MARCONI 2440 OCXO UPGRADE |
While using the MARCONI 2440 frequency counter, I noticed the poor stability of the internal time base. Even though I use an external GPSDO as a reference for precise measurements, I preferred to upgrade it by using a much more accurate OCXO.
Thus, I replaced the original 10MHz 44990-418V OCXO with a modern CTI OSC5A2B02 OCXO (easily found at a low price). Of course, other equivalent models can also be used (example: NDK ENE3311B)
To install the new 5V OCXO, an adapter PCB and a switching mode voltage regulator (LMR50410Y5FQDBVRQ1) were required, as the available supply voltage is 10.5V.
The auxiliary 5V voltage regulator IC2 78L05 cannot be used due to its low current capacity and limited power dissipation.
I avoided making any other modifications, the original trimmer for frequency calibration (R25) and its voltage reference (D6 1N825) were left unchanged.
Due to the limited space and the different footprint of the new OCXO, the adapter PCB is interconnected with the frequency counter's main board using a few pins. Pay attention to the pins installed beneath the CTI OCXO. I ultimately used Kapton tape to ensure there is no short circuit with the metal casing of the oscillator.
The performance achieved is excellent, with the new OCXO's warm-up time being approximately >3 minutes. Stability is very good, and I highly recommend this upgrade.
Marconi OCXO adapter schematic
Marconi OCXO adapter PCB (Gerber)
Marconi 2440 OCXO schematic
10MHz OCXO 44990-418V
