What is it?

GPS trained super high precision frequency counter. All the operation is trained by atomic clock of GPS system.

Why did you make it?

Recently I got a programmable crystal oscillator, but actually I cannot evaluate its frequency accuracy without reliable frequency standard, for example recently calibrated rubidium oscillator. Second-hand rubidium can be found even in eBay but rubidium oscillator has "aging" and need to be calibrated if we need "exact" frequency source. (Of course, even after aging, it seems still they has a good accuracy though..). In addition, several hundreds dollars is not easy pay for "hobby", and I am willing to get if it is less than $50! Very recently I got to know u-blox's GPS module can generate "GPS trained" signal more than 1Hz, even up to 10MHz! Indeed 10MHz is enough to drive AVR Atmega328 and if Atmega328 is driven by the GPS trained exact clock, and if executing frequency counter program, we will get "frequency counter with very high accuracy"!! Yeah, this is the background and objective of this item!

What makes it special?

• All-in-one high precision frequency counter.

• MCU is popular ATmega328 with Arduino bootloader (Pro/Mini 3.3V/8MHz with fuse bit setting to external oscillator).

• On-board USB-UART interface (CP2102N) enables GPS monitoring or New firmware uploading through USB.

• On-board Lipo battery charger provides portable application. Power source is selectable (Lipo battery or USB bus power).

• Compact active GPS antenna (with embedded 2-stage LNA) is included in the item.

• Firmware is open source, you can find it at my GitHub repository.

• Onboard crystal is prepared, for accuracy comparison with GPS or non GPS signal available situation.

• Maximum input frequency is 4MHz. Please consider to attach proper pre-scaler for higher frequency measurement.

Two SMA connectors are GPS antenna and frequency counter input, connecting to PD5 of ATmega328 through single gate inverter element. Rotary encoder change gate time for frequency measurement. Coin battery (CR1220) keeps setting of NEO6M (8MHz clock generation..). The RED toggle switch can select the connection between CP2102N to NEO6M or ATmega328, which enables GPS monitor or Arduino sketch upload through USB. As same as most of my board, onboard Lipo battery charger is prepared. The Yellow toggle switch change the clock source of ATmega328, on-board 8MHz crystal or GPS trained signal. Actually NEO6M generate 8MHz even before GPS locking (with low precision) and after GPS clock capturing, it goes to atomic clock precision!.

What is included in this item

• Fully completed and tested frequency counter board

• Two stage LNA embedded active GPS antenna (3m cable, IP67, embedded amplifier: MAX2659 two stage, Gain 28dB / NF <1.5dB)

What is NOT included in this item

• Lipo battery. It is not essential for operation for USB bus power mode but you may need it for portable application