(27-06-2011, 07:17 AM)fido Wrote: A few months ago I purchased a very basic valve RF signal generator. It works but checking it with the 'scope shows the sine wave to be quite distorted. I suppose this might be more of a problem with an AF sig. gen. but how concerned should I be about this distortion? I don't have a frequency counter and I think that is my next item on the test equipment shopping list.
Just a thought, but when you say 'distorted' do you mean that the peaks and troughs are wide rather than a fine line? If so, that may be because the modulation of the sig genny is switched on. If you switch it off, you'll see a sine wave, unless of course the generator really is faulty! To illustrate the effect of modulation on a sine wave, , I've attached below two scope traces. They're actually of a 650kHz VFO in a MW modulator that I made to enable signals from any source (I-pod DAB, AM/FM) to be fed into a radio (legally) tuned to 650 kHz in the MW band. It's an alternative to a 'micro transmitter' which are unlicensable anywhere in Europe, not that anyone worries too much about that! The modulator works by the audio input from a device such as an i-pod, FM radio, CD player or whatever, modulating the 650kHz VFO, then playing this through the antenna socket of a vintage radio tuned to the 650kHz. (460M). The modulator in tunable from 430M - 490M. (It was built on three homebrew PCBs - the rest of the bits mostly came from my spares box).
The first trace shows the unmodulated sine wave, which is what you should see from a signal generator, the second is the 650kHz sine wave modulated with a 1kHz tone. On most signal generators you can adjust the depth of modulation.
I've attached a couple of other pics - one showing how you can check the frequency of an oscillator on a frequency counter simply by placing a loop of insulated wire close to the oscilator, another pic shows my homebrrew 200MHz frequency counter which I built in 1976, when such commercial instruments were far beyond the pocket of constructors, but since I'm a constructor, I constructed one! I later built a 1gHz counter, but theres' not much point these days when you can buy a hand-held one for about £30.
Hope that's of interest, and helps a bit.
David
