Reverse Engineering

CMAC R2133 20.48MHz TCVCXO

I have two PCBs from a piece of defunct Nortel equipment with this oscillator. CMAC R2133 Freq 20.48 MHz. Looking at the circuitry around it, there’s a 16-bit DAC (AD7846), a 5V regulator (78L05) and a buffer (74F00). Google could not find me a datasheet, so I removed the oscillator to trace the pinouts (Nortel used the outside layers as groundplane, you will see nearly every trace goes from a component to a via. Makes it difficult).

Pin 1 is 0V, pin 2 is the output, pin 3 is 5V, and pin 16 is the control voltage from the DAC which is configured to supply 0-5V out via the 51R1 resistor R40 and a 47u capacitor C77. The output goes to pin 2 of the 74F00 via 33R2 R53.

I posted the above pic to Facebook, within an hour or two someone sent me the datasheet (well, the Nortel spec sheet, which is basically the datasheet in reverse). It is indeed a TCVCXO, pinouts as I’d already determined (pins 4-15 are N/C), giving -10 to -15 ppm with 0V on the control pin and +10 to +15 ppm with 4V on the control pin.

Stick this thing through an 11 stage binary counter and you get 10 kHz, add a PIC and a GPS and it could be the most accurate thing on your bench.