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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 ...

The LTC6957 comes with 3 selectable narrowband filter options of 500MHz, 160MHz and 50MHz. The best choice will depend on the input signal so I’ve opted to connect the filter pins to MCU GPIO pins so I can offer the choice of filter through the user interface.


LP5907

I’m powering the LTC6957 from an LP5907 ultra-low noise LDO regulator from Texas Instruments.

The input stage in the illustration is the sample input. The external reference input is basically identical. The only difference is that I’ve connected the LTC6957 output shutdown pin to an MCU GPIO pin so that I can shutdown the output when the onboard reference is being used.

The onboard reference oscillator

The stability and tolerance of the reference clock is key to the accuracy of the frequency measurements. Clearly I need something more accurate than the crystals that I seek to measure which means in practice that I need to use either a temperature compensated oscillator (TCXO) or an ovenised oscillator (OCXO).

The price of these devices is in direct proportion to their stability and they get quite expensive really quickly. Since I’m providing the ability to supply an external reference when exceptional accuracy is required I’ve decided to go for a reasonably high end TCXO, the Connor Winfield M100F, which has a stability of ±100ppb and costs about £14 plus tax from Digikey.



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More stable TCXOs exist, and then there are the very stable OCXOs but they’re beyond the scope of my requirements for this article. Cheap used OCXOs and VCOCXOs abound on ebay but I’d be very wary of them since the reason they’ve been decommissioned is that their age means that they could have drifted out of spec and for the VCOCXOs that means they could have drifted out of the range of voltage correction.


Connor Winfield M100F

The TCXO works by applying compensation to the output frequency based on a curve of temperature vs. offset that’s preprogrammed at the factory. A TCXO will still drift with change in temperature but it will do so at a much lower rate than a standard crystal (we’ll have some fun demonstrating this in the videos that accompany this article). Better immunity to ambient temperature changes can be had with an OCXO but they’re really expensive from the usual distributors —.

The datasheet for the M100 specifies an output load capacitance of 15pF. Deviation from this value could alter the output frequency by up to 20ppb per pF. I consulted the datasheet for my FPGA and found an input capacitance range of 3-10pF for input pins with no typical value specified. Hoping that it’ll be somewhere around 5pF I added C25 to the output to provide the additional load. It doesn’t matter too much if I’m off by a bit because the error is a constant and can be compensated for in calibration.

 

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The PLL

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