Archive for August 19th, 2016

FG085 Function Generator

The topic of function generators came up at Squidwrench a while ago (Sophi was tinkering with LCD shutters) and I finally picked up one of those JYE Tech FG085 DDS function generators to see how they work:

FG085 Fn Gen - in case

FG085 Fn Gen – in case

Short answer: adequate, if you’re not too fussy.

The board arrived with a bizarre solder defect. It seems a solder stalk yanked one terminal off a ceramic SMD caps:

FG085 - Solder stalk - C26

FG085 – Solder stalk – C26

The schematic and adjacent parts suggested the victim was a 10 uF cap, so I replaced it with one from my stash that worked fine.

However, after soldering enough of the switches to do something useful, the board wouldn’t power up. With a bit of poking around, I discovered the power jack had +15 V from the wall wart, but the center terminals on the DPDT power switch that should have been connected to the jack showed maybe 0.3 V. Jumpering around the failed via and a short trace on the bottom surface let the board power up correctly:

FG085 - Jumpered power trace

FG085 – Jumpered power trace

If you’re building one of these, solder one pin of each switch, push all the switch caps in place, shove the faceplate over all of them, tape it to the PCB, make sure all the switches are push-able, then solder the remainder of the switch pins. If you do them one by one, you’re certain to end up with a few mis-aligned switches that will either prevent the faceplate from sliding over them or wedge firmly against the side of their assigned hole. Just sayin’.

It lives in a case from Thingiverse:

FG085enclosure - 1268379

FG085enclosure – 1268379

I tweaked the dimensions slightly to fit the (slightly larger, possibly new, maybe tolerance-eased) front panel, but the bottom mounting screw hole spacing depends on the front panel size, not a specific set of dimensions, leading me to relocate those holes by abrasive adjustment. I didn’t bother with the lid (which doesn’t clear the BNC jack anyway) or the printed plastic feet (having a supply of silicone rubber feet).

The fancy vent gridwork along the sides printed surprisingly well, even in PETG. I’d have gone with larger slots, although I doubt the thing really needs vents in the first place.

The DDS sine wave output is rough, to say the least:

FG085 Fn Gen - 60 kHz sine

FG085 Fn Gen – 60 kHz sine

The spectrum shows oodles of harmonic content:

FG085 Fn Gen - 60 kHz sine - spectrum

FG085 Fn Gen – 60 kHz sine – spectrum

A closer look:

FG085 Fn Gen - 60 kHz sine - spectrum - detail

FG085 Fn Gen – 60 kHz sine – spectrum – detail

Stepping back a bit shows harmonics of (and around) the 2.5 MHz DDS sampling frequency:

FG085 Fn Gen - 60 kHz sine - spectrum - 10 MHz

FG085 Fn Gen – 60 kHz sine – spectrum – 10 MHz

For comparison, my old Fordham FG-801 analog function generator has nice smooth harmonics:

FG-801 Fn Gen - 60 kHz sine - spectrum

FG-801 Fn Gen – 60 kHz sine – spectrum

Closer in:

FG-801 Fn Gen - 60 kHz sine - spectrum - detail

FG-801 Fn Gen – 60 kHz sine – spectrum – detail

Of course, that crusty old analog dial doesn’t provide nearly the set-ability of a nice digital display.