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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 Circuit Design

So, considering all that I’ve learned so far I came up with a circuit design for the complete system. This isn’t going to be a bargain-basement design. My requirement for high accuracy means that top quality components from the likes of Linear Technology and Analog Devices are going to be making an appearance on this board.




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The USB port and power entry

Power is supplied through a USB mini B socket. The potentially noisy 5V line is smoothed using capacitors and a ferrite bead. I know from my previous experience with FPGAs that they can be rather power hungry. If the power consumption goes above 500mA then I’ll need to restrict usage to USB 3 ports or charging plugs.

I’ve opted to hook up the D+ and D- data lines even though I may not use them in the final design. The USBLC6 ESD protection device from ST Micro provides protection against spikes on the line that may occur during the insertion and removal of the plug.



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The sample and reference input stages

The input stage is based around the LTC6957 from Linear Technology. This device is a very low noise buffer and signal distribution device that can accept any sine or logic level less than 2Vp-p and output a fixed logical level.

It’s available in 4 separate versions that can output LVPECL, LVDS or CMOS logic levels. I’ve selected the LTC6957IMS-3 variant that has CMOS output levels and costs about £5 plus tax from Digikey.

Linear Technology produce a very useful design note, DN514, that presents an example input circuit for the LTC6957 that does exactly what I need.

The spark gap, SG2 and ESD protection diode, D2, provide a level of defense against input voltage spikes that could occur during human interaction with the BNC connector. The capacitor C7 AC couples the input signal before it passes through the transformer, T2 and the schottky diodes provide voltage limiting to the inputs of the LTC6957. That’s a very brief summary of the operation. I’d encourage you to read DN514 for the full details as presented by Linear Tech’s engineers.

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