Audio Oscillator Project

Lately I’ve been loading some of the earliest SolderSmoke episodes into my mp3 player. I’ve really been enjoying them. In the very first, Bill mentioned that he was warming up the soldering iron because he just felt like making something and melting some solder. With that in mind this weekend, I decided that I was just going to build something.

So I opened up EMRFD and turned to chapter 7, which I knew had some simple projects like oscillators, rf probes, watt meters, etc. I came down on the simple one frequency audio oscillator (page 7.13, Fig 7.24) that they wrote was useful for testing audio sections of a radio. (Keeps you from having to whistle in the the mic.) Since I am aspiring to one day building a reciever from scratch, and maybe even an SSB transmitter I figured I’d give it a shot.

I’ve never built anything in the “true” ugly style with a plain groundplane. I also decided to begin the project with the end in mind, connectors and controls. In the case it was pretty simple. A pot for output level and an output jack of some kind. So, I also added the flange at the beginning. Here is a picture of what I ended up with. I think it compares to the photo in EMRFD, if I say so myself 🙂 ).

530 Hz Audio Oscillator

This is actually the first sine wave oscillator I’ve ever successfully built. In the past I’ve always had to fall back on the 555 timer square wave oscillator.

Since I didn’t have the capacitors used in the EMRFD version, and because they wrote that it could be built with whatever was on hand, I decided to use four of the 22nF ceramic disc capacitors from my junk box. Beyond that I didn’t know how to substitute my capacitors into the design. I knew that this was a phase shift oscillator that worked by feeding an inverting amplifier with a feedback loop made of a phase shifting RC network. And that to oscillate, the network needed to shift the phase of a particular frequency an additional 180 degrees. But how did I pick resistors to select a frequency?

I found a web site that talked about this and used an equation from it. f=1/(2*pi*R*C) Being young and dumb I took the equation and rearranged it to find R and slapped my values in: R = 1/(2*pi*1200Hz*22nF). Using my trusty calculator I found that roughly R=6k Ohms. I selected 5.6 as the standard value resistor and started building. (Note if you are reading this to figure out how to substitute in your own capacitors that this did not work. You can see on the photo that I ended up with an oscillator for 530Hz, not 1200Hz that I was shooting for.)

I built the oscillator in a hour or two plugged in the battery and got nothing. No voltage (AC or DC) at the output. Bummer! I checked the DC voltages and resistances of the various nodes and nothing seemed amiss. Still no output. Well it was getting late so I slept on it.

The next day I searched the web again and struck gold. I found an oscillator design that had a small section talking about adjustment of the emitter resistance to get the least distortion. It said to adjust the resistance so it was just below the resistance that would stop oscillation. Ok, now I had something the try. My emitter resistance was 10k Ohms. I used a jumper with clips to add an 18k resistor in parallel and it worked! I played with the resistor value and ended up adding a 22k resistor in parallel for 6.8k Ohms total or so.

A sine wave.!I could hear it! Seemed a little low though.

I plugged the output into my computer’s “mic in” and fired up Spectrogram and Soundcard Scope. Yep it was nice and clean with the second harmonic about 32dB down from the primary. But the primary frequency was 530Hz. Why?

On further reading it appears that I should have used the equation f=1/(2*pi*sqrt(6)*R*C) which is further down on the first site. However, that would actually make the error worse, wouldn’t it? I compared schematics between EMRFD and the web site and found that EMRFD had one more capacitor in the phase shift network. I shorted out that last capacitor with a jumper and … killed the oscillations.

So I don’t know exactly what is happening, but I suspect that the two problems I’ve had are related.

Anyone got any ideas? I’m all ears.

UPDATE 4/25/08:  I’ve drawn up a schematic as of the oscillator as built;  Here it is:


When am I going to get some exercise?

Matt made a new post on his blog Runner’s Journey this weekend that got me wondering (again) what I am going to do about getting some exercise.

The last time I ran was 5 or 6 years ago. At that time I had the urge to lose some weight and try to feel better. I did pretty well. I lost about 20 lbs and was able to run around my local lake (Lake Merced: 4-5 miles). Then, I had a surgery which made running uncomfortable for a few months and I’ve never gotten back out since.

