Why Do I Use So many Bypass Capacitors?

When you look at the design requirements of most ICs they commonly require a .1uF bypass capacitor, with one larger bypass capacitor (on the order of a few uFs) for every 3 or 5 ICs. I use a .1uF and a 2.2uF capacitor for every IC. That seems a bit extravagant both in capacitor costs and board area. So why do I do it?

Let us think about it. An IC draws a pulse of current due to a change of state. This pulse causes a change in voltage on the .1uF bypass capacitor. By having a 20 times larger capacitor right next to the .1uF the change in voltage will be 20 times smaller (actually a bit less due to the frequency response of the larger capacitor). This effectively reduces power supply noise for the IC by a factor of 20. It also decreases the peak of the current pulse needed to recharge the capacitor (the recharge will also take longer). Lowering the peak current also reduces emitted EMI.

If I was going into volume production and had a good EMI lab I could reduce the number of 2.2uF capacitors until it started making a measurable difference. Since I don't have an EMI lab in my shop - I just keep those extra capacitors and come in with a lower noise design. This is especially necessary for high frequency noise since most chips are not very good at reducing that kind of noise.

So why not throw out the .1uF capacitors and just use the 2.2uF caps? Because capacitors are to some extent frequency sensitive. The .1uF caps will respond to fast transients that will not (immediately) affect the 2.2uF capacitors.

Engineering is the art of making what you want from what you can get at a profit.

Crystal Radios

Ben Tongue of Blonder-Tongue Laboratories has written extensively on crystal radios and crystal detectors. You can find the articles here. Ben is deceased. I have collected the articles and turned them into pdfs along with some other crystal radio stuff. As soon as I up load them I will post a link to the package.

The zipped package of Crystal Radio pdfs can be downloaded here.

What got me started on this was Diode Detectors for RF Measurement. And what got me into that was the design of an SWR Meter. I will publish that design when the board layout is complete.

Engineering is the art of making what you want from what you can get at a profit.

Radio Receiver Update

I have the very initial schematic of the radio receiver done. You can see the first page here.

Click on the image for a larger view)
And the rest of the schematic (in pdf) is here.

Comments and suggestions welcome. I will be doing a clean up (assigning part numbers and other stuff) over the next few days and will then begin layout.

Engineering is the art of making what you want from what you can get at a profit.

A Little Confection

While I'm working on a few projects, I thought it would be good to get something done. So I did. A little logic level tester. This one is a little different. It tests for voltages. Less than 0.5 volts. More than 2.0 volts. And more than 4.0 volts. And yes there is a light for each. Good for looking at mixed voltage logic. And there is also a light for greater than 0.5 volts but less than 2.0 volts. So you get a light for ambiguous (in some situations) voltages. The schematic looks like this:

Click on the image for a larger view.

The circuit will run on 3.0 volts (nominal) but the lights will be a little dim. The voltage response will be the same as it is on 5.0 volt power. The magic of voltage references and comparators.

The complete package including schematics, parts layout, and parts list (in pdf) can be found here.

Engineering is the art of making what you want from what you can get at a profit.

A Radio Receiver

I have a long time interest in amateur radio. This got me noodling around the 'net and I found the most interesting circuit. A SSB receiver. And the most interesting thing about the circuit is the frequency control device. The CS2000. It generates a frequency from 6 to 75MHz with just a frequency input and a digital word. Mouser has them in stock for $8.93 each in lots of one. Not a bad price for such a wide ranging frequency source.

There are of course other ways of doing things. this guy likes the AD9854. It looks good. And costs over $50 for one. A little out of my price range this week.

Naturally there were some circuit modifications to be made. Like directly driving 5 volt logic from a 3.3 volt logic source. It works - usually - but is not guaranteed. In those places (where necessary - some 5V logic is designed to respond to TTL levels. A relic from another era. But very useful in this one.) I added a TTL level compliant buffer. S0 .8V for a logic low and 2.4V for a logic high are guaranteed.

Also the detector circuit requires matched capacitors. Four for the detectors and eight for the phase shift (all pass) filters. So I have designed a capacitor matcher. With it you can read out a .1 uF capacitor to parts per ten million - or better. The accuracy is much less. But for matching lots of resolution is good. It also gives an interesting view of the world. The changes that can be detected when bodies move.

Design of the all pass filter was simple using the (free) design software found here. You need to install it on your computer. I have installed it on mine.

I'm working on schematics, parts lists, and board layouts for all of this. I will post them here. When they are done. About a week or two.

Engineering is the art of making what you want from what you can get at a profit.