Noise presence at a 433Mhz receiver

Question

I'm playing around with these 433Mhz transmitter and receiver modules in combination with and Arduino Uno.

I first soldered a 17cm long helical antenna to both modules. After I while I noticed that a lot of noise is generated while both modules are working. I've identified two reasons for this:

1. The antennas themselves: If I removed the antennas from the modules, there was considerably less noise.
2. The Uno was connected to the USB port of the PC (I wanted to see the output in the serial console). The USB port of the computer seemed to also have an impact on the quantity of the noise.

When I removed both antennas and powered the Uno via an external power supply, no noise was present. Since I still wanted to see the output of the receiver I connected the TX and GND pin of the Uno to the PC via an FTDI cable.

My question is: Why was noise present when the Uno was connected to the PC via an A-B USB cable, but was not present when I connected the TX and GND pins of the Uno to the PC via a FTDI cable?

Answer 1

The receiver (on the left) is just an RF amplifer followed by a simple demodulator followed by a data slicer or data comparator. They do not recognize data and they do not understand received signal level - they will max-out the RF amplifier until the output signal (noise when there is no potentially valid data transmission being received) is quite high and, when a carrier is present somewhere within the pass-band, its fairly large amplitude will cause the RF gain to drop and the output will take a more conventional shape and look like data.

If you are not transmitting data from its counterpart transmitter then expect purely noise unless of course you remove the antenna and severely limit the ability of noise or a potentially valid transmission to be received.

As for potential interference from your connected equipment yes, this can be a problem - try transmitting data and see what happens to the output.

The next question I guess you might ask is "if there's a bunch of noise outputted when nothing is being transmitted then how can I tell it isn't data?"