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How Frequency Counter Works? And Build a Nanocounter using an FPGA, STM32 and a Bluetooth Android App

How Frequency Counter Works? And Build a Nanocounter using an FPGA, STM32 and a Bluetooth Androi ...

Calibration

Calibrating the device requires me to use an external frequency standard to measure the actual performance of the onboard TCXO. The easiest way to do this is to connect up the frequency standard to the sample input port.


Measuring the onboard TCXO with the app running on my phone

The frequency standard that I’m using is a GPSDO with a 10MHz output. GPSDOs are very interesting devices and I’ll be publishing reviews and videos on the one that I’ve got in a future article.

Because the frequency standard is known to be as near-as-who-cares to 10MHz the value that you see on the display is the performance of the onboard oscillator. If we rearrange the sample frequency equation given at the start of this article so that it yields the reference frequency before the x20 factor of the PLL we have:



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Comparing the two equations shows that the frequency of the onboard TCXO is 10MHz+(10MHz-displayed value). In other words, it’s 10,000,004Hz at my current room temperature of 21.6°C.

To see how that value of 379ppb compares to the specifications produced by Connor Winfield we need to refer to the datasheet.

Unfortunately I can’t do an accurate calculation because all the reference figures are given at 25°C but I can see that my figure of 379ppb is well within the ±1ppm factory calibration range.

 

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Video and Design Files

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