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Transistor switching RF noise?

Latch
On one of my prototypes I have a uC sending out a continually pulsing signal, just letting other peripherals know it is still there. The signal is generated using 2 transistors switching. (One on 2 different lines). The transmissions are equal and opposite to each other.
 
The issue is that the pulse signal can be heard on an AM radio. My circuit is throwing off intense RF just from the 2 transistors. How can I suppress this effect?
 
Latch
  • 0

    You have to get rid off the sharp edges of the pulses. The wires  between the devices will act as a transmitting antenna.. Just place an inductor that has a high impedance at radio-frequencies  and a low impedance at your pulse frequency in series  as close to the signal source as possible. There are inductors that have the shape of a resistor. Sometimes a resistor will also do the job, Especially when the "antenna" has a very low impedance at radio frequencies and the impedance of the pulse receiver is high. (for instance a C-mos gate) A 100 Ohm resistor will not affect the pulse, but will decrease the interference. Higher resistor values will make the receiver more sensitieve for noise, but in some cases you can use 100 k-Ohm resistors.  If your pulse receiver has a low impedance, like the LED in an opto coupler, it will help to use a twisted pair of wires for the signal and the return. The return circuit not connected to ground at the receiver side.

  • 0 in reply to jvdberg@ieee.org

    Johan,

     

    Thanks for the ideas.

     

    I first placed a 100pf capacitor between the base-collector to get rid of RF interference at the amp. Another was placed across the base-emitter to round off the rise and fall times. It helped, but I still heard the interference.

     

    I places the transistor pair close to where I think the issue may be. It definitely helped. I can faintly hear the sound.

     

    I think I may have to adjust the positions of all the components on the PCB to change the amount of RF noise it produces. I may even switch to surface mount.

     

    Latch.

  • 0 in reply to Latch

    The use of only capacitors does not help in most cases. Sometimes a capacitor with a resistor in series does help. These devices are used in devices like vacuum cleaners. The function of the resistor is to dissipate the RF power. A capacitor alone will reflect the RF power. Capacitors sometimes make things worse, becaurse they make the switching device to draw more current at the switching moment, causing more interference. The use of a series resistor or inductor does help in most cases. Only in worst cases, filters made of inductors and capacitors must be used.

  • 0 in reply to jvdberg@ieee.org

    Johan,

     

    Since the post, I moved all components closer to each other  and switch to surface mount transistors. I effectively removed any  antenna like traces.

     

    I added you snubber circuit to the transistors for good measure. (resistor-capacitor in series.)

    I was considering a voltage suppression diode, but this will work.

     

    Now I have to move the circuit right next to a radio to even hear a faint sound.

     

    Thanks for the help.

     

    Latch