used in VCO. Since VHF radios are doing fine, they should be taken as a reference in the process of finding the root cause on UHF radios. But there are too many variables. Still do not know which particular area that had potentially caused the problem.

It has nothing to do with the radio being UHF or VHF, and the keys are not interfering with the RF signal, nor are they generating significant EMI.

The analog wireless microphones employ varying degrees and quality of audio compression. The ultrasonics from the keys confuse the circuit / algorithm. This works for some bells and chimes too.

This is known as the "key test" among wireless microphone manufacturers.

Frank Raffaeli

So why does it happen with the mic unplugged?

Betcha the radios are microphonic.

Sit one UHF radio on top of the other, disconnect the microphones, turn the volume up all the way, and enable the transmitter on one. They will probably howl and sing to each other nicely.

It can be observed with two radios, or when one radio sings to itself. Speaker vibration - modulates Rx LO VCO - amplified by audio amp - back to speaker - etc... yuck.

Try tuning one at max volume to a CW carrier. It might not be as quiet as you think.

UHF radios are more susceptible because the RF frequency is higher.

Less mechanical vibration required to induce the required delta-C into the Tx VCO circuit. If the VCOs are built on fiberglass FR-4, it's hard to get them mechanically stable enough.

Higher end radios will usually use a ceramic hybrid VCO to gain higher performance and better mechanical stability.

Your explanation does not explain why he saw the effect with the microphone unplugged.

Another UHF radio with different design did not show any problem as mentioned. 'Good' UHF radio uses resonator at VCO tank circuit while the 'bad' one uses large High Q coils. VHF radio uses chip inductor at VCO tank and all VCO circuits are shielded. Any chance of coil's magnetic field interaction with shield and RF caused problem? I'm totally bewildered. It's good if RF is visible to our eyes so that I could see what actually had happened between the 'good' and 'bad' radio. How do we make sure the VCO is mechanically stable? More shielding? I have no idea of any solution since I do not know the root cause yet.

We use the "key test" to show that our current wireless microphone systems don't employ an audio compandor while all others do. However, I think what is being described here has nothing to do with this, since the mic is not hooked up. I would guess that the keys are reflecting/affecting the RF from the transmitter, then the transmitter case is picking it up, which then affects the transmitted signal.

Karl Winkler Lectrosonics, Inc.

I'm convinced this is related to ultrasonics, not RF interference. Jingling keys are LOUD at high audio frequencies.

Your "high Q coil" resonant element in the transmitter tank circuit may be picking up ultrasonics directly, or through the PC board.

Try this test: Enclose the keys in foam padding, while letting them free to jingle. The padding will absorb some of the ultrasonics, without changing the RF features significantly.

If the coil is picking up the ultrasonics directly, the transmitter may be sending out-of-band emissions due to the high modulation frequency, causing distortion at the receiver after band-limiting of the channel.

Frank Raffaeli