A few months ago, I got an HP 8904A Multifunction Synthesizer, from DC to 600kHz (from eBay). Got it for a good price, fully loaded with options. The unit has (2) balanced/floating outputs, (4) internal channels, and can do “noise”, square, ramp, triangle up to 50kHz, and sine waves up to 600kHz. It also has some other “odd” functions like Tone, DTMF, digital and Hop Ram sequences. It will also do AM, FM, Pulse and other type of modulations (some I haven’t even heard of before).
At first glance, this test equipment seems limited (can only do up to 600kHz), but this equipment is not designed for lots of Megahertz of output. This is specially made for audio equipment testing. The floating ground feature alone is not something you’d find in most signal generators. This allows the unit to be referenced to an external ground rather than the chassis ground. This floating ground configuration eliminates ground loops between the HP8904A and the Device Under Test.
The 8904A can also the option of having a 600:600 ohm output transformer installed! Pretty unique!
So basically, the 4 internal channels can be combined together, and the summed output can be sent to any of the 2 outputs. You can get really wild here combining different types of waveform, that you can then use to “hammer” your audio equipment under testing and see if the audio equipment can handle these kinds of difficult signals and still be able to reproduce them faithfully!
In the photo below, the top waveform is the input signal, and bottom waveform is the output signal from my Discrete DOA-12 OpAmp.
My only gripe about this test equipment is the lack of easy-to-adjust knobs, and everything is input driven via numeric keypads. It’s very tedious to setup. It’s a very capable and advanced machine, but the User Interface is difficult for such a complicated machine. If only this machine was designed a few decades later, and controllable via USB and a PC/Mac software, it will be more easier to operate.
I bet you’d be interested what the above waveform sounds like. Here it is… unfortunately, the iPhone video didn’t capture the low end, the harmonics, subtleties and richness of the sound. But this gives you a rough idea.
