Avian’s Blog

I've been missing a good function generator for a while now. Over time I've accumulated an assortment of various ad-hoc 555 oscillators and microcontroller hacks I've cobbled together whenever I needed to generate some kind of a signal to test a circuit I worked on. For a while I've been considering joining all of these in one box and slap some kind of an interface on top. I even bought a nice back-lit LCD display and some buttons for the front panel.

Then at one point more or less by accident I stumbled upon this low-end line of function generators from GW Instek and they looked cheaper and more versatile than what I was building myself. From what opinions I could find on the web about it they mostly appeared positive. Last week when it came in stock in this corner of the world I bought one.

After a couple of hours of playing with it I can say it's a perfectly functional instrument that as far as I can see works as specified. However as you might expect, the fact that it costs around 150 € less than the next cheapest arbitrary waveform generator definitely shows itself.

Interestingly, my complaints mostly have to deal with the user interface. For instance, the first thing I noticed is that there is no visual indicator whether the display is set to show signal amplitude for a 50 Ω or high-impedance output termination. Since this means a factor of two in output voltage level a wrong setting can easily be fatal for some circuits. I guess resetting to high-Z setting before turning off the generator would be a prudent habit to keep.

One thing that sold this device for me is the USB connectivity. After being spoiled by the equipment I have at work, I missed being capable of running some automated tests for projects at home. While the manual doesn't mention the word USBTMC anywhere, the remote-control commands look suspiciously identical to the venerable industry-standard GPIB language.

When I connected AFG-2005 to my computer, first thing I noticed is that it doesn't actually use USBTMC protocol. Instead, it's behaves like a serial modem with USB ACM interface. As far as Linux user space is concerned, this doesn't seem to make a big difference - both kernel drivers expose a simple character device for interaction with the device. For USBTMC I know that when the write() syscall returns, the device has already accepted the command and you can expect it will be giving the signal you programmed. But I'm not sure whether this also holds in this case.

The implementation of the protocol is somewhat buggy though. First of all, commands require a terminating ASCII line feed (\n) character, which isn't the case with USBTMC devices. Invalid commands are simply ignored - there are no error or diagnostic messages at all.

My device identifies itself like this for the *IDN? command:

GW INSTEK,AFG-2005,SN:EN810021,V1.11

It's quite easy to crash the instrument's firmware by feeding it malformed commands. In that case the front panel stops working and the only way to restore it back to normal is by using the hardware power button.

The front panel display itself is somewhat sluggish and if you issue a lot of commands through the USB it might take a while to catch up with all the settings, even though if you watch the signal generator's output you can see they are applied much faster. The multi-purpose knob has similar issues and if you turn it too fast the values start jumping around instead of going in one direction (this seem to be a common issue though - I've seen a lot of different devices have a problem with that).

Arbitrary function generator works nicely and can be easily programmed from a PC using a DATA:DAC command. What I don't quite understand is the syntax there. In contrast to other instruments I worked with, the manual says that the first argument is the Start address of the arbitrary waveform. It gives an example where this address is 1000, but that doesn't seem to work at all with my instrument. The only address where the ARB output works as expected is 0. Using other addresses I either get garbage output with no correlation to what I wanted to do or crash the firmware.

Arbitrary function generator has a 20 MHz maximum sampling frequency and firmware allows you to fully exploit that, even when the frequency of the signal exceeds 5 MHz maximum for pre-programmed waveforms. The results in that case are less than perfect of course. The most obvious problem is the significant DAC jitter. Here is how a square wave looks like at 10 MHz:

In conclusion, this instrument gives you quite a lot of value for its price. But unsurprisingly, it's not on the level of instruments that have a few zeros more on the left of the Euro sign. I can't stop myself thinking though that a lot of these problems would take a simple firmware patch to fix. If only the manufacturer would provide source and simple means of reprogramming.