Help Regarding details of RF 433 Mhz modules

[Krishna Patil]

Hello Everyone ,

I'm using RF 433Mhz modules like this one https://www.sunrom.com/143 . What will be bandwidth I'll be getting ?and what is the data rate ? In this datasheet https://www.robokits.co.in/datasheets/TXC1.pdf it is mentioned that 1kbps . How it is calculated ?

Does the data rate vary over distance for these modules like it happens in a Wifi Connection ? If it'll be varying how can I measure the data rate vs distance ?


Thank you,

 

[Krishna Patil]

I gathered information from googling. But still have doubts . Please help me with this

I'm using a 433 Mhz Tx/Rx Pair for RF Communication and HT12E and HT12D for encoding and decoding. I wanted to know more about data rate and setting it for faithful transreception.

And bandwidth for ASK is 2*Bit rate which is 2KHz. But it is mentioned in data sheet as 2Mhz

Why it is always suggested that we use 1Mohm as Rosc ? can I use a 10M ohm resistor to reduce fosc and get a lower data rate which reduces bandwidth and increases receiver sensitivity which ultimately increases range of RF transmission ?

Can I use a resistor less than 470k and get a faster data rate in short range ?

Can I get a graph of how range decreases with data rate ?

 

[betwixt]

These are AM devices so the bandwidth will be approximately twice the data rate (carrier +/- modulation). The maximum data rate is governed by the time taken for the carrier to reach peak and zero amplitude when turned on and off at the transmitter and by the response time at the receiver. Most receivers have some kind of filter or monostable in them to limit the speed so the effects of noise are reduced.

If you look at the data sheets, inside the HT12 devices there is a divide by 3 circuit between the oscillator and shift register, that's why the 3KHz to 1Kbps is shown.

I doubt you could increase range by reducing the bandwidth. It is primarily set by the filtering in the receiver and to redesign those would be a big problem. I think the resistors are to set the data rate (bits per second) by adjusting the uart clock, not the bandwidth per se.

The distance it will cover is depends on many factors, including the environment and other users in the same frequency band, there is no direct relationship between rate and distance for this kind of device. If you really need long distance data coverage at those frequencies you should look at the more exotic modulation schemes and DSP demodulators, even then you would suffer interference from other users. All these parts are intended for simple remote control applications where reliability isn't too important.

Brian.

 

[Krishna Patil]

I doubt you could increase range by reducing the bandwidth. It is primarily set by the filtering in the receiver and to redesign those would be a big problem. I think the resistors are to set the data rate (bits per second) by adjusting the uart clock, not the bandwidth per se.

Thank you very much for answering. Actually I'm using this for a remote control application itself. But I want a little more range because of RCC(reinforced cement concrete) in between Tx and Rx . So when I look at datasheet of HT12E there is a provision of lowering data rate there is graph of fosc vs voltage and corresponding resistor values.

If as you said no relationship between rate and distance then why would they provide different rate in the data sheet of Tx. Max is 9.6kbps and min is 300bps. Lowering data rate reduces bandwidth, less power , less sensitivity. Since power transmitted is same it should provide a little more range. Am I right ?

It is primarily set by the filtering in the receiver

Please Explain me this . Is the range been set for these kind of devices ?

 

[betwixt]

In an ideal system, lowering the data rate and therefore bandwidth would help but you would also reduce the receiver bandwidth to match the transmission. The problem here is the receiver bandwidth is quite wide and decided by the filtering in the receiver circuit. As the transmitter initial frequency accuracy and drift are relatively poor, the manufacturers make receivers with wider bandwidth than you might expect, typically they are several hundred KHz wide and have poor blocking performance. In other words, narrowing your transmission bandwidth simply makes it less likely to be seen against the other signals, interference and noise in that part of the frequency spectrum.

The main reason for altering the data rate would be to make it incompatible with other devices using the same frequency. The receiver is not selective so it will respond to any signal it can hear within it's bandwidth. Even in my remote location I can hear many signals around 433.92MHZ and they sound quite different to each other because the maufacturers of whatever they are try to be unique to avoid cross-interference.

The only way to confirm the range is by experimentation. 433MHz will travel through concrete reasonably well but if it is reinforced by steel rods they will act like a screen. How much they reduce the signal will depend on things like the rod lengths, how many there are and whether they are horizontal, vertical or both. You can increase the range by adding a suitable antenna, try at the transmitter first and only add one at the receiver if that isn't sufficient. Although a receiver antenna will help it pick up more of your signal, it will also pick up more of everyone elses too.

Brian.

 

[Krishna Patil]

Thank you for your detailed answer that helped me a lot . I started building this circuit so I can experiment with the range. Once I complete it I'll post the results and if I have any doubts I'll ask .
Thank you again betwixt

 

[Krishna Patil]

With experimentation I found that, what betwixt stated is exactly right. There is no much improvement in the range even if I change data rate from minimum to maximum , may be only 2-3 meters difference. I'm totally convinced by the reason

The problem here is the receiver bandwidth is quite wide and decided by the filtering in the receiver circuit. As the transmitter initial frequency accuracy and drift are relatively poor, the manufacturers make receivers with wider bandwidth than you might expect, typically they are several hundred KHz wide and have poor blocking performance. In other words, narrowing your transmission bandwidth simply makes it less likely to be seen against the other signals, interference and noise in that part of the frequency spectrum.

With 17cm Single strand wire as antenna on both Tx/Rx I'm able to get 15m range with typical 1kbps data rate. With or without wall range doesn't change. But as betwixt said it would change with reinforced concrete. So still lesser range.

So I need to use a directional antenna to further improve range.

 

[betwixt]

At 433MHz directional antennas are still quite large. You need at least a reflector, a driven dipole and one director. Adding extra directors will increase the directivity and apparent gain. The best source of information on 433MHZ antennas is Amateur radio web sites because the frequency falls within one of their allocated bands (70cm band).

Brian.

 

[chuckey]

A group A TV yagi might work.
Frank

 

[Krishna Patil]

Thanks Brain and Frank . Yes, I have searched amateur radio web sites and found yagi antenna is best. I'm a newbie in antennas. I'm supposed to use yagi on receiver side or transmitter or both ? Do I need a connect co axial cable to feed or receive from the antenna ?

If in case, I go with wired connection can I use a cat 2 cable to transmit serial data from output of HT12E. Can a 50 meter long wire get reliable reception at 1kbps ? How about using a Co axial ?

 

[betwixt]

If you use the RF method try a Yagi at the transmitter first, obviously pointing at the receiver. This will direct more of the transmission in the direction you need it and less elesewhere so the receiver should see a stronger signal. If that doesn't work, try a Yagi at the receiver as well, obviously pointing toward the transmitter. The Yagi will dramatically increase the range but try only at the transmitting end first, there is a danger that using one at the receiver will increase unwanted interference as well, leaving you no better off.

A wired connection will almost always work better. Over 50 metres I suggest you use CAT-5 cable and RS422 signalling. You can forget the present encoder and decoder and use an RS422 driver and receiver instead. Over 50m it should easily work at more than 100Kbps, maybe more than 200Kbps. You would have a single data input and a single data output without needing any encoding or signal conditioning. If you need it, you can add more receivers by wiring them in parallel.

Brian.