HP3325A Synthesized Function Generator


The 3325A was one if not the first commercial available Function Generator based on the Fractional-N technology. Developed around the end of the 1970s its operation is controlled fully digitial, it has an simple HP-IB Interface (GP-IB, but HP called it different at least at the beginning) and is for example able to do the following:

There are also two available options which my unit do not have. Option 1 is a high stability frequency reference, and Option 2 is the high voltage output. This high voltage output allows to drive a signal from 4mVp-p up to 40Vp-p into a load of > 500 ohms and < 500pF, but with the downside that the maximum possible signal frequency is now 1 MHz for sine and square wave.



General weak points or drawbacks of the 3325A seem to be

The pictures below show the top side of the unit with the power supply PCB, the control board (largest board on topside) and the VCO board (middle picture). Changing the mains voltage setting for Europe was very easy, just two switch positions had to be set in the correct way. Other than that the conversion only consisted in changing the main fuse to the correct rating for 240V mains voltage. The mains voltage selection switches are located on the power supply PCB which is shown below on the right most picture.



The operation of the 3325a is controlled by the control board on the top with lots of different digital devices. Here we find also the HP part "1820-1691", the so called nanoprocessor, a microprozessor specificly designed and used in some products by HP from this time. Interesting about this prozessor is that it is a NMOS device, and that it also needs a negative supply voltage for proper operation. I was reading in the internet that the first nanoprozessors which were produced had a higher production variance of the needed negative supply voltage. Therefore each produced device was handcharacterized and then the correct negative supply voltage for each device was written down by hand on the package. Only later HP improved the production process and the correct negative voltage was stabilized to be -5.0V. This information could be useful for someone that has to change the prozessor...it could be not enough to simply swap the prozessor. If the new prozessor needs a different negative supply voltage then the old one than a resistive divider in the unit has to be adjusted. My unit seems to be a newer one, because my prozessor has no handwritten note for the correct voltage on it anymore, and the -5.0V are already printed with the part name onto the device package.



If the 3325A is flipped on its other side then more PCBs can be found (see picture above on the right). The bottom half of the instrument contains the mixer board (left picture), the attenuator PCB (picture in the middle), and the largest PCB which is the output board with the final output stage circuitry (right picture).



I really like my 3325A, but I am also sure that it is not the right instrument for everybody. If somebody prefers more functions and a newly built unit then maybe it is a better idea to get something new. But I like the HP built quality, the construction and I also like the fact that my generator is most likely more than 30 years old and still works. Of course it is a risk that something old like this can break any day, but I am also not sure if something built today will even work anymore after that lifespan. An if it breaks I still have the full service-manual with all circuit diagrams, and probably the chance to fix it, if not a custom HP part breaks down. I hope my 3325A will still have a lot of life time left in it.