How to make a simple FM modulator?

[SsZERO]

Ok, I know I'm not the ONLY guy here who plays with R/C cars. I'm not talking about the ones you get at Toys R Us, I'm talking about "hobby grade" R/C, like the ones made by HPI (www.hpiracing.com) and whatnot.

Now the radio and receiver for these RC cars use a crystal to modulate a 75.xxx MHz FM frequency. IT is possible to pop these crystals out and put a new one in to change the channel, so as not to interfere with other peoples' cars. Now the crystal kit costs like $30...so I'm thinking, there has to be a RadioShack way to make a pair of adjustable tuners that replace the fixed freq. crystal -- one for the Rx and one for the Tx.

What parts would I need from the shack to make something like this, and how would I go about assembling it? It would only need to tune RC freqs on the 75 Mhz band. I'd also like this to be a high quality deal, so if there is a place on the web that sells better quality stuff, then by all means, point the way.

I'd also want this to be small, so the tuning would not be done by a big dial, rather it would use two small screw-type adjusters with 10 positions (0-9). Setting one to 3 and the other to 6 would be channel 36.

-= SsZERO =-

 

[blahblah99]

I think the easiest and cheapest way to go is to just go out and buy the crystals yourself and replace them. They are like $2 each on digikey. How often are you going to be changing your carrier frequency?

 

[Superdoopercooper]

<< I think the easiest and cheapest way to go is to just go out and buy the crystals yourself and replace them. They are like $2 each on digikey. How often are you going to be changing your carrier frequency? >>



The reason why the crystal sets from Futabe/Airtronics/Hi-tech are so expensive is because they are MATCHED. If you order two randome 75.235MHz crystals from Digi-Key, you may not get a good communication between radio and receiver unless you know that BOTH crystal oscillate in the target circuits at the same frequency. Of course, there can be some margin of error, but just Crystal A and Crystal B picked from the same box of 75.XX MHz parts may not cut it. You can try... at least it'd only be a $5 mistake if it doesn't.

 

[SsZERO]

Well, I still think it would be better to make the synthesized tuner. I don't change my channel often, but I'd like to be able to do it quick and easy if necessary. Also, if those $5 crystals crap out while I'm driving, my car might end up blasting off at full throttle into some place it shouldn't...and now this $5 experiment has turned into a $500 experiment.

-= SsZERO =-

 

[Superdoopercooper]

SsZero,

the way to TEST the $5 experiment would be to power the reciever with an external 4.8-6V pack... and disconnect the BEC circuitry/Racing Pack. If your steering servo is responsive up to 100 feet away or whatever with the antenna down, you should be FINE. Just make sure you are relatively glitch free.

I'm not an FM expert... but making a synthesizer isn't as easy as you think. We had some students try to do some FM stuff in the lab class I TA'd in collge, and it turned out less than stellar. Of course they were using proto-kits, and not a nicely laid-out copper clad board.

Good luck to you on this.... I think I'd just do the $5 experiment... or drop the $30 on the Futaba crystals. I'm an EE, and rarely do I fart around with my own circuitry. Although that will probably change when I get more time and desire to tinker outside of work, esp. now that I work for a microcontroller company... and I'm learning all the nifty stuff I can do with them.

 

[Superdoopercooper]

I read your original post again... and now I see what you want to do... You want a user selectable frequency generating thingy. I somehow missed your last two paragraphs the first time I read this thread. Serves me right for scimming.

I still think it will be difficult and hard to match the receiver to the transmitter (if you only needed one... I would whip you up a circuit that should work, but since you need a TX/RX pair that have matched frequencies... that is tougher).

Try to PM a dude named "pm". He works for intel and seems to be wicked smart... he is an IC designer but probably knows about phase-locked loops or something that would do the trick for you. I just don't have that experience. I think a PLL is probably the way you'd want to go. Sounds like and interesting project.

A voltage controlled oscillator would probably work... but not sure it'll be easy to get up to 75MHz. I used a chip about 6 years ago that was fairly accurate and worked up to 10MHz. I'm sure it was some PLL based kind of thing... don't remember now though.

 

[SsZERO]

Well, wouldn't this basically work like a tuner in a stereo, just a bit more precise? I'm thinking that a PLL would be needed to synthesize the frequency...and then there would be two adustable oscillators to control the frequency. The actual receiver itself is about 1 inch by 1.5 inches in size, so I don't think the tuner would need ot be much bigger. I'll go check out some stuff on the web.

-= SsZERO =-

 

[davesaudio]

it has to be PLL to achieve any level of stability
If you don't find an off the shelf PLL with the required frequency divider/multipliers built in -I think you could end
up with a substantial chipcount.
in the old TTL days I seem to remember this took about a dozen TTL chips(hows that for dating myself)

 

[schmedy]

A way to do it and keep it stable is to have to have a base crystal of 30mhz and and variable circuit to multiply it by to get the desired frequency, with an AFC loop back to keep it on freq. Prob more then you want to do and/or can get parts for, I have them here at work andnot too hard to make, but how much space do you have to work with in a RC car? And if you are doing all this you should have a good counter to make sure you tune them right as well as a communications test set for injecting signals and testing power out, both of wich are way more then 30 bucks for a pair of crystals.

 

[Jerboy]

<< Ok, I know I'm not the ONLY guy here who plays with R/C cars. I'm not talking about the ones you get at Toys R Us, I'm talking about "hobby grade" R/C, like the ones made by HPI (www.hpiracing.com) and whatnot.

Now the radio and receiver for these RC cars use a crystal to modulate a 75.xxx MHz FM frequency. IT is possible to pop these crystals out and put a new one in to change the channel, so as not to interfere with other peoples' cars. Now the crystal kit costs like $30...so I'm thinking, there has to be a RadioShack way to make a pair of adjustable tuners that replace the fixed freq. crystal -- one for the Rx and one for the Tx.

What parts would I need from the shack to make something like this, and how would I go about assembling it? It would only need to tune RC freqs on the 75 Mhz band. I'd also like this to be a high quality deal, so if there is a place on the web that sells better quality stuff, then by all means, point the way.

I'd also want this to be small, so the tuning would not be done by a big dial, rather it would use two small screw-type adjusters with 10 positions (0-9). Setting one to 3 and the other to 6 would be channel 36.

-= SsZERO =- >>



IIRC, you need to change the crystal to match and even then, you can only change a few channels without having to send the units in for factory adjustment. I wouldn't tamper with it anyhow. If it gets glitchy you're risking serious damage to expensive model cars and AMA(Academy of Model Aeronautics, provides liability insurance for all kinds of R/C accidents) denies your claim if your radio system has been tampered with.

 

[Tiroloco2000]

Try http://hem2.passagen.se/sm0vpo/conv/syn-info.htm (sorry, don't know how to linkify, so just go and do the old cut and paste). This page has a nice toturial on synthesizers and other communication stuff. If you can make your own synthesizer.. cool! good for you. I'd just buy the right crystals at $30 they are worth it if your toys run at $500 or more.