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Saturated input in an RF receiver?

Saturated input in an RF receiver?

Saturated input in an RF receiver?

I'm using a Linx single chip receiver to control an electric motor.  The motor puts out a lot of electrical noise.  Shielding it helps considerably.  I'm wondering what might be going on in the receiver when it cuts out.  It acts like it might be going into saturation somewhere. The motor will run for awhile, the receiver will cut out (motor stops), the motor will start up again and will quickly cut out again.  Once it starts it seems to occur quickly again.  Looks to me like something is saturating.  The front end of the receiver has a SAW bandpass filter followed by a low noise amplifier.  Any opinions on what might be going on?

RE: Saturated input in an RF receiver?

Would be helpful to know which Linx you were using as that would provide info on frequency and modulation.

Noise can affect your receiver by two means: 1)Directly at the RF input frequency, or 2)Through connecting wires or directly coupling via electric or magnetic field directly onto the receiver circuits at about any frequency.

1) Noise at the RF input frequency
The SAW input filter should do a good job of filtering incoming RF to the frequency band used. This reduces the possibility of the antenna picking up out-of-band signals that might swamp the receiver input.

The lower cost wireless use ASK and OOK modulation which are the most sensitive to noise at the input frequency. FM, FHSS, or DSSS modulation wireless devices are much more immune.

AC Motor noise is most likely 60 Hz power line and harmonics of 60 Hz. It is unlikely that power line harmonics are reaching up to the receive input frequency. However, if the motor is controlled by a speed controller there can be significant spurious noise up the spectrum, especially if the receiver is close to the motor.

2) Noise directly coupling into the receiver
Near the motor, there will be strong magnetic and electric fields. These fields can couple directly to the receiver IF, VCO, or demodulated output in strength to sufficiently jam the receiver. These external fields can also couple to wires you have connected to the receiver, and from there into the receiver.
A) Shield the receiver. You indicated you found some improvement from this. However, so close to the motor, an aluminum or copper shield will only block the E field. Try a ferrous metal shield (sheet steel) as well to block the possibility of magnetic coupling.
B) Filter wires connected to the receiver. Connect small bypass capacitors directly at the receiver between each wire and ground with very short leads. Try 100 pF on the more sensitive digital or analog wires, and 100 pf along with .01 uf and 1uf for the power wires. If the noise environment is really serious, you can add a series resistor of 10 to 100 ohms on digital and analog wires (depends on if the signal will tolerate the resistor).

RE: Saturated input in an RF receiver?

The Linx device is the ES series, 916 Mhz, FSK.  I think the noise is from the electric field produced by the motor.  The motor is a universal type.  The noise source is the brush/commutator sparking.  The motor powers a winch.  When lifting a load (high amps, low speed) the RF unit works well.  When lowering the load (low current, high speed) the receiver will cut out when the transmitter is far enough away. Cleaning the commutator with a commutator stone helps significantly but it is a short term fix.  A thick aluminum shield using feedthru capacitors helps considerably.  Unfortunately that is expensive (and it makes the air cooling for the motor difficult).  I was hoping maybe some combination of attenuation/filtering/amplifying between the antenna and the module might help.

I do have the receiver module's power supply terminals bypassed to ground with a .1 uF capacitor.  I'll try a parallel, smaller one as well as the resistors.  I'll also try them on the other terminals of the module.

The control with the receiver is close to the motor.  The control's enclosure is a thick aluminum casting.  I hadn't thought about the RF noise dirctly coupling to the module.  I'll try a shield around the module (cheaper than a shield around the motor).

Thanks for the ideas -

RE: Saturated input in an RF receiver?

I gather from your post that the RF module may be inside the control enclosure? If so, the receiver could be picking up noise directly from the control wiring to the motor. Shielding just the module will help.

Also, is the antenna outside the control enclosure? I deal with wireless devices, and it is surprising how often people install them inside a NEMA box and then wonder why it doesn't work well - it's such a simple thing to overlook. In such a situation, the receiver is trying to pick up the very weak RF control signal indirectly carried on the wires entering and exiting the box. The signal attenuation is so great that the control signal is at the level of all sorts of noise.

Finally, you can use ferrite beads instead of resistors. Resistors (if your control lines can handle it) have the advantage of providing constant impedance regardless of frequency where ferrite beads do not work well at the lower frequencies. This improves the filtering action of the capacitors.

RE: Saturated input in an RF receiver?

Yes, the module is inside our control and the antenna is outside the control.  I've tried ferite beads on the motor lead wires.  I hadn't thought about using them on the control lines coming into the control.  That would be an easy thing to try - Thanks.

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