Making Waves

Right now my function generator only makes wave. As in singular, not plural. This is because it has only one output hooked up to only one DAC board.

But I'd like it to make waves, plural, and there's room on the front panel for a second set of outputs, and room inside for a second DAC board. So I got to wondering: can I simply install another DAC board to enable the second set of outputs? The internet holds no clues on this matter, and it's not addressed in any detail in the service manual.

So let's find out the fun way!

For the record, this board is HP part number 08904-60203.

And it's supposed to have a shield on the back but for some reason eBay scrappers always seem to pull it off and discard it, but oh well it'll be fine for my uses.

Anyway, as we can see it's identical to the one I already have.

So let's stick it in and see what happens.

Now I don't (yet) have a second pair of SMC to BNC cables, but the "Low" output is actually just a floating ground that you can bypass to chassis ground, so it's not essential for most of the uses I have, and certainly not essential for testing out if the two boards will work together, so I can temporarily reroute it to the second board.

The other connection I need is a 12 inch 34-pin ribbon cable, which is very easy to source.

Ideally this cable would have one of the connectors flipped around to the opposite side so that the ribbon cable exits the other side of the connector, but it'll work just fine as-is.

Routing this cable is probably the most difficult part of the install. I had to loosen both the DAC board and the main board on the other side of the chassis and then wiggle the cable in from the mainboard side.

But I got it in there.

Then it's just a matter of plugging the board in, snapping it into place on the standoffs, and we're ready to tickle it with electrons and see if it'll dance.

So good news off the bat, without any extra configuration it lets me now select Out2 as a destination.

And if I take a peek at the scope...

It is, in fact, making waves (plural)! Perfect.

Now this does leave one minor issue, which is that I don't have any 19mm spacers to go between the boards to prevent them from jiggling loose and shorting out against each other.

But then, I do have a 3d printer.

Uh, excuse me a moment...

Right, as I was saying, I do have a 3d printer.

So let's make it squirt out some plastic.

So why did I make these spacers hexagonal? Well the answer is simple: hexagons are bestagons.

Anyway, I'll trim away the brim and slide them down over the standoffs.

And then the second DAC board snaps into place without any danger of contacting the first.

I do still need to get a second pair of SMC to BNC cables but I can get to that whenever I feel like it. In the meantime, I'm calling this upgrade a success!

Feet Pics

In a previous blog post I introduced the function generator that I recently picked up. Having been formerly used in a rack mount setup, it didn't have any feet, and so was sitting directly on its bare chassis on the overhead shelf of my workbench.

This isn't especially ideal given that there's cooling vents on the bottom of the unit, so I ordered up some feet to lift it back up to its proper height.

They just snap into the trio of holes on the bottom of the unit.

And now the function generator is sitting much nicer on the shelf with plenty of air space underneath.

I'm still contemplating one other upgrade for this unit, but that will have to wait for a future blog post, depending on how things work out.

Generating Functions

Ok it's not actually generating functions, but instead a function generator.

I picked up an HP 8904A off of FleaBay because I really don't like my cheap Chinese function generator. The latter has a shitty UI, and despite having two channels it's incapable of modulating one with the other. It does have outputs on the front, though, which this one curiously lacks.

But if we peek around back, there they are.

Routing the IO on the back of an instrument is quite common in rack mounted setups where you arrange a bunch of instruments in a fixed configuration, but for more ad-hoc usage it's really a pain. Luckily, the connectors are just routed inside pretty simply with a pair of coax leads.

So it's just a matter of unscrewing them from the back and screwing them in at the front.

And of course we'll transfer the blanking plugs to the rear, though I had to heat them up with my hot air station to get the little plastic catches to stretch back into place after decades of being compressed.

But with everything back together, we get a lovely 1kHz sine wave at 1v peak-to-peak.

Which is, of course, configurable for amplitude, frequency, and so on, as are the many other wave shapes it can generate.

What we don't get, though, is modulation. You see, that's an option that you have to pay HP to enable, and whoever owned this instrument in the past never paid HP to enable it, which means the HP tech never came out and pressed the power button (on), shift, service, shift, f4, 092448, shift, service, next, next, next, f1, 3ffc, f2, ff, f4, f4, power button (off), which would have allowed the instrument to perform amplitude modulation like this.

Or frequency modulation like this.

Or many other types of more complex modulations.

What a shame, I guess I'll have to just live without.