3 channel FM transmitter/receiver

Thread Starter

jan.rover

Joined Jun 18, 2011
19
hey, I need to construct an FM Transceiver and I want to know if I can use a separate FM transmitter and FM Receiver with the same frequency and try to 'merge' them.
But I the schematic I have only caters one channel, and I need to modify it to three.

I want to ask if I have to make 3 separate circuits for both Tx and Rx?
In the Rx circuit, pins 5 and 6 are the LC connections right? for the frequency channel? HOw do I modify it so that I can come up with 3 channels?

This is the Rx image:


This is the Tx image:
http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/88-108MHz_25mW_FM_TX.gif

And this is the proposed connection for Tx and Rx 'merging'. Is this correct?
http://2.bp.blogspot.com/_FLJi_sW6vA0/TJLp_xS-3TI/AAAAAAAAAWU/FLrWaX3ugrw/s1600/trans2.JPG



Pls...!!! I really need some help with this ASAP. T_T
 

praondevou

Joined Jul 9, 2011
2,942
If this is a walkie-talkie scheme with half-duplex communication, yes you'll use one receiver and one transmitter on each side, all being adjusted to the same frequency.

Neither your transmitter nor your receiver has an option to switch channels/frequencies, there are certainly options to achieve this with the circuits you have but it won't be simple. So you will probably have to build 3 of those circuits and adjust them accordingly. Not a good idea if you ask me. Your circuit is going to be huge...

Aren't there any ICs designed which already have a multiple channel option?

Do you really want to work on the frequencies from 88MHz to 108Mhz? There are so many broadcasting stations, if you live in a city, this won't work on a bigger distance.
 

PeeSeeBee

Joined Jun 17, 2011
56
For walkie talkies you need a crystal controlled TX & RX. LC tuned circuits are simply not stable enough for walkie talkie use.

If you find a circuit for crystal controlled RX & TX, you can change channels by simply switching in different crystals.
 

Thread Starter

jan.rover

Joined Jun 18, 2011
19
If this is a walkie-talkie scheme with half-duplex communication, yes you'll use one receiver and one transmitter on each side, all being adjusted to the same frequency.

Neither your transmitter nor your receiver has an option to switch channels/frequencies, there are certainly options to achieve this with the circuits you have but it won't be simple. So you will probably have to build 3 of those circuits and adjust them accordingly. Not a good idea if you ask me. Your circuit is going to be huge...

Aren't there any ICs designed which already have a multiple channel option?

Do you really want to work on the frequencies from 88MHz to 108Mhz? There are so many broadcasting stations, if you live in a city, this won't work on a bigger distance.

yes, I've thought about that one option, and it really is going to be huge and troublesome.
There are ICs for multiple channel option,T've read about them; but I haven't seen a schematic that used one.
The distance doesn't matter much, because this is only for a school project. Even minimum of 2m is enough. I just hope there's enough frequency in between FM radio stations that could cater our Tx/Rx without interference. I live in a city.
 

Thread Starter

jan.rover

Joined Jun 18, 2011
19
For walkie talkies you need a crystal controlled TX & RX. LC tuned circuits are simply not stable enough for walkie talkie use.

If you find a circuit for crystal controlled RX & TX, you can change channels by simply switching in different crystals.
Ah, yes. I've read about LC not being stable enough for changing channels. I've seen this schematic a few days ago, and I just noticed the different crystals at the lower left of the picture.

Will this one do for an 88-108Mhz FM TxRx? It does use multiple crystals for switching...
here's the pic:




??
 

praondevou

Joined Jul 9, 2011
2,942
WOW, if this is your first RF-project.. good luck ;)

It says "2m". If he means the 2m amateur band (144MHz to 148MHz), then this is not meant to work on frequencies between 88MHz - 108MHz. And there are certainly legal restrictions for this frequency band, too.
Somehow I doubt that you will find a schematic for a transceiver working in this frequency band.

The site you found is great ! Doesn't any of the other transceivers in different freq bands fit your needs?
 

Thread Starter

jan.rover

Joined Jun 18, 2011
19
WOW, if this is your first RF-project.. good luck ;)

It says "2m". If he means the 2m amateur band (144MHz to 148MHz), then this is not meant to work on frequencies between 88MHz - 108MHz. And there are certainly legal restrictions for this frequency band, too.
Somehow I doubt that you will find a schematic for a transceiver working in this frequency band.

The site you found is great ! Doesn't any of the other transceivers in different freq bands fit your needs?

..yeah, I think he meant 144Mhz. But it's still Frequency right? I've read something about Frequency Modulation being used not only in the 88-108Mhz range, is that correct?

The site I've found is really good, quite helpful if as it offers a lot of options. But so far the schematic above is the closest I've got.


how about these?




http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/144MHz_SSB-FM_TRX_DC6HL.gif


this is my first RF project indeed. Pls reply... I really need every help that I can get. :l
 

praondevou

Joined Jul 9, 2011
2,942
??? You think you will be able to build this??? It doesn't even fit on my screen!!! And it's in german...:confused:

I think we should really be trying to find something else.
 

wayneh

Joined Sep 9, 2010
17,155
I think we should really be trying to find something else.
+1
Those are really complex and would require a lot of experience and equipment to test if/when anything went wrong. A simple circuit times 3 would be far simpler and raise the odds of success. That one circuit uses the MC3362. I've got some of those but I'm pretty sure they're obsolete.
 

Thread Starter

jan.rover

Joined Jun 18, 2011
19
I've seen a lot of transceiver schematics, but I'm not confident if they fit the requirements. About the 88-108Mhz range, I think I've read something about FM being used even in other ranges right. Is that right?


:l
 

Thread Starter

jan.rover

Joined Jun 18, 2011
19
??? You think you will be able to build this??? It doesn't even fit on my screen!!! And it's in german...:confused:

I think we should really be trying to find something else.

hahaha! you're absolutely right. hmmm. I'll refer the first schematic to my instructor, maybe it'll do.
hey, FM can be used even outside the 88-108Mhz range, right?
 

Thread Starter

jan.rover

Joined Jun 18, 2011
19
This kit is available from Ramsey Electronics. Bulky, but it has decent transmit distance.
It's really helpful, 'cause there's a PCB layout with it. But we're not allowed to use Programmable IC; which, I think, made the project quite more difficult since I've encountered other schematics for FM transceiver that's simpler but utilized PIC.
thanks, though. :)
 
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praondevou

Joined Jul 9, 2011
2,942
To be honest I like more the schematics Radiohead posted, even though I didn't analyze in depth. There is for example a PIC involved.. software??

The schematics you post, do you have a part list too?? There is no specs of the inductors / capacitors, wattage of resistors, instruction on how to adjust everything.... apart from the (inexistent) layout where it becomes more and more important to follow specific rules when it comes to RF.

Really if this is the first RF project u should stick to something simpler.

edit: just found this one: http://www.pira.cz/entx3.htm It's not a small project but it has a complete instruction how to build it, part list, schematics, pcb. It works on FM 88-108MHz, frequency can be adjusted via dip switches (PLL). The dip-switches could be replaced with another digital circuit and then you have your multichannel fm transmitter.. Receiver should be easier to accomplish.

found the link on this site: http://www.discovercircuits.com/F/fm-tranms.htm
 
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