What RF trancievers will work...?

[CPW]

What I need for my project is some simple RF transcievers. Here is a list of the things they need to have/do:
-Crystal socket for XTAL crystals in the 27 mhz range
-4 simple pins/connections: positive, negative, data to send, data received
-Place to solder antenna to (obviously)
-Small (2x2 inches MAX)
-Range of a few hundred feet (at least 100 ft)

All of the data sent and received will be processed externally by microcontrollers. All I need the trancievers for is to "convert" electric pulses (my data) to RF and vice versa. They will serve as the wireless link between two circuits (obviously). They need to do nothing more. I simply want to hook them up to power. Then I will have a microcontroller that sends a series of pulses to the pin/connection for data to be sent, and the tranciever will send the data via RF. The same will happen for incoming data. The tranciever will pick up the RF signal and will convert it to electric pulses, and the microcontroller will read this data by connecting to the pin/connection for data received.

The quantity, frequency, etc. of pulses will be controlled entirely by external microcontrollers. The trancievers need to do nothing but send the data the microcontroller gives and receive the data it picks up via RF. They need to be that simple.

My question is, what trancievers fit this decription and cost very little (less than $10 a piece, preferrably a few bucks a piece)? Where could I get those trancievers? Thank you so much for the help.

[audioguru]

Hi CPW,
Does any wireless module manufacturer make anything for 27MHz anymore? They have many on 433MHz and 900MHz.

I don't think it is legal to transmit data at 27MHz. Old wireless home phones, radio controlled toys and models and old CB radios use 27MHz.

You might need to transmit on 433MHz and receive on 900MHz so that the nearby receiver doesn't get overloaded.

[CPW]

I was thinking of using this to "upgrade" a radio-controlled car with a better radio system for fun. It uses interchangeable crystals in the 27 mhz range, so that's why I said I wanted that.

However, after looking into it, I decided it might be wiser to go with 2.4 ghz, as that seems to be the easiest way. I found some neat little chips that you may have heard of. They are nRF24L01 chips from Nordic Semiconductors. Here is a link: http://www.nvlsi.no/index.cfm?obj=product&act=display&pro=89 They seem to have tons of features and are very cheap (a few bucks) and require very few external parts. I am thinking about buying some and playing around with them. Right now, I would like to get a simple two unit test thing going where one unit would send a number or something to the other unit, and the other unit would turn an LED on or off or something. That's what I would like to accomplish right now: something very simple just so I can learn how to do it.

Actually, I don't really have a good way to use a chip like that because I have no way to connect to the pins. So while I was looking around I found this: http://www.sparkfun.com/commerce/product_info.php?products_id=705 It's perfect for what I need. It is a simple tranceiver module using the chip I mentioned earlier. It has eight easy connections and an antenna jack. Its basically exactly what I was originally looking for, only it is 2.4 ghz and it doesn't have a crystal socket (not even necessary or possible). If I got two module and two antennae (antennas?), I would be able to easily conduct the tests I want. Its supposed to have a range of up to 80 meters, which is plenty even for when I use it in my radio-controlled car (if I ever get that far).

What are your opinions on this module?

[audioguru]

Hi CPW,
The 2.4GHz module looks good.
It won't work the same as an RC system for good models that use a form of pulse width control for motor speed and servo control. So you will need to invent an encoder and decoder yourself.

[CPW]

That's ok. I was planning on that anyway. I figured I would just send the values as numbers (from 0 to 255?) and then convert them back to the right pulse width at the end. It shouldn't be that hard, so I'm not worried about it.

Now all I need to do is wait until they get those in stock.

One question though. On the receiver side, I will need an antenna other than the RP-SMA (I think that's the right combination of letters) antenna. I will need to use either the thin metal wire antenna the car already has (it can be cut shorter), or a ceramic antenna. Do you think I could somehow accomplish that with the module (preferrably the thin metal wire antenna)?

Thanks.

[audioguru]

I think the receiving antenna should be rigid so it stands vertical. Then your transmitting antenna should also be vertical.

[CPW]

The transmitter's antenna will definetely be vertical. As for the receiver, the metal antenna is vertical, so if I could use that, things would work well. Otherwise I could just use a wire and somehow make that stand up. Whatever I do, both the antenna on the transmitter and the one on the receiver will be standing up.

It would work to use just a wire, right? I don't necessarily need a special RP-SMA antenna on the receiver. They use plain wires (obviously insulated wire) all the time in RC, so it should be no problem, right?

Thanks for your help.

[audioguru]

The cable feednig the antenna should be coax with the recommended impedance and the length of the antenna should match the frequency.

[CPW]

I know about the length of the antenna already, and that is no problem.

The antenna will be hooked directly to the board, so there will be no cable going from the board to the antenna.

My question is though, for the antenna, will my thin metal wire antenna that is already on the car work ok? It is stiff, so it stands straight up by itself (it bends back when you drive, but that's a given). My other option is to use a piece of wire that is the right length. Obviously, it will not be stiff or stand up, but I could probably use a plastic tube like a lot of RC cars use to hold up their wire antennas.

