I was watching Ebay for a basic signal generator to test aftermarket car ecu set ups on the bench by injecting waveforms to mimic Hall effect or magnetic sensors, then realised that these are now available as USB add ons for lap tops and desk top PC's. Apart from being tied to using one with a PC, are there other disadvantages? I wondered if their voltage and current output is limited? If used for other work, are they stable and accurate? If I got a bench type signal generator I would want a digital frequency display, and many of those seem a bit over the top, functionality and complexity wise, for what I probably need. Thanks. I am in the UK, and new to this forum, but try not to hold that against me

 

USB is limited to 500mA at maybe 5V so max is 2.5W and realistically less, so unless the device has an additional power source that is all the available power for not only the output but the control.

However, this is really too open ended a question to have any answer. It will vary from unit to unit. If you see a particular unit you are interested in then post a link so we can toss stones at it, I mean, provide an independent evaluation of it's feature set.

 

If you are looking for something like variable duty cycle square waves and low frequencies, you may be able to build one yourself with 555 timers and Bill Marden's blog.

If you are looking for specific ramp times, hold times, and aperiodic waveforms, then an arbitrary waveform generator will cost you quite a bit of cash.

If you are looking for a byte/word digital generator, those can be built or bought, depending on the interface you need.

More specifics would help. Disadvantages of USB would almost certainly need an external supply to go over 5V or deliver much current, also somewhat frequency limited in many cases.
Advantages of USB is more advanced waveforms can be made with the Windows Software and powerful CPU telling the USB DAC what to make for less money than a standalone generator.

 

USB or other 'virtual instruments' are limited in the speed of manipulating the controls. Its faster to turn and adjust a physical knob than a virtual knob, such as in many controls on LCD TVs. On a standard laptop, you are constrained to having only one mouse pointer, and can turn one knob at a time, with a real device you could manipulate several as fast as you hands or fingers can move them. Even touch screens are limited to one active point at a time.

The actual voltage output isn't material, even its say 5V 500mA, it can easily have onboard voltage multipliers to output what it needs, it however, is limited in overall power by its source.

A main advantage is its programmable flexibility. Nearly all USB or similar devices can be programmed, then linked together by the controlling PC but it take programming work, something that is LabView's bread and butter. This is helpful in a production environment or if you have some set up that requires large data collection sets so inputting them into say Excel, would be quite a pain. For example, if you were characterizing a design's stability over 1 day, 90 days to 1 year, the amount of data accumulated is nearly impossible to enter if done with non-networked equipment.

That said, many stand alone devices can have different levels of connectivity, others not.

Another disadvantage of USB or any VI is its dependence on the PC, its OS and the drivers. As you know, OS are updated fairly often, and at somepoint the drivers or client software may no longer run if not updated. If the manufacturer ceases support for your gear, its very difficult to get updated client software unless the manufacturer releases the SDK or programming specification for the device. If USB itself is abandoned in the future, then you have to keep an old PC to insure your devices are still runnable; in the past this happened to folks who are tied to the old ISA bus.

With stand alone instruments, you can have gear nearly 40 years old still usable, if maintained right. Many metrological quality instruments are still worth maintaining, providing top end specifications and may just lack or use obsolete connectivity like GPIB or similar.

 

I hate PC driven instruments.

You find yourself not wanting to do the things you need to do because you don't feel like dragging out the gear, AND the PC, tripping over all the wires, fighting your way through the hoop-jumping menus and the like.

A dedicated instrument improves your workflow, and your sanity.