Tuesday, June 12, 2007

Wes (W7ZOI) on Oscilloscopes and Test Instruments

In a recent mail from Wes (W7ZOI), he discussed oscilloscopes and test instruments. I thought it was worth sharing his comments with the readers.

Wes wrote -

I used to argue that there were three classes of oscilloscope:
  1. No scope at all
  2. A scope that would provide an indication of what was happening in a circuit
  3. A scope that was capable of measurement.
There can be real danger when using a scope though. A major flaw is the "perfect sine wave" syndrome. Folks will look at the output of their 14 MHz transmitter on a 20 MHz scope and see a sinewave. Of course it's a sinewave, for all of the harmonics are well into the attenuation part of the scope bandwidth. That 20 MHz scope is not really useful to see all of the details of a waveform beyond perhaps something for the 160 or 80 meter bands. You need that 465 for anything in the HF spectrum. Still, you can do really a lot with a very modest box.

For years my home 'scope was a Tek T-922, which is a dual trace 15 MHz box. It has a plastic box, so it is a problem for transmitter testing. Still, I could do a lot with it. The key is to understand something of the limitations.

I agree with the digital versus analog meter. An analog meter is a really useful thing. But the DVM is also handy and they are so cheap that one can have both. A frequency counter is, as Farhan noted, extremely useful. If you are going to do work with crystal filters, or oscillator stability, you want it to go down to 1 Hz resolution. That is so easy these days that you may as well build it that way. Don't bother with a grid dipper (or anything else that might dip.) That was for the beginning of the last century.

One of the blog readers commented that a high Z volt meter was a good thing. Na, not really. Yes it is good, but you can get there with an op-amp and a 1000 Ohm per volt meter. So I would opt for a rugged, dependable meter that you could use to measure lots of current or normal voltages. Then I would build a high Z input section for the sensitive measurements, and would also buy a cheap DVM. Actually, I have a Fluke 73 DVM that I have really enjoyed. In an earlier time, a VTVM was the center of my lab, but that ended when I got my hands on a proper 'scope.

Monday, June 4, 2007

DC 40 - A Direct Conversion Receiver

I have just posted a new project on my website:
A simple direct conversion receiver
  • You can assemble it in one evening. Just six transistors and any audio amp IC.
  • It does not use any special components. All out of your junk box.
  • Quite a performer. It will probably beat your general coverage, synthesized all mode transceiver with its reception.
The article also traces our journey from idea to finishing the project.