I was planing the frequency counter basically to be modular.
So I had some idea in my head for accessories:
1. Precision timebase Actually I'm using a calibrated but not thermally compensated simple 4.194MHz XTAL oscillator. This needs some uplift in frequency anyhow (I tried it but I've failed on the breadboard),
and as an option I would need an OCXO timebase. I bought OCXO for this already:
In addition to the OCXO this accessory requires a frequency divider to 1Hz. It came to my mind to create the divider from an MCU (an MSP430G2230 or an NXP LPC810) instead of discrete logic. This solution has an additional advantage, that I can put a digital thermal sensor IC under the OCXO case (if I choose the MSP430G2230, it can be put under the OCXO itself because of the integrated thermometer) and an additional LED to check if the OCXO achieved the operating temperature.
A few month ago I bought a Xilinx CPLD development board.
My goal was to learn using the programmable logic ecosystem (CPLD, FPGA, etc.). My second goal with it to create a prescaler for the frequency counter.
If I understand correctly this series of CPLD is able to handle signals up to 400MHz (maybe wrong) and the smallest version with 32 macrocells cost around €1.
3. Input circuit
I continuously feel the absence of a good input circuit. I was searching around the internet but haven't found solution (maybe just because I didn't built any of the available schematics). It is still fuzzy to me what would be the requirements.
The only thought what I have about it to pick a high speed integrated comparator and add some input protection, a shortable DC decoupling capacitor and a rail-to-ral variable threshold level to it.
4.PSUI need a PSU (may a directly mains connected one) for all of above. Not all of the listed things (the OCXO for sure, the input circuit may) are able to work from a single 3.3V. This makes it necessary to have a multi-output PSU.
I was thinking to put all of the accessories one by one on PCBs equally sized to the frequency counter itself. This can be modularly connected on top of each other to the counter.