How to make a radio transmiter to have a specific frequency?

[ikuu]

I want to have 50 mhz or 500 mhz emitted frequency, I know that the antenna has to be 1/2 1/4 of the wavelenght but i havent found about how you determine de circuit that you need, what frequency does the crystal oscillator frequncy has to be

 

[cowboybob]

Welcome to ETO, ikuu!

I want to have 50 mhz or 500 mhz emitted frequency, I know that the antenna has to be 1/2 1/4 of the wavelenght but i havent found about how you determine de circuit that you need, what frequency does the crystal oscillator frequncy has to be

Your post begs the question; Which came first: the Frequency (transmitter issue) or the Wavelength (antenna issue)?

Build (design) the transmitter first - the frequency that the transmitter oscillates at will be used to determine the antenna design specifications.

Maybe start at your local Library. If they have a copy, check out any edition of The Radio Amateur's Handbook.

 

[unclejed613]

I want to have 50 mhz or 500 mhz emitted frequency

while a physicist is happy if their calculations are accurate to within an order of magnitude, the same isn't true of techies and engineers. we like to get things pinned down a bit closer than that. the wavelength at 50Mhz is 6 meters, and the wavelength at 500Mhz is 60cm. there are a lot of users of the spectrum space between 50 and 500Mhz.

there are sections of spectrum that are used for specific purposes and services, and these are allocated by international agreements, so your choice of operating frequency will be determined by what you want your device to do. for instance, making a RADAR that operates at 100Mhz would be a bad idea because that's in the middle of the FM broadcast band. so, the operating frequency of your transmitter can't be determined at random. in many cases, you will need some form of license for your transmitter, and there will be certain technical standards the transmitter needs to adhere to (frequency stability, bandwidth, elimination of out-of-band signals, etc...). if you get an amateur radio license, you get a bit more leeway of the types of experimental systems you can build and test (as long as they are not for business purposes), and there are quite a few portions of spectrum open for amateur radio in the 50-500Mhz range.

it would make things easier if you told us what you want to make. we could give you advice and info better suited for your purposes. without more specifics, the answers you get will be a bit of a "shotgun" effect.

 

[RadioRon]

This is a big question. There are entire books that address it, but let's try a summary. Typically, you create an RF signal by first considering what kind of modulation you want and what frequency you will transmit on. These two factors will tell you what frequency stability you need, which often leads to the decision to have crystal control. Crystal oscillators can be designed to operate up to a few hundred MHz but when the frequency gets too high, the crystal becomes impossible to manufacture. There are two ways that we commonly get crystal stability at very high frequencies. The most popular method now is to use a phase locked loop synthesizer. The other way is to use frequency multipliers. Which one is the best approach depends on what kind of modulation you will have. If your frequency is relatively low, like below 100 MHz, then it is feasible to set your crystal oscillator frequency at the same as your transmitter output frequency, which eliminates the need for synthesizers or frequency multipliers.

So far, we've talked about how to generate a carrier frequency. That isn't much use unless you can modulate it with information. The techniques for generating modulation are very different depending on the modulation type. For analog FM, you can modulate your crystal oscillator or your frequency synthesizer directly. For phase modulation, you often use a mixer or upconverter. For multicarrier (like OFDM), you would probably use a DSP with A/D converter to generate the signal. So, it depends. What kind of modulation do you want?

In the real world, the transmit frequency choice is driven by government regulations. The government determines what ranges of transmit frequencies are to be used for specific services, so we have to consult the rules to find out what frequency our transmitter will be allowed to transmit on. This begs the question...why is your transmit frequency going to be 50 MHz or 500 MHz?

 

[davenn]

I want to have 50 mhz or 500 mhz emitted frequency,

This begs the question...why is your transmit frequency going to be 50 MHz or 500 MHz?

and it also begs the usual question ... Do you have licence(s) to transmit on these frequencies ?

 

[ikuu]

i dont have license , but the frequency is not important it doesnt have to be one which is regulated,

 

[alec_t]

but the frequency is not important

Yes it is. Harmonics of the generated frequency may well be present and fall in a range which requires a licence.

 

[unclejed613]

i dont have license , but the frequency is not important it doesnt have to be one which is regulated,

well, it is important, since spectrum space is managed based on international treaties and agreements. countries follow those treaties and guidelines, and depending on the government, any given country can have rules that are almost nonexistent (a bare minimum to meet treaty obligations) to downright draconian (we detected an unauthorized transmission coming from your house at 0.5 microvolt per meter, so we're going to smash your equipment and put you in jail for 10 years), and everything in between. you may be in one of those places where radio is the "wild west" where they don't care unless you are interfering with government radio links. you really should try to find out first.... an as alec_t mentioned, if you are emitting harmonics or intermod on other frequencies than what you are allowed to use, that's usually treated as a violation of the rules in any country. emitting unwanted signals in the aero band in the US is what they usually double fines for when they catch somebody running an unlicensed FM radio station in the US.

 

[audioguru]

When I was a teenager the RF cops caught me with a badly sparking fan motor that caused radio and TV interference to many people. I was lucky they did not jail me, instead they bought me a new fan.

A few years later I designed and tested an FM transmitter. Most of the FM band was used by radio stations in my city so I used the frequency or a foreign language station that hardly anybody listens to.