changing transmit freq

Thread Starter


Joined Jun 28, 2023
This is with an inexpensive driveway alarm. The xmtr is at 433 Mhz. If I changed the crystal to 13.2256 Mhz would it likely accept that and still work or am I going too far with the change in freq for the circuit?


Joined Jan 23, 2018
Several questions, the first one: 433.???,??? and is that crystal controlled? Then the receiver for that same selected frequency may also be crystal controlled. The but the actual crystal may be marked "433" but the actual crystal frequency is much lower, with multiplier stages between. I am guessing that the receiver is also crystal controlled, but maybe not.
So another question is just what frequency do you want to use? And why??


Joined Jan 27, 2019
Welcome to AAC.

As @MrChips said, it will not work but if it did you’d be illegally transmitting on whatever frequency it ended up on.

The device uses 433MHz because that’s a band that international treaties (the ITU) and federal regulators (the FCC) have allocated for unlicensed operation of low powered transmitters for short, infrequent remote control signals. There is another band the device might have used and that’s 27MHz which has some overlap in regulations.

But, that’s irrelevant since the regulations also prohibit modifying the transmitter in any way. Once that’s done, it becomes illegal to operate on any band. So, it really doesn’t matter if it works or not—preactially.

Still, it’s good to know why it wouldn’t work. In a nutshell, the crystal in a crystal oscillator does keep it on frequency but to do that the circuit it is part of must be resonant at that frequency. A tank circuit is a resonant circuit made of a capacitance and an inductance that naturally responds to a particular frequency due to reactance increasing with frequency in an inductor and decreasing in a capacitor.

When the frequency is just right, the reactances are matched and the circuit resonates. An oscillator can be more complex that a simple tank but the idea is the same. For the crystal to “work”, the circuit in which it is placed has to have components whose values allow them to resonate at the crystal’s frequency.

It’s not just a matter of them being “close”. If you had a 433MHz crystal controlled oscillator and wanted to put a 400MHz crystal in its place, you would still have to change components though not entire PCB layouts as you would for 433MHz --> 13MHz would require.

So consider resonant circuits as very delicate when it comes to components and layouts. A variable capacitor can make an oscillator move a little, but the more the leeway in making such changes the worse the oscillator is at whatever it is set to.


Joined Jan 23, 2018
No. It will not work.
Changing the transmit frequency to 13.2256 Mhz will not be possible without as complete rework and rebuild of the transmitter. And why that particular freequency? That would seem to be the frequency prior to multiplication. So what is the actual intended frequency to be transmitted???