Thursday, August 29, 2013

Low pass filter and VCO update

VCO update

I added an LED to indicate the oscillation frequency of the VCO.  I also found an old power amplifier in the electronics junk room in the basement. From that I harvested a whole bunch of RCA jacks and a couple of very nice potentiometers. The jacks will be great for connecting modules, since RCA cables are shielded and easy to find/cheap. I disconnected the voltage source for the VCO, so now you have a choice of connecting either the voltage divider or another external voltage source to control the VCO.

I also discovered that when connecting the triangle wave output to a speaker (+ amplifier) i got some high frequency oscillations superimposed upon the triangle wave. I think these oscillations arise due to driving a capacitive load which is in the cabling or perhaps a DC-block capacitor in the amplifier. I'm going to deal with this later, but the TLC721 is a programmable opamp and is currently programmed as low biased. This is the slowest and most energy effective mode with a phase margin of 34°. The high bias mode has a phase margin of 47°, which might make the difference in this case. A first I thought that the output needed to be buffered, so I added an emitter follower opamp. This removed the oscillations from the original output opamp, but transferred the problem to the buffer output instead.

Superimposed oscillations on sawtooth wave. Upper waveform is from stage connected to amplifier. Lower waveform is from stage before output buffer.

Active Low Pass Filter

From The Art of Electronics, I took the simplest active filter circuit i could find. It's a two pole low pass Butterworth filter, which consists of two identical capacitors and two identical resistors, making up the filter, one opamp and two resistors for gain control.
Having two identical resistors that make up the filter gives a problem when making a variable filter, since you will need a double gang potentiometer. Lucky enough I found a couple of those in the power amplifier I mentioned. I took one that goes to 50k\(\Omega\). The -3dB frequency of the filter is given by
\[f_0 = \frac{1}{2 \pi RC}\]
with the lowest frequency being 20Hz, that gives me:
\[C = \frac{1}{2\pi 50000\Omega 20\text{Hz}} = 150 \text{nF}\]
with higher cutoff frequency with lower resistance in the potentiometer.

I added RCA jacks and the whole thing looks like this:
Active low pass filter schematic

Active low pass filter to the right

works like expected as well...

Monday, August 19, 2013

Voltage Controlled Oscillator

I'm looking into how analogue synthesizers work and plan to build a couple of modules. The first module to build is a VCO, Voltage Controlled Oscillator. I found a simple one in The art of Electronics which i built on a copper plane, Jim Williams style.
 Schematics and images are shown below.
VCO on copper plane. A potentiometer is used as voltage source.

VCO schematic based on design from The Art of Electronics.
In The Art of Electronics, 1% resistors are used in the input stage of the first Op-Amp. I didn't have any such resistors at home, but it seems to work well with 5% resistors. Max frequency is about 50Hz, which means this design isn't very useful as instrument, but it gave me something basic to start with. Also, voltage output from sawtooth and square wave where quite different. The square wave comes from a comparator with the full supply rail as swing.
Sawtooth and square wave output from VCO.

In the junk room in my cellar, I found an old power amplifier with a lot of RCA jacks on the back, which I'll use to connect the different modules to each other. More on this topic later on.

Tuesday, August 6, 2013

Fluke 8050A Repair, Part 2

I tried out the modification suggested in the last post, but somehow I couldn't get it working. The layout of the board in the modification instruction is not the same as mine, and I guess I didn't figure out all the differences.
 Finally I ended up buying som NiCd batteries with solderlugs. I didn't buy the more expensive C-size ones, but the cheapest ones that would fit, which cost about 24€. The DMM seems to be quite good, so I figured it was worth the new batteries.
 Installing wasn't more difficult than soldering the old wires the the new batteries and now the DMM works great, both on AC and battery. The new batteries are specified to almost the double amount of charge, so I expect the DMM to be going for quite some time between charge.
Measuring voltage running on battery power