RF Receiver GND N.C

Beginners
#1

Background Information (for my question on the pins of the RF receiver)

I have an RF transmitter connected to an arduino uno and an RF receiver connected to another arduino uno. I am successfully sending strings of text from the arduino with the transmitter to the arduino with the receiver and everything is working as expected.

Two of the pins on the receiver (pin 2, and pin 3) are labelled on the pin description (image attached) as “GND1 (for RF only, please setup the pin on N.C when it did not used)”.

The receiver has two output pins, Pin 16 is digital output and Pin 15 is analog output. I am using Pin 16 (digital output).

  • My initial experimentation had both pin 2 and pin 3 connected to ground, and worked well.
  • I then did an experiment where I disconnected pin 2 from ground. Everything still worked.
  • I then reconnected pin 2 to ground and disconnected pin 3 from ground. This did not work.

I deduced (even though I haven’t tested the analog output) that pin 3 is the GND required for digital output and pin 2 is the GND required for analog output.

I have done a lot of google searching and cannot find much, if any information related to this receiver or any receiver in as far as N.C goes – mind you I am a beginner in circuits etc. so I may be naive with my search terms.

The question:
Can someone hazard a guess if by the pin description that they mean:
1) If you connect pin 2 and pin 3 to ground that damage will occur over the long term?
2) They are simply saying that only use the one you need? This is assuming of course, that my above assumption is correct about the purpose of these pins as I cannot find any documentation telling me about their relationship to digital and analog output.
3) Lastly, as well as only connecting the one required, do you think they also mean even if only one is connected to ground it should only be for short periods of time to prevent damage? If this is the case, then I assume some cycle is required to ground the pin every so often to check for data to receive - after all, the only way to receive anything is by listening.

#2

The module has two sides separated by a gap in the row of pins; on the left is radiofrequency (RF) and on the right is everything else; supply and signal. Pins 3 and 4 are supply feed for the radio.

  1. If you connect pin 2 and pin 3 to ground that damage will occur over the long term?

No.

Pins 1, 2 and 3 form the RF input of the module.

Pins 2 and 3 should be connected together on your PCB or protoboard.

Pins 2 and 3 should be connected to the outer conductors of your RF transmission line, which is the coax or PCB stripes between the antenna and the module. For increased sensitivity, this transmission line should be matched.

There should be no other connection between pins 2 and 3 and your digital and supply ground used on the demodulator side of the module. You’ll notice how they are really well separated.

Making a connection like you did won’t cause damage, but it is not how the module is designed to be used.

When you find your system won’t work without that connection, may mean your digital and supply ground has become part of the RF path, which means your antenna and transmission line is not properly matched.

NC means not connected. In the context of the table in the datasheet it means “do not connect”. In other words, if you don’t need an RF ground for your transmission line or antenna, don’t connect the pins to anything except each other and the radio supply ground.

Pin 3 is the negative return for the radio power supply.

  1. They are simply saying that only use the one you need? This is assuming of course, that my above assumption is correct about the purpose of these pins as I cannot find any documentation telling me about their relationship to digital and analog output.

There should be no connection between the RF side and the digital side of the module.

On the other hand, you might be able to combine the two power supplies into one, if sensitivity is not important.

The module datasheet presumes much knowledge of RF design.

  1. Lastly, as well as only connecting the one required, do you think they also mean even if only one is connected to ground it should only be for short periods of time to prevent damage? If this is the case, then I assume some cycle is required to ground the pin every so often to check for data to receive - after all, the only way to receive anything is by listening.

No.

Hope that helps!

#3

Once again, you make perfect sense. But I have a few follow up questions if I may.

Power Supply

  1. In other words, you are saying that I should have two separate 5v circuits? That the left hand side should have a separate power supply than the right hand side? If this is the case, then I assume I will need more than just the power supply coming from the arduino - or is it possible to segregate power from the arduino as such and have two independent 5v circuits?

