72mhz frequency synthesizer

[bearcreek]

I would like to modify my radio transmitter to be able to quickly switch between many frequency channels in a fraction of a second. I have some old transmitters with simple circuitry (no IC's) and I can understand some of the oscillation circuitry by looking at it, and I have some newer TX's with IC's, but those may be more difficult to change. Could I put a variable voltage source or variable capacitor in the circuit or does anyone know of any RF synthesizers that could be used to quickly switch channels?

[Nikolash]

Such modification is Illegal in US

For newer tx
I dont know how long does need futaba synth module to settle frequency , after turning dials, but dials could be easyly bridged to make close to fingers.

for older tx
Another minimal efort option is to have bunch of tx Xtals plugged in a matrix and than engage one you like by turning dial (like on voltmeter)
You can try with only 2 xtals to proove concept an than upgrade to more.

All of those solution are crude, great chances of Rf noise, but simple to perform as experiment.

[vintage1]

Ther is actually a simpler way..you simply build a 72/3Mhz-ish VCO , divide down by a preset number and phase lock to a stable Xtal - probably 1Mhz or so.

If you first divide the 1mhz down to 3.3khz, and use a 24Mhz oscillator divided by a variable amount to compare and lock to that, then a frequency tripler and an output stage.

Its not trivial, but its doable with freq counter test gear, a scope and some fairly high speed logic.

Probably need a spectrum analyser test to set up final tuning for street legal performance.

[pmackenzie]

FWIW, the JR synth module requires you to cycle the power before it will change frequencies. Futaba might be the same.


I have to ask, what is the possible application of such a device?

About the only one I can think of is to deliberately interfere with other users.

Not sure any more help should be provided unless you are willing to divulge details of your application.

Pat MacKenzie

[bearcreek]

I am trying to build a vehicle that would use extra channels without having to supply many crystals. I would not want to cause any crashes. There are some DDS (direct digital synthesizer) IC's that can be set with a microcontroller. Also I have tried removing the crystal from my old radio and tune it to the receiver frequency with variable capacitors with no luck yet, but maybe I should get a frequency counter to check it.

[mem]

It’s still not clear what problem you are trying to solve. Assuming you want to control more servos, it may be easier to modify the Tx so that it transmits more channels (ie PWM pulses within the 20ms frame) then to change transmit frequencies on the fly.

[Hugh P]

I do lots of electronic design and have all the test gear you would ever need to do your project.

However when I wanted to update an old Royal Omega TX, hey it's one of the first protypes I got before it was sold as a kit, real metal gimbles with ball bearings etc. I simply bought a HiTech 72 synth transmitter module $70ish at LHS. Also got a HiTech FM synth receiver for about the same money.

Pulled the old AM TX deck out.

Ten minutes work installing a new plug header on the existing cable from the encoder and plug it in to the new FM module, solder a wire to the existing antenna, set the channel and power up.

Job done after I mounted the module to the case with Dubro velcro.

It just wasn't worth the time to do a design and build when off the shelf was available.

Hugh

[hadihf]

It is just fun making one of your own. It is also a good learning..

This one is located at www.mstar2k.com

Regards