# Basic Oscillator Question

Discussion in 'General Electronics Discussion' started by mtr00, May 27, 2010.

1. ### mtr00

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May 27, 2010
Hi,

I saw another post about multiple frequency FM transmission but it was a bit different. I'm looking to design a simple circuit modifying a single-frequency FM transmitter to 3-4 different frequencies.

Assuming I already have a signal at say, 100MHz, from what I understand I need to create a few other signals to multiply with it; e.g, at 1,3,5 MHz. I'm a bit confused about how to generate these signals. I've been told to use a crystal oscillator, but It's such a huge scope that I don't know where to start.

Basically I'm wondering how to get started on making these other sine signals. Any help would be appreciated.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Please tell us what you're trying to do. Your shopping list of requirements does not make sense.

Are you trying to make a tuneable FM transmitter for voice, or are you trying to make a variable frequency signal source in the range of (say) 1 to 10 MHz?

3. ### mtr00

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May 27, 2010
Hi Steve,

Thanks for the quick reply. Right now I have an FM transmitter that I use for my ipod with a 3.5mm male to male stereo audio cable. I'd like for it to work for voice as well; is that easily done?

I want to modify the device so that it transmitts over multiple frequencies as supposed to just one. From what I understand, I have to multiply the original signal (say 100MHz) with a few small signals (1,3,5) so that I can transmit on say, 101, 103, 105, and then add them up.

As a first step, I don't really understand how to create the 1,3, and 5 MHz signals. I've been told to use a crystal oscillator, but don't understand where to start with that. As I don't really understand the basics, my first question would be how to create that 1MHz signal?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
It's not that easy. And your ipod transmitter is probably a marvel of miniaturization that would be very difficult to modify.

If the only connection to the ipod is the 3.5mm plug, then it should be as simple as connecting this to another signal source.

There are plenty of fm transmitters designed for MP3 players that would suit your purpose and cost very little. I suggest you look at them first.

Remember that most FM transmitters for MP3 players are pretty much designed to operate right next to the receiver so their range is probably less than the distance over which you can talk to someone, and certainly less than the distance you could shout.

5. ### davennModerator

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Sep 5, 2009
You dont want to create those 3 separate 1,3,5 MHz signals. For simultaneous transmission on all 3 freq's 101,103, 105 MHz you are really looking at 3 separate transmitters all fed with the same audio signal from your MP3 player and it would be split (in a balanced way) 3 ways to each transmitter.

As Steve said you are not likely to be able to do much with one of those small transmitters, not really easily modifyable.
the easiest way would be to buy 2 more of those transmitters set them to 3 different freq's and split the audio between them.

cheers
Dave
VK2TDN

6. ### mtr00

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May 27, 2010
Hey guys, maybe I'm not explaining it properly, because my professor said it wouldn't be too hard to do. He said all I have to do is take the signal out, multiply that by a few signals, and put them through an adder.

Putting the project as a whole aside though, my problem right now is getting started and creating those signals that I have to multiply with. How do I create a signal of say, 3Mhz? What design steps should I complete before actually buying a quartz crystal of some sort and how do I go about choosing the crystal?

**Edit:
By the way this project isn't for practical purposes. It's just intended to be a learning experience.
The FM transmitter that I'm working with is found at this link. http://www.canakit.com/hi-fi-stereo-fm-transmitter-kit-ck222b-uk222b-ck222-uk222.html
**

Last edited: May 28, 2010
7. ### davennModerator

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Sep 5, 2009

the main encoder/transmitter chip in that project .. BA1404 is a single freq transmitter
at any one time. Yes it can be used over the 88-108MHz band, but on one freq at a time
which is dependant on the tuned circuit based around pins 9 and 10.
if you want 3 simultaneous transmission freqs you need 3 of those transmitters, each one with its own set of tuned circuit components to set that single freq required.
that chip CANNOT produce multiple freq's out (other than unwanted spurri and harmonics)
in the way that you want.

your teacher, if a professor, doesnt seem to understand the operation of that stereo modulator/transmitter chip.
look at this drawing I have done its the only way you could produce the 3 different signals
it would be much work to complete and very little power out compared to the TX unit operating on a single freq. Because the TX output is being divided 3 ways.
The mixing of the 3 output signals could produce all sorts of unwanted signals even with the low pass flters in place.
it would ... as I originally suggested .... much much easier and cheaper to use 3 of those units one tuned for each required freq .

8. ### mtr00

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May 27, 2010
The professor is world-renowned in the field of communication...

Anyways,

I understand that it can only produce 1 signal- I am supposed to carry out something like what is drawn in your picture. Does it really matter if there's a bit of noise elsewhere? Assuming I'm tuned into 105,103,101, why would a few extra signals matter? My point is only to be able to transmit the signal properly over 105,103,101, for example.

My sticking point right now though is, how do I make those oscillators?