Pin 2, Pin 3, and the Antenna
2) If I am using “Standard 22 AWG solid” hookup wire at the correct length for my antenna, then I assume you mean that Pin 2 and Pin 3 should connect to each other and to the ground of the power supply of that side of the circuit board. If using a COAX cable for an antenna, then as well as connecting Pin 2 and Pin 3 together and to the ground, they should also connect to the shielding of the COAX wire. Do you know if there is much to be gained by using COAX and grounding the shield instead of the using the hookup wire in the way I have chosen?

#4

Power supply

The module has two power inputs, one for the radio and one for the demodulator. These parts of the module induce different signals on the power supply, as they vary their current draw according to the phase of the signals being dealt with in the parts.

Yes, you can use the power supply from an Arduino, but for best sensitivity you should suppress the noise generated by the module parts.

That suppression then gives you three power supplies; one for the Arduino, one for the radio, and one for the demodulator.

Th suppression depends on the nature of the noise; which can be measured by oscilloscope.

The capacitance can be calculated using design practice that takes into account the voltage, current, and frequency of the noise. The radio will generate noise at some intermediate frequency related to the RF frequency of 433 MHz. The demodulator will generate noise at frequencies of the signal you have transmitted; e.g. 1 kHz. It may also generate noise when nothing is being received.

Usually a 0.1 microfarad capacitor will give adequate suppression, so give that a try. Place it between the RF GND and RF VCC pins.

By doing that, you have two supplies. Terminology becomes a bit of a quibble.

Pin 2, 3, and antenna

For a wire antenna, yes, as you say.

For a coax transmission line leading to an antenna, yes, as you say.

Coax can’t be used alone. You either attach an antenna, or coax then an antenna. The advantage of coax is to separate the radio from the antenna. You may not need to do this. My preference is to design the antenna as directly connected.

The antenna works best if it is separated from all other metal (apart from the RF ground) by at least the wavelength of the signal; 69cm. This is hard to achieve on a prototype, unless you use coax. It is impractical on protoboards, so you have inevitable loss of signal power.

You might increase the sensitivity of your antenna by providing a ground wire at 180 degrees to the antenna.

#5

Power Supply
Looks like I already have separate power supplies (terminology withstanding). As mentioned, I did quite a lot of google searching before the fact, and found a few designs in where they highlight the use of capacitors (albeit lacking a description of separating the power supply, but rather cleaning the power). I have been using 0.1 microfarad (104 blue) capacitors. I have one between the GND and VCC pins on the demodulator side, and one between the VCC and one of the GND pins on the RF side. Do I also need one to connect the VCC to the other GND pin on the RF side?

Antenna
Once again, it looks like I am all okay – during my search I found mention that the antenna pin should not be inserted in a breadboard. With my mini-breadboard the gap between the out-most pin column and the edge was small enough for me to hang the pin over.

#6

I have been using 0.1 microfarad (104 blue) capacitors. I have one between the GND and VCC pins on the demodulator side, and one between the VCC and one of the GND pins on the RF side.

Use an oscilloscope to make sure this is enough capacitance; the voltage at the input to the module shouldn’t vary by much. Exactly how much depends on the module, and the datasheet doesn’t say. But if you can decrease the variation by using a larger capacitor, then do so.

Do I also need one to connect the VCC to the other GND pin on the RF side?

You’ve lost me there. That would cause a fault current.

#7

thanks. I’m not sure if I lost you by my improper question writing or with my lack of electrical understanding. I was spun out when I first realised that a capacitor joining positive and negative was allowed (or maybe I’m showing my lack of understanding again), but to me it seems as if the negative and the positive are being bridged by the capacitor which in my mind would create a short circuit - but I think I may get it now - in a way it does create a short circuit and becomes the power source for the circuit it shorted off (the other side).
I have attached a diagram of what my previous question was asking about the placement of capacitors. I assume by your reply that you mean that either C1 or C2 in my diagram should not exist. So I should either have “C1 and C3”, or “C2 and C3”.

