420-450MHz, High-Low state detection (without interference).

[Sender123]

TL;DR Summary

rf, analog, transmitter, receiver

Hello all,

Im trying to come up with a simple method (and design) for detecting (mostly and foremost next to transmitting) a (continued) HIGH or LOW state through a specific frequency, most likely in the 420-450Mhz range. Without the possibility of interference. Preferably analog, at least on the receiver side.

The range has to be a minimum of 1000m (but not much more) and the speed has to be at least around 1Mbits per second (although I am currently trying to avoid sending actual bits, as said above).

On the receiver side of things I thought, for a moment, about using a (possibly envelop detector like) design which would require receiving two separate frequencies at once (on two separate antennas) in combination with an AND gate. Which assumes a HIGH state if the two frequencies are present and the LOW state if they are not. However this still has a risk of interference.

Which might force me into a digital receiver (or transceiver) type of design with "bit" verification for the signal. The downside is that this adds complexity in several ways. It, for instance and among others, almost always requires a additional MCU on the receiver side, which is something I would like to avoid. Also the speed is not high enough on most of the receiver ICs in the suggested RF range. I didnt look at receiving transceivers for now as they always require MCUs. Again, I am not looking to receive information, only a continued HIGH or LOW state.

On the transmitter side it would likely (if technically feasible) require a more standard IC transmitter/transceiver type of design which would be driven from a MCU (CPU). However, again, these are also mostly made to send out bits, so this could turn out to be a challenge. The receiver would have to be able to translate a (possibly digital) signal into a analog HIGH state.

Does anyone have any suggestions on how to approach the (receiver and to a lesser degree transmitter) design in order to reach the requirements? Can I, for instance, add logic on the receiving side to detect the (analog or digital and logically modulated) signal? Modulation does require additional speed but it possibly can't be avoided to prevent interference?

Or should I direct my time and resources on a dual transceiver IC design (if actually available and feasible for this type of operation)? Also considering FCC regulations and such, although a simple analog receiver design in this range should not be a problem I think.

I sincerely appreciate any and all suggestions. Thanks.

 

[berkeman]

Welcome to PhysicsForums.

Summary:: rf, analog, transmitter, receiver

On the transmitter side it would likely (if technically feasible) require a more standard IC transmitter/transceiver type of design which would be driven from a MCU (CPU). However, again, these are also mostly made to send out bits, so this could turn out to be a challenge. The receiver would have to be able to translate a (possibly digital) signal into a analog HIGH state.

Which licensed RF band are you planning on using? Do you already have the license to transmit in that band?

 

[anorlunda]

Why RF? Can't you do better at non-interference with a laser?

 

[Sender123]

Thanks berkeman. I```m only considering the frequencies that are free/open for use and suitable for the purpose in terms of range, speed, availability of materials, etc. Any suggestions on suitable frequencies outside of the 420-450Mhz range are welcome.

RF is required because interference from objects is likely. I like the laser suggestion but it is not an option here. Also in terms of cost.

 

[DaveE]

I think you just described my garage door opener. The design should be pretty straightforward, not counting dealing with the lawyers you might attract.

 

[berkeman]

Thanks berkeman. I```m only considering the frequencies that are free/open for use and suitable for the purpose in terms of range, speed, availability of materials, etc. Any suggestions on suitable frequencies outside of the 420-450Mhz range are welcome.

The range has to be a minimum of 1000m (but not much more) and the speed has to be at least around 1Mbits per second (although I am currently trying to avoid sending actual bits, as said above).

So that is an unlicensed band in your country? Even if it's unlicensed, there will be limits on Tx power and out-of-band interference. Have you designed any RF transmitters in that frequency range before? What is the limit on in-band power for this band in your country? Is the band already used for other things since it is unlicensed? What kind of other things?

To achieve 1km range, you will likely need to use high-gain Yagi Tx/Rx antennas. Do you have experience with Yagis? I'm assuming that you have a line-of-sight clear path between the Tx and Rx locations...

 

[hutchphd]

What have you researched? There are already (as per @DaveE ) many Frequency Shift Keying schema.

