Marc, Thanks for the info. I also got an email from James Miller and he gave me some other bits of info. What is accurate amongst all the replies I do not know, maybe all of it. ;) >From what I have read, you can put replace a 10544A with a 10811 without any problems (backwards compatible), but the reverse is not always true. Anyhow, James gave this bit of info: In reply to your Time-Nuts question. The smaller PCB was used in early equipment (such as the HP5328A counter), with the 10544A oscillator. It has just the basic elements to support the oscillator; essentially a voltage regulator and a little filtering here and there. The larger board was introduced later and included the above, but also extra stuff to provide: * Automatic detection of external 10 MHz source * Buffered 1 MHz and 10 MHz to the external world. * Support for 10544A and 10811A oscillator (little difference electrically) > Hello group. > > At least one question I can answer ! > Both boards correspond to the A3 board for an HP 5328A counter. > A3 is used for OCXO (option 10.) > > Board 1 is the A3 board for military versions of the 5328A: the AN-USM 459. > It has extra controls for voltage and fine frequency adjust (multiturn pot) > > Board 2 is the A3 board for the regular 5328A, it is described in the 05328-90011 PDF available from the Agilent website. > It is supposed to drive a 10544A, but I have a 5328A with a 10811 connected to it and running OK. > > Hope this helps. > > -Marc-