Oscope advice please

[NorCalTuna]

I myself just bought my first scope and was wondering if anybody in the know could tell me if there is anything wrong with the 62 dollar apex or tenma generators? the compact size is certainly attractive, as is the price! Has anyone had any success with using a computer as a source?

thanks!

[Enzo]

Which one, the 72-490? The 72-505?

The 490 would be minimalist, but would allow oyu to applu a sine or square throughtout eh audio range and likely at high enough levels for your needs.

For that money you could also find an old used Heathkit with more features. And there are plenty of good quality commercial test equipment models out there for cheap. Old HP units abound.

COmputers do a lot, but for me, an audio generator needs to have a knob. I can sweep the frequency up an down if I want to look for a resonant peak or a vibrational sensitive spot. To me it is kinda like tuning a guitar - a lot easier to just tweak the tuning peg than it would be to adjust a parameter a the screen. (assuming I have a computer tuning guitar)

[Steve Conner]

I'm with you on the knob thing. I've used computers, and generators that need the frequency punched in on a keypad, but it's not as convenient as an analog generator with a knob and a range switch.

At the moment I have a digital signal generator with a knob and a keypad, but it can be annoying. The knob is an encoder that jumps in discrete steps, and to change the size of the steps you need to prod another couple of buttons.

On those old HP things and their clones, the knob turned a big variable capacitor that was part of the time constant in the oscillator circuit. Hard to get simpler than that. With a bit of practice, you can gain major geek points by bringing it to jam sessions and playing it like a Theremin

You can pick them up used pretty cheap, too: I paid 20 bucks for my last one, but when I turned it on, the electrolytic caps sizzled and shot out gloop all over the insides. Definitely overdue for a cap job!

Computers have their place in the lab, where they really excel is using FFT techniques to measure distortion, frequency response, and the like. With a pseudo-random noise generator and an averaging FFT, you can see the response of your experimental tone stack or EQ, updating in real time as you mess with the controls.

If you use them to generate waveforms, though, the output is only as good as your generator software. Freeware written by amateurs can have horrible artifacts. And even if the software were perfect, the ADCs and DACs in all modern soundcards use digital filters that put scary ripples and ringing onto square waves. Also, thanks to our old buddy Nyquist, you can't output any frequencies above half the sampling rate. If you try to output a 20kHz square wave, it will look like a sine wave, because the harmonics can't be reproduced.