 

[Sender123]

No the garage door opener is too slow, it therefore lacks reliabilty as it requires the sending and error free reception of bits or bytes within a specific time frame, also it does not have the range required although that is not a problem for the 420-450Mhz frequency range in general.

OOK keying might be a suitable modulation method if I decide to go digital but it seems to be too slow and also not free from interference. Most modulation techniques are not designed for this HIGH-LOW switching purpose but I am looking into it in more detail. I also considered using OOK keying on two parallel frequencies simultaneously to decrease interference.

It seems that sending a continues analog frequency signal might indeed be illegal. So that would only leave the digital option.

 

[Sender123]

@berkeman: Mostly unlicensed yes. Depending on the actual frequency selected it is amateur, industrial and medical, mostly free from license or it requires an easy to obtain license. In most countries. No I have not designed one before. The limits on power are only reached by the MCU controlled transmitter which would possibly be a standard type and would therefore fall within range of the legally allowed. The type of transmitter depends of the receiver design, it might be either continues frequency transmission or some kind of modulation. Analog or digital, it has to meet the requirements.

@hutchphd: I am looking at all types of modulation, mostly digital, but I would like to go without modulation. FSK is possibly an option, next to OOK, however it is more prone to errors, etc.

Currently I am looking at an analog solution first although I understand it might be difficult.

 

[hutchphd]

So how do you encode information without modulation? And what is at 2MHz?

 

[Sender123]

@hutchphd: Sorry, to be more clear: I am not sending information. Only a simple continued HIGH or LOW state which switches states within the microsecond range. I am not sure what you mean with it being at 2 MHz, it drives a power state if that answers your question.

I should also add that there are multiple detectors/receivers looking for the same input.

 

[Baluncore]

Im not sending information. Only a simple continued HIGH or LOW state which switches states within the microsecond range.

That is an application for Direct-Sequence Spread Spectrum, which is not used below 400 MHz.

The transmitter is spread over a 10 MHz or wider band using a PRBS key. The signal falls into the noise floor. The receiver de-spreads the signal, lifting it out of the noise floor, by locking to the phase of the PRBS key with a correlator, and demodulates the status of the information bit. Incident RF interference is spread into the noise floor at the receiver.
https://en.wikipedia.org/wiki/Spread_spectrum
https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum
https://en.wikipedia.org/wiki/Pseudorandom_binary_sequence

 

[Averagesupernova]

@hutchphd: Sorry, to be more clear: I am not sending information. Only a simple continued HIGH or LOW state which switches states within the microsecond range. I am not sure what you mean with it being at 2 MHz, it drives a power state if that answers your question.

I should also add that there are multiple detectors/receivers looking for the same input.

You ARE sending information. You ARE modulating. You cannot do anything to a carrier without creating sidebands. Even on/off keying takes spectrum.

 

[Sender123]

@Baluncore: That sounds like a good read. Let me look into it. Still probably slightly complex for what I need to achieve. If it is the only way to do it then this is so.

It is probably too much to expect a new type of basic analog of modulation for my purposes. But maybe my (or someones) brain flares up with a suggestion.

@ Averagesupernova: Yes and no. I would like to avoid modulation as I do not need to protect information and I need to keep things simple. I am not sending information esssentially. It only becomes information once it turns (or I make it) into a modulated signal. For instance let's say I want to send the HIGH state, to achieve this I send out a specific frequency signal continuously, I want the LOW state, I remove the signal or send a new signal in case of digital means.

Im not sure what you mean with it needing spectrum. Every signal uses a specific or band of frequency, sure. My thought for a moment was to use a very specific and possibly modulated frequency only or even multiple specific frequencies simultaneously, to reduce the risk of (regular atmospheric) interference, not jamming or anything. However most are pointing towards some form of digital modulation to identify the transmitted signal in order to prevent interference. However the needs are for a specific speed and range also.

 

[berkeman]

@Sender123 -- What reading have you been doing so far about RF Tx/Rx channels, Link Budgets, Modulation techniques, etc. I get the sense that this is the first time you've tried to do something like this.

I also am confused by your repeatedly saying that you just want to send a continuous square wave transmission. Such transmissions are not generally allowed, even in unlicensed bands.