(Thank you for taking the time to discuss this with me by the way).

Edit*:: Isn't there any way that I'd be able to increase the power?
Also, the amount of work isn't a problem- I'd like to get as much learning experience as I can

Last edited: May 28, 2010
9. ### davennModerator

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Sep 5, 2009

The professor is world-renowned in the field of communication
if he cant figure out the operation of such a basic chip some how I doubt your claim
and if he is such an expert why then isnt he giving you all these answers ??

Does it really matter if there's a bit of noise elsewhere?

YES, of course it does, if you want to upset all your neighbours and have them screaming at you

Edit*:: Isn't there any way that I'd be able to increase the power?

Yes, and then you will be upsetting more than just your immediate neighbours
you could start intereferring with commercial services and get into serious legal trouble

If you build radio transmitters you are responsible for and have an obligation to make sure you transmitter has a clean, spurii and harmonic free output and if it is above a certain power level set by your country's communication authority, then you need to have a licence to use it.

My sticking point right now though is, how do I make those oscillators?

well you need 3 crystals and build 3 separate crystal oscillators as in my diagram
you could use Colpitts Osc's look them up on google and get you "expert Prof. " to help you build them.
you need to buy make a balanced splitter, buy 3 balanced diode mixers, see Mini-Circuits Ltd on the WWW You need to make 3 Low Pass Filters, in a simple Pi format would probably do ... look up RF LPF's on the web

(Thank you for taking the time to discuss this with me by the way).

you are welcome but you must understand with your admitted lack of knowledge you are taking on a serious project. this isnt a "10 minute" (simple) job

Dave
VK2TDN

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
What happens to the difference signal in all of these cases?

11. ### mtr00

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May 27, 2010
Hey Dave,

http://webdocs.cs.ualberta.ca/~schlegel/publications.html

Any lack of understanding is on my part, not his.. He's a super busy guy so that's why I'd like to have a pretty good understanding of the project before I ask him about stuff, or else I'd be at the office a few times a day asking questions

Regarding the oscillator circuits, to choose the values of the elements, do I basically choose any LC combination that yields a 1,3 or 5 MHz frequency by f = 1/(2pi*sqrt(LC))?
How are the resistor values chosen?

As for the diode mixers, do I just buy those or can I build a few multipliers with op amps or something else? I'd prefer to keep things as close to "building from basics" as possible.

Thanks

12. ### mtr00

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May 27, 2010
Is this the signal at 99,97,95? If so, I'd like to use those signals too if they're viable.

If that's not what you're talking about, pie on my face, don't know what the difference signal is, haha.

13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The problem is that you mention using low lass filters to get the 101 MHz signal from the mix of 100 and 1 MHz signals.

Logic suggests that it is a high pass filter you want to extract the higher frequency.

RF is not my area, so I'll just ask the silly question and wait for someone smarter than me to answer

14. ### davennModerator

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Sep 5, 2009
Hi Steve and mtr00

what you really need are band pass filters which have to be narrow enough to try an aviod TX signal mixing but wide enough to allow the 200kc odd FM stereo bandwidth very very difficult when the chosen TX freqs are so close to each other

My block diag was primarily to show a basic way of producing the 3 diff freq's required .... in practice there's a lot of work to be done filling in the details

all the hi pass filters on the output of each stage do is prevent the transmission of harmonics further up the spectrum from each of those freqs.
and yes there is going to be for say 100MHz a 99 and a 101 MHz signal present
as with the other chosen freqs 103 105 etc they all will have their +- 3 and 5 components the mixing products are going to be horrific.

mtr00 again I say to you what you are wanting to do is really really bad .... not dangerous just RF bad ... youare going to produce RF garbage all across the immediate spectrum. and you may find it very difficult to actually get clean signals on the freq's you want because of all the adjecent noise its going to be intermod hell !!!! ....
Intermod ... the mixing of multiple TX freqs to produce yet more and undesirable transmitted freqs.

Why is not transmitting on just 1 freq enough ? what are yo trying to achieve by transmitting on multiple freqs after all you have only one audio source ?

Dave

Last edited: May 29, 2010
15. ### mtr00

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May 27, 2010
Hey Dave,

Like I said it's just a project for fun to learn and get some experience. Even if I can't get it to work cleanly, I'll still be better off for taking a shot at it.

Anyhow, what if we center it at say, 98 and do a 95/9101 and 91/105? Is it realistic to use the negative image at the same time? That way the spacing is a bit better.

Also, I am reposting two earlier questions:
"Regarding the oscillator circuits, to choose the values of the elements, do I basically choose any LC combination that yields an (e.g, 1,3 or 5 MHz) frequency by f = 1/(2pi*sqrt(LC))?
How are the resistor values chosen?

As for the diode mixers, do I just buy those or can I build a few multipliers with op amps or something else? I'd prefer to keep things as close to "building from basics" as possible."