#8

sorry for the double reply, but I have a question regarding oscilloscopes too… is there much difference between these two devices oscilloscope wise besides one having a PC interface and Multi-meter built in?

http://www.jaycar.com.au/Test-%26-Measurement/Oscilloscopes/LCD/10MHz-4000-Count-Handheld-Scope-DMM/p/QM1577
http://www.jaycar.com.au/Test-%26-Measurement/Oscilloscopes/LCD/10MHz-Velleman-Rechargeable-Handheld-Pocket-Scope/p/QC1914

Thanks… on way to shop now :slight_smile:

#9

Yes, it is normal to place a capacitor across a supply. What it does is slow the initial rise of the supply voltage, and slow any fall of the supply voltage. Become familiar with other circuit diagrams and you’ll see how they are used.

Your diagram is correct, but you have not shown the other connections. I’d omit C1, connect pins 2, 3 and 17 together, connect pins 4 and 14 together, then I’d bring the power supply ground to pin 17, and the power supply positive to pin 14.

I’ve no comment about those test instruments, sorry. I’ve not used those ones.

#10

Thanks… I bought http://www.jaycar.com.au/Test-%26-Measurement/Oscilloscopes/LCD/10MHz-Velleman-Rechargeable-Handheld-Pocket-Scope/p/QC1914
I’ve attached a diagram of how I think you mean the wiring should be - are you able to verify I have you correct?
Thanks.

#11

Yes, that looks correct, assuming the datasheet is correct.

#12

Cool… Thanks. I’ve been playing around with my oscillator today… Big learning curve on this one :slight_smile: Also I just realised the shield that’s attached to my arduino has two 0.1uF capacitors, which I’m assuming would have an effect on the sizing of the capacitors I’m using… I’ll do some more oscilloscope testing tomorrow now that I have half a handle on how to use it. Tuning to see the signal was a bit tedious as I was not to sure what variance was acceptable for my circuit and also what time divider and voltage divider is appropriate. I was able to get the signal to display within a 1 volt divider (or may have been 2 volt) and it looked fairly straight, only a pixel difference or so out of the ~10 pixels per divider.
I’m assuming that capacitors on the transmitter arduino is also a good idea.

#13

A piece of wire, or a copper trace on a circuit board, or the header pins on the shield, or the header socket on the Arduino; all these have a non-zero resistance. Very small, but still non-zero. By ohms law, when current flows through a resistance the voltage will drop.

The capacitors on the shield are there to compensate for the extension of the power supply to the shield from the Arduino board. They compensate for the non-zero resistance of the copper trace on the Arduino, header pins, header socket, and then copper trace on the shield.

When you extend the power supply to the receiver module from the shield, the extension has resistance, and you may have to add capacitors to smooth the voltage. The capacitors on the shield have no real effect on what you are to choose for the receiver module.

The receiver module likely also has capacitors to compensate for the header pins and copper trace on it.

So, the power supply voltage at the receiver module will fall when the receiver is active. It might not fall enough to be a problem. That’s what you should test. Apart from minimum operating voltage, the datasheet is mute on how much fall is acceptable. That’s normal for the datasheet of very cheap module.

You should use your instrument to prove that the voltage does not fall below the minimum. If it does not fall at all, do not add capacitance. If it does fall a bit, maybe add capacitance. If it does fall below the minimum, certainly add capacitance.

You have to do this test while the module is powered waiting for a signal, or powered and receiving a signal.

You have to understand how to configure your instrument and read the display so you can say something like “the voltage fell from 5.15 V to 5.10 V for 10 milliseconds and then recovered”.

It may also help to trigger the instrument to record just before a signal is received by the module. This is so the recording has the time period of interest.

Finally, this power supply tuning probably won’t be important unless you are trying to operate the module near the limits of sensitivity. If your radio link is very short, say a few metres, then there’s a good chance it will work regardless of power supply noise.

#14

great information. thanks! sorry for the delay in reply, work has been hectic!