So either my stiff metal wire antenna or just a piece of wire (not bare wire) that is the right length will work, right?

Thanks.

[audioguru]

The wire for an antenna should not be too thin. If it can hold itself up in a wind then it is OK.

[CPW]

Ok, thanks. That's what I wanted to know.

I have a few more questions.

The datasheet says to use a resistor to ground the "ground" pin of the chip. What is the purpose of this resistor and how would I determine the value of it?

The datasheet's example schematic also shows that you should use a bunch of capacitors in a certain configuration around the the antenna pins and the antenna (see the datasheet). In general, what are all those capacitors for, and again, how would I determine their values?

Last question. How would I determine what crystal to use with the chip (see datasheet)?

Thank you so much for your help.  ;D

[audioguru]

Hi CPW,
I don't see a "ground" pin on the chip. It has four VSS pins.
If you don't know what the bunch of capacitors are for and don't know what is a 16MHz crystal, then mayb you should buy the assembled module.

Is this the schematic that you are talking about?

[CPW]

I guess I was getting things mixed up because I didn't actually look at the schematic while I was explaining, I was just going by memory. But yes, that is the schematic I was talking about.

I understand what a 16mhz crystal is and how to identify it, I was just asking how I know what value of crystal to use. I didn't realize it said to use a 16mhz one, so nevermind on that quesion.  ;)

As for the capacitors and resistors. I understand what each does, but I've never really been able to figure out why they are used where they are. Obviously resistors are used to limit current, so in some situations I understand their use. But when they are used to connect, say, the IREF pin to ground, I have no idea why and it makes things confusing for me.

Could you explain the basic idea/reason for why you would use resistors going to ground such as in IREF's case? And could you please explain the basic idea for why you would use a bunch of capacitors going to ground as they are being used, in say the upper left section of the schematic? Capacitors are generally used to keep things smooth and to reduce sudden changes in voltage, aren't they?

I am going to buy the assembled modules, but in the future I will want to build my own so I can learn more in electronics, and because it will be more convenient in my projects.

I am also having trouble understaning what some of the pins are for because I can't find a good description and I don't know what they mean.

Here is a list of the pin functions from the datasheet. I have deleted all of the ones whose function I fully understand, and left the ones I am still a little (or a lot) confused about.

Pin Name Pin function Description
2 CSN Digital Input SPI Chip Select
3 SCK Digital Input SPI Clock
4 MOSI Digital Input SPI Slave Data Input
5 MISO Digital Output SPI Slave Data Output, with tri-state option
6 IRQ Digital Output Maskable interrupt pin
11 VDD_PA Power Output Power Supply (+1.8V) to Power Amplifier
16 IREF Analog Input Reference current
19 DVDD Power Output Positive Digital Supply output for de-coupling purposes

These are my guesses as to what the pins are for, please tell me if I am right.
2 CSN Digital Input SPI Chip Select -
3 SCK Digital Input SPI Clock - Obviously something with the clock, but what is inputted here?
4 MOSI Digital Input SPI Slave Data Input - My guess is that its that data you want sent out
5 MISO Digital Output SPI Slave Data Output, with tri-state option - My guess is its the data received by the transceiver
6 IRQ Digital Output Maskable interrupt pin - What is an interrupt? I've never understood that
11 VDD_PA Power Output Power Supply (+1.8V) to Power Amplifier -
16 IREF Analog Input Reference current - I know what it is, but whats it for?
19 DVDD Power Output Positive Digital Supply output for de-coupling purposes - What's de-coupling?

Thanks SO much for your help. I'm trying to learn more about electronics, but I can't find good explanations for things, especially terms. Thanks for all your time and help.

[audioguru]

Hi CPW,
I didn't read all 43 pages of the datasheet so I don't know what most pins are for.
Decoupling capacitors filter a voltage so it doesn't change quickly. At 2.4GHz just a short piece of wire is an inductor that can resonate and be a high impedance on a line that must be a low impedance, so a decoupling capacitor to ground creates a low impedance on it.
A resistor connected to a reference current pin usually sets the amount of current in a device. For the amount of output power?

[CPW]

Thats ok, I didn't expect you to know everything about it. I just need some help learning, piece by piece.

So most (or all) of the capictors are there to lower impedance? That makes a little more sense now.

Just a few more questions.

What is the purpose of the crystal? I kind of understand what a crystal does, but I don't really understand its use here. Could you explain that a little?

Last question. Inductors filter out signals of the wrong frequency, do they not? So is that what they are helping with here, limiting the input signal from the antenna to what is needed?

[audioguru]

A crystal makes a very stable frequency that is accurate and doesn't drift. The IC multiplies it so the output is an accurate and stable 2.4GHz.
The inductors tune the receiver to to 2.4GHz with a capacitor or two, and are transmitting filters so double and triple 2.4GHz harmonics are reduced from causing interference.

[CPW]

Ok, I see know. This is beginning to make more sense. I still couldn't figure out what parts to use all by myself, but now I have a better understanding of what the parts are for. Thank you so much for helping me understand more.  ;D