HP 8904A Review

I recently acquired two new function generators, one being the HP 8904A. As there is not a lot of consumer information on the net regarding the HP 8904A, at least that I could find, I thought it would be reasonable to start with that.

There are several options available for this unit and Option 001 provides 3 additional "internal channels" - B, C and D - that can modulate or sum with eachother to create unique outputs. For example, here Channel B is a 10 kHz sine wave which modulates Channel A - a 100 kHz sine wave - to output an AM signal (top).

Option 002 provides a second output which you can pipe your various channels out to. Only output one can be modulated, but you can "sum" signals to output 2 (bottom trace, above). I believe that adding option 002 adds at least a second channel as it would be useless without it. Looking inside you can see that option 002 actually requires a second board and so is not actually a software option.

As a stock unit the HP 8904A does not seem very impressive in my opinion. Without any options it is merely a 0-600kHz sine or 0-50kHz square, ramp and triangle generator. The floating output is a major feature, but if it's not something you need then a stock HP 8904A really does look rather ho-hum. My one major complaint though, is that it is not able to generate a square wave with any duty cycle other than 50% with a single channel. You must sum two channels and vary their phase to create a non-50% duty cycle. Fortunately my unit has both options 001 and 002.

Above is an image of an AD565AJD chip on one of the output boards, it is a 12-bit D/A converter and I think it is reasonable to assume that it is one of the major work-horses in the function generator. I really love these ceramic / gold packages, just because they look so cool! Below is a picture of the top board. I haven't gone through all the ICs, but I would guess mostly processor and ram stuff.

There is a lithium battery here, most likely used to store settings and operating state in RAM. It seems to be working as my settings have been retained although irritatingly a "special" function had to be changed so that it would power on to the last used state. It seems the battery may have leaked so I may replace it and clean the board.

Overall I'm very happy with my HP 8904A. I don't think I would have gone for it without any options, and especially not at the original price, but with the options 001 and 002 it is a very capable unit. Major drawbacks are the RAM based memory and lack of single channel duty cycle. The only other complaint I have with it is that it is very "menu driven" and can be tedious and non-intuitive to set up exactly the signal you want. I wouldn't want to use it for example to make a bode-plot necessarily, but again all in all I think it is a very niece piece of test equipment.

For more information you can Google search: Keysight 8904A to find their scans of the original documentation (Keysight is formerly Agilent is formerly HP aka Hewlett-Packard).