Is there an amateur radio (HAM) club in your area? You might be able to see what kind of projects they have built, and get some good Mentor advice from an "Elmer" about the projects you want to pursue.

 

[Sender123]

@berkeman: Sorry, maybe I need to be more clear: it is a continuous square wave switching states in the microsecond range. Preferably represented by a (form of) analog frequency presence or absence and the detection of these states. There are multiple receivers/detectors looking for the same transmitted signal.

Or by using a form of digitally encoded and modulated signals to induce the HIGH and another signal to induce LOW state. This is not preferred however as there are numerous forms of encoding and modulation with different and often complex purposes, applications and implementations.

Im not trying to send information and it therefore essentially does not have to be encoded. The signal only has to survive general atmospheric interference.

Its probably best to go into details only once the basic solution to the problem is clear. There is no use for Link Budgets if the frequency or transmission (and modulation) methods have not been decided on, agreed?

Im not stating that I know everything there is to know about RF or even something close to it, especially on the theoretical and design side, I will need to detail out more once I get to that point. Hence the questions.

Allowed power levels for the required distance (and speed) does not seem to be a problem at first glance for this frequency range. And it is probably most relevant to the transmitter. Which will likely be a more standard of the shelf type of solution, also due to global regulations.

I did look into modulation, mostly OOK keying as it is the most straightforward method. But IC part availability, considering the requirements, might be a challenge.

Im reading up on all the modulation types out there as there are numerous.

Much appreciated and thanks again.

 

[berkeman]

Its probably best to go into details only once the basic solution to the problem is clear. There is no use for Link Budgets if the frequency or transmission (and modulation) methods have not been decided on, agreed?

Of course not. You have stated that you are worried about atmospheric noise (at 400MHz?) and other noise sources, so channel characterization and link budget are very important.

Maybe you could please stop dancing around and tell us what you want to achieve? That will help to stop wasting the time of a lot of people who want to help you.

 

[Sender123]

Im not dancing around and I am not looking to waste time.

Not random noise but mostly interference from other signals at the same frequency, there are long range transmitters out there. I did not say they are unimportant, I said there is an order in deciding on them.

Ok, let's say I use a simple (possibly envelope like) detector to sniff out a specific frequency and filter out the rest. Which then outputs a voltage translated through a OPAMP configuration to provide a HIGH state if the signal is present and a LOW when its not. That is essentially all I need. Now I need to be able to somehow isolate this frequency signal from other atmospheric interference (mostly other signals at roughly the same frequency). For instance by encoding in it a (simple, for example, 4 bit) signal which I decode with analog or basic digital means (hopefully not requiring an MCU, possibly logic gates). Or by sending a more complex pattern of frequencies which I somehow detect for a duration of time. Like the suggested DSSS (Direct-Sequence Spread Spectrum).

 

[Merlin3189]

I'm still not clear on what you're trying to do - as opposed to how you're trying to do it. Maybe some description of that would help.

Most of the obvious points, re. your solutions, have been dealt with.
But I'm wondering what you mean by "switching states in the microsecond range"?
That sounds like simple post-receiver processing to clean up the signal - say a fast Schmidt trigger.
Maybe you mean you want your continuous unmodulated signal to switch states a million times per second? Well that's down to your base frequency limiting your modulation options.
If you want a response within a microsecond, over 1000m range, I think you'll be out of luck whatever you do!

Edit: I see I've duplicated berkemans comment while I was writing.

 

[berkeman]

Im not dancing around and I am not looking to waste time.

Well you are. Thread is closed temporarily until I can figure out if we can help you or not.

Please send me a Private Message (PM -- click on my avatar and Start a Conversation) with more information about your project. I'm happy to help you update this thread so we can give you the best help possible.

In your PM please answer the questions you've been asked in the thread so far but have not addressed, such as:

• What is your channel characterization (noise, BW, modulation, power, link budget, allowed BER, Checksum/FEC scheme, etc.)?
• Why do you keep saying that you don't care about the data content, and only want to Tx a square wave?
• Please include links to the reading you've been doing so far on this project -- we still have no idea of your background...

 

[berkeman]

At the OP's request, this thread will remain closed. Thank you to all those who tried to help.