If Matt keeps it up, I may find myself out on a run some chilly morning. Keep it up Matt.

New Power Meter

In my last blog I talked about the way I had a power meter nearly build itself. (In retrospect I remember a SolderSmoke episode where Bill claimed the same sort of thing happening with a different version of a power meter.) Friday night I got the chance to build the power meter from my parts.

It only took an hour or so get it put together. The hole for the meter turned out to not be a big deal. The trickiest part was actually getting the BNC jack through the hole in the side. The dummy load was too big and the hole too small to angle the PC board in. I ended up filing one of the corners and put a little force to get it in.

Here is what I ended up with.

Here are some shots of the power meter with my K2. The power meter is connected to the K2 antenna jack and the K2 is keyed. The K2 reports the power from it’s internal power meter. So you can see the K2 calculated power with the deflection of the meter in the new power meter.

The readings are actually fairly interesting. By a serendipitous coincidence 0.5W reads as ~10uA, 2W reads 20uA, 5W reads 30uA and 10W reads 40uA. Couldn’t have gotten that sort of correspondence if I had tried.

And finally after building it I had to plug every transmitter I have into the power meter. The sw20+ showed around 2 watts. Here is a picture of the power meter measuring the output of my homebrew 30m CW crystal transmitter Reads about .5W, which is what I expected but now I have confirmation.

So, all in all I’d have to say I am very pleased. I am going to find some Avery labels and label this guy so when someone goes through the shack after I pass they will know what it is. Not to mention, keeping me from toasting it because I was sure I built it for 100W. Other than that I am ready to build another RF generator to plug in.


New Project falling together

So it has been a while since I’ve done anything actively in Amateur Radio. If you had seen my workbench before this weekend you would not have needed to be told that. At the same time you would have known what one of the big hurdles was. (I know my K2 is in there somewhere.)

Well, I checked EMRFD out from the library a few weeks ago. I hoped it would spark some interest. It did.

As an aside, if you are a homebrewer and don’t have access to Experimental Methods in Radio Frequency Design you should get access to the book. It is a great book to read and I hope to build out of.

One of the first projects recommended in EMRFD is an RF power meter. It will help with the question, “Is this thing doing anything?” I haven’t had one. I have the power meter in my antenna tuner, but normally I have used a NORCAL dummy load with a DVM or my trusty Simpson meter. But the “required hand count” goes up steeply with each probe that needs to be held in place. Pretty awkward.

As I was reading the section on power meters again, I realized that the only thing I use my dummy load for is testing circuits. So why not just build it into an enclosure and include a voltmeter in the box. Then I can just plug in my circuit and the meter just sits there telling me how many watts and providing a load. No hands required. Cool!

Well to keep this story short, I spent last night at my newly cleaned bench picking parts out of the parts bin to produce a power meter. Here is the list pf parts.

  1. The dummy load from NORCAL provides the 50 ohm load and has the peak detector on it. It also has a mountable BNC jack that adequately mounts the whole thing. The peak detector and dummy load is good for 1-10W in short bursts.
  2. The meter is a 0-50uA meter I got from my dad. It is brand new in the box.
  3. A couple of 1.5M ohm resistors in parallel give ~750 k Ohms of resistance in series with the meter to provide a roughly 0-36V volt meter.
  4. The plastic Hammond case was a surplus hand-me-down from a friend at a testing lab.

Calibration: It will be as accurate as my K2. I intend to plug my K2 into it and energize with varying power levels. Then I will probably just mark the meter face at 1/2 watt intervals.

So when I mount everything I will have a 1-10W power meter that is in a small (but big enough) box to stand on the bench while I am tinkering.

The box will also have emough room to put in a more sensitive milliW meter circuit that reuses the meter at a later date. That will take a little more soldering. I haven’t decided whether to just do another uncompensated meter or use something like my compensated RF probe, but that will come later.

So stay tuned and I will get some photos up when it is done.