# Which frequency measurement method is more suitable for RF signals from 200MHz to 915MHz?

I need to build embedded circuit, an RF frequency meter (also known as RF frequency counter) to measure frequencies of remote key fobs. The frequency range I need to measure is 200MHz to approximately 1GHz

I found a good reference, the following book:

George Kamas, Time and frequency user’s manual, May 1977.

In this reference, are described the following methods:

• Direct Measurement
• Prescaling
• Heterodyne Converters
• Transfer Oscillators

I would like to know which of these methods would be more suitable? Is there another method that might be better for the case?

EDITED:

A tolerance of +/- 1MHz would be acceptable. These key fobs operates at different frequencies, for example: 290MHz, 315MHz, 433MHz, 915MHz. I dont need a very accurate measure, but it needs to be sufficient accurate to distingish between the different frequencies. So, a good cost-accuracy relation would be enough.

The user need know the frequency. So, He or she, will press a button on the key fob, point it to the device and the device will inform the frequency of that key fob

• You're not going to direct sample a 1ghz carrier frequency without at least a 2-3 gsps adc. Which I doubt you're prepared to spend 50K on a scope which does this. Commented Aug 31, 2020 at 16:38
• What's important to you? Cost, resolution, size, reading speed, power consumption, operation in a high signal clean environment or off-air low level and polluted signals? There is no 'best' until you're clear what you want, and share that with us. Commented Aug 31, 2020 at 16:42
• @Neil_UK . A tolerance of +/- 1MHz would be acceptable. These key fobs operates at different frequencies, for example: 290MHz, 315MHz, 433MHz, 915MHz. I dont need a very accurate measure, but it needs to be sufficient accurate to distingish between the different frequencies. So, a good cost-accuracy relation would be enough. Commented Aug 31, 2020 at 16:53
• Voting to close as the question is opinion-based. If you can reword the question so that you are asking for information and not opinions, that would be good. Commented Aug 31, 2020 at 16:57
• "Point the fob at the device" implies a wrong approach. If you're going to measure some arbitrary signal over that wide a range, you're going to have to be right next to the pick-up antenna. "Hold the fob against the device and press the button" may work. Commented Aug 31, 2020 at 17:18

Your 43 year old book might be just a little bit behind the times. But in those terms, "prescaling" is your best bet, because it involves the least amount of circuitry that needs to handle the high frequency input.

Since you want to pick the signal out of the air, you'll need an antenna, a buffer amplifier and a "slicer" to convert the RF signal to digital, all running at the carrier frequency. You feed that to a prescaler chip, and then do frequency counting on the output of that chip, probably using a microcontroller that has suitable timer/counter hardware on-board.

Your MCU firmware performs the measurement and formats it for your display device.

• yes, this book is a bit old, but was the more complete reference that I found. At first, the prescaling method seemed to me as the more suitable, however, I'm afraid about noise effects. I never analized the signal right after the antenna of reception, and I don't have equipement to do it either, so, I can not estimate the amount of noise will be present. So I was in doubt if the heterodine converter would be a better approach than prescaling. Commented Aug 31, 2020 at 18:26
• Some key fobs has OOK(On-off keying) modulation. Can it be an issue to measure the frequency using the prescaling approach? Commented Sep 1, 2020 at 12:25
• It can be an issue for any of the approaches. Commented Sep 1, 2020 at 14:53