How do you multiply the frequency of an oscillator?

the_emi_guy

Exactly which HAM band are you targeting?

Why not use 10meters (28MHz)?

tackyattack

1.25 meters. I would use 10 meters, but my receiver can only receive between 52 - 2200 MHz.

the_emi_guy

Your balloon is TX only right? So on the ground you just need a receiver.

You can get a 10meter receiver kit for $50.

Plus, it is much easier to work at 28MHz vs. 222MHz (less worries about layout, parasitics etc.).

Since it is a balloon the antenna can be pretty long.

tackyattack

I already have a setup that I put money into though. :(
Would a $50 kit even be enough to receive low power signals from that far?

the_emi_guy

What is the receive sensitivity of your receiver at 52MHz?

I'm guessing that even cheap kits give you < 1uV at 28MHz.

(Do you still have the receipts?)

davenn

strangely enough the system on the page he linked to uses 10 metre band ... 29.xxx MHz
with an incredibly simple transmitter circuit


tackyattack --- surely one of your mates has a shortwave receiver
you are not the only ham in the area are you? dont you have an HF transceiver ?


Dave

tackyattack

I'm a 16 year old teen, my friends aren't exactly into that kind of hobby. The closest ham group to me is too far. Actually, I'm using an SDR as my receiver. That's why the frequency range isn't too fantastic.

davenn

OK, no probs :)

then Its good you are giving things a try, but be aware of biting off more than you can chew.
Building a VHF or UHF transmitter that has a clean output isnt a first timer starter project
Building a receiver for 29MHz would be much easier. and wouldnt cause hassles to other band users if things didnt work properly :)

Dave

vk6kro

This might be a good time to mention that most countries have pretty strict rules about transmitting.

You don't get to choose which part of the spectrum you want to transmit on.

Even if you have a Ham licence, this would be classed as an unattended beacon and it would need to meet some strict conditions dealing with unwanted emissions.

This may vary with where you live, but you should check on your local rules. Penalties in most countries include serious fines or time in jail. So it isn't worth the trouble you could get in to.

berkeman

@tackyattack -- vk6kro brings up some good points. Also, what are you planning on broadcasting from your module? Will it be telemetry or slow-scan video, or both? How often will you broadcast your callsign from the module?

BTW, I think it is great that you are working on such an ambitious project. If you manage to get it to work (within the legal issues), just think of the kinds of projects you will be getting to work during your college years!

tackyattack

Thanks, that makes sense! I guess I should probably stick with pre-made transmitters for the VHF and beyond frequencies. It's too bad building a disposable VHF module is nearly impossible! Maybe once I have some low frequency transmitter experience under my belt I can safely move up to the higher bands. Or maybe I should just get a lower frequency receiver ;)

Right. I'll make sure to look into the laws for my areas more carefully. I have learned a bit about them for my type of project, but probably not enough. Thanks for the warning! I do have one question: isn't there a point above the earth that radio laws no longer apply? There has to be some height where they are no longer valid because it's not like there are laws for radio communications on the moon. Right?

I plan on transmitting temperature and pressure telemetry through a kind of OOK binary transmission. As far as my callsign, I plan on broadcasting it at the end of each transmission. That would be the easiest.

Thank you for your kind words! I am very excited for college!

Enthalpy

This is the standard way to build a frequency doubler:
http://electriciantraining.tpub.com/...s/14181_95.htm
put two of them to quadruple the frequency.

A PLL does it also, but for a fixed ratio I feel it overkill.

tackyattack

Wow, that is very simple! So adding more than two would cause the signal to diminish to nearly nothing. But, what if you amplified before adding into the next multiplier? Would there be a point that too much noise is introduced?

davenn

the transistor in the doubler cct will produce some amplification

the cct shown is a pretty standard way of what is used in a multi stage multiplier
when starting with a low freq crystal oscillator.
for the 144MHz ham band the crystal freq was commonly 12MHz tripled to 36MHz doubled to 72MHz and finally doubled again to 144MHz

Dave

berkeman

So for that circuit, you put in a big enough signal to get the transistor to conduct on the + half cycles, and it's cut off for the - half cycle? And that drives the resonant circuit to ring at its tuned 2x frequency? Interesting.

Enthalpy

That's it. The circuit exploits the transistor's nonlinearity.

Vbe's threshold would reduce the conduction angle below 180°, making the circuit more efficient at producing 2nd and 3rd harmonic, but the transistor's limited speed makes the waveform smoother and less harmonic-rich.

I used it up to the 5th harmonic, but this is exaggerated. The main limit is the selectivity, hence accuracy, of the output filter. A multistage filter gets any desired selectivity, but only if it pick the proper harmonic under any circumstance. As a doubler or tripler, two LC are a good value, then Q=10 uses to be more than enough.

Before chips were available, PLL were too complicated hence uncommon, and this circuit was standard practice. Still nicely simple, and it introduces less phase noise than a PLL.

Symmetric circuits, with two transistors, reduce the odd or even harmonics.

davenn

The other common way for freq multiplication these days is to use MMIC devices like MAR or ERA etc series

You drive the MMIC hard from the initial osc cct to give a good rich harmonic output
filter this output with a bandpass filter to isolate the wanted harmonic, before going into the next multiplier stage.
A number of amateur transverters use this scheme eg the Downeast Microwave transverters

cheers
Dave