In my design, I have a transitter and receiver on the same PCB (the terminal) and a seperate transmitter located many feet away from the terminal. The goal is to have the terminal signal a PICAXE when to transmit its data.
After reading the forums, I see that my transmitter needs to be located at least six feet from my reciever. If the transmitter on the terminal is not transmitting, is there still an issue with the transmitter and reciever being next to each other? In other words, can a transmitter that’s not transmitting be located next to a reciever that’s trying to reciever data from a second transmitter many feet away?
The transmitter is low power and I don’t see why you can’t have the receiver on the same board. You need to use separate antennas for each or use an RF switch. The important point is that you want the receiver not to be swamped by the transmitter, reducing its sensitivity to receive the remote data. I’d try removing the power to the receiver when the transmitter is active, connect the power after the transmitter is done, and wait a short time (50ms?) for the receiver to stabilize. Your protocol should have your terminal send the transmit command and the remote receiver should wait a short period of time (500ms?) before sending the requested data.
I was just thinking of using a relay to control power to the transmitter (great minds think alike). I still have to figure out my source of noise, but since I’ve read the two devices need to be at least six feet apart and that the reciever will generate noise when nothing transmits, I may have most of the problem worked out.
The receiver does not have a squelch circuit but it does have an automatic gain circuit (AGC) that will adjust its sensitivity (increased with low or no signal, and decreased with a strong signal). Expect to see noise coming out of the receiver if you don’t employ a squelch circuit. That is why the software looks for preamble bytes to know when it is receiving an incoming signal.
The transmitter is a OOK/ASK type which means the transmitter is keyed on and off with the data stream. It is better not to use a relay since the contacts have to switch on and off many times a second. You can key the transmitter directly off a microcontroller I/O pin.
I got the system to work now. The remote station transmits the following byte pattern.
170,170,170,D,Data-1,Data-2,check,D
The 170’s are bit 1010101010 and gives the receiver a chance to warm up. The D is the beginning of the data stream. Data-1 and 2 are my two bytes of data (temperature and light level). The check is a byte result of multiplying Data-1 and 2 together (ignoring overflows), and the last D is the end of the packet and padding for the last of the important data.
Initially, I had the receiving station too close to the transmitter, moving it away helps.
Ultimately, the receiving station is going to transmit and well as receive. The TX and RX have separate antennas in close proximity and the RX and TX boards are next to each other on the PCB. So I was thinking about having the micro turn on and off the RX and TX as they are needed. That way the receiving station’s RX won’t be overwhelmed when the TX transmits. I figure I’d switch on a relay to power up the TX, wait a second, transmit a command, wait 250 ms, then switch off the TX. The remote station would wait one second before it begins transmitting data. It too would switch on and off its RX and TX because it’s baords and antennas are in close proximity.
I don’t think you will need to worry about powering the transmitter on and off.
As riden stated:
riden:
The transmitter is a OOK/ASK type which means the transmitter is keyed on and off with the data stream.
Meaning that the carrier is turned on and off based upon the data line. So long as the data line is held low, the transmitter should not emit any RF (well, the RF we’re interested in…) whatsoever.
That makes sense, thanks. That also simplifies my design. While the TX is not being used, I’ll low the line. Right now I don’t send a LOW command. But since it’s a PICAXE and the particular pin is only an output, the pin must be low by default.
Now what about the receiver? I wonder if it could be damaged having the transmitter antenna right next to its antenna.
Although I’m not 100% sure, I’ll take a stab at this one too.
Theoretically speaking, I guess it would be possible to couple enough energy into the antenna such that it would break/fry one of the components in the receiver. However, what is more likely is that one would simply be pushing one of the components out of spec and it would fail to operate normally (most likely whatever circuit is used for amplification, the AGC).
Further, if the transmitter is capable of outputting this much power, well… that’s just not gonna happen, not with these modules at least.
So, in short, no.
Someone please correct me if I’m wrong, but I believe my statements to be correct.
There just isn’t enough power to do that. As previously mentioned, the only problem is swamping the receiver if the transmitter is on whilst receiving.