A modern frequency reference for an ELV FC-500 frequency counter #EE
Petteri Aimonen looked to update a 20 year old ELV FC-500 frequency counter made out of a kit.
The original design uses a basic 4.096 MHz clock crystal, combined with a trimmer capacitor. The capacitor allows adjusting the meter using a known reference source. However there is no temperature compensation for the crystal, which has about 0.2 ppm/°C slope. My FC-500 had a few ppm of inaccuracy in room temperature before this project started.
Temperature compensated crystal oscillators are a reasonable alternative. For a device like this, you want a VCTCXO: VC for voltage controlled, TC for temperature compensated, XO for crystal oscillator. Voltage control allows adjusting the oscillator for initial calibration. Temperature compensation should keep it accurate after that.
A complication is that VCTCXOs with 4.096 MHz frequency are quite rare. Fortunately 16.384 MHz is better available, and a divide-by-four circuit can be built with two flipflops.
As possible options, Petteri identified the following parts:
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