Unstable opamps, VCO and low pass filter

As I mentioned earlier there was some oscillation problems with the VCO sawtooth output. The problem was easily solved by biasing the opamp high. This makes the opamp faster and gives it a higher phase margin, thus making it more stable when driving capacitive loads. Since this solved the problem, I removed the opamp buffer from the output. The low pass filter had the same kind of issue, and it was solved in the same way.
 The filter also suffered from some clipping, which I solved by replacing R1 with a potentiometer and trying out the best value, which was 3.45k\(\Omega\). When P1 was set to the lowest resistance, there where some oscillations in the output. This was solved by putting 1k\(\Omega\) in series with each potentiometer terminal. I'll see if I can get my function generator to sweep out a bode plot of the filter, to see some details on it's characteristics.

Updated active filter schematics

 To get some higher tones out of the VCO, replaced C1 with a smaller capacitor. I needed a few tries to find a good match. A small capacitor gave a higher frequency, but the wave forms got scewed, probably due to unmatched resistors in the input stage. A smaller capacitor also raised the lower end frequency too high. This shows that I probably need to find a better VCO design, to get a higher bandwidth.

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...

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.

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

Fluke 8050A Repair, Part 1

We scrapped an old Fluke 8050A, Digital Multimeter. It's and old bench multimeter with 4.5 digits. This one has the built in rechargable battery option.

The instrument was simply dead, so I took it home to try to fix it. Accompanying the DMM was an excellent manual, with schematics and service instructions.

 Taking a look at the motherboard showed some blackness around a LM317 linear regulator, so I took that out and measured the resistance at the mounting wholes of the regulator, to see if anything had shorted out. Resistance was as expected. I replaced the regulator with one I found in my junk pile. No difference. The regulator got really hot. 

Replaced regulator is the TO package located just below transformer.

I measured some test voltage points that where described in the manual, and they all showed a somewhat lower voltage than expected. So the problem is probably with the power delivery.

 I looked around the net and found some tips about modifying the 8050A so that it works without batteries. Hm, hadn't thought about it not working without it's batteries. Of course, Ni Cd batteries from the late 70's, early 80's, won't work properly today. Because of this, I had already taken the batteries out, thinking that it would work without them, as many modern pieces of equipment does.

 To test if this was the problem i connected an external 4.8V power supply to the battery terminals, to emulate a working battery. Voila! The display lights up, but is missing some segments.

 The segments are fixed by removing the LCD display and cleaning the contact surfaces. Here's the result:

Fluke 8050A, with emulated battery, measuring the resistance of a resistor.

So the question is now, do I replace the old batteries? Four new Ni Cd C batteries cost about 40€, which  I should think is very close to the value of a working instrument... It would be nice to have the old instrument in original condition though. Modifying the DMM to work without batteries is pretty easy and is described here. I will have to think about what to do, and will return in part two to tell what I ended up doing.

Finally, a proper workbench

i finally got a proper workbench installed at home! This is really good for me, since I usually only have an hour or so on my projects, after my kids are put to bed. Before I had all my equipment in a cupboard. So to start working, I needed to set everything up on the kitchen table. When I'd finished working, everything needed to be put back. So, not a lot of time for working...
 Now I've got a workbench where I can leave projects lying and work on them for five minutes, if that's what I have.

The metal detector is yet to be finished, I'll do that soon. Right now, as can be seen in the picture, I'm trying to fix a broken Fluke 8050A, which I salvaged from work. That will be in my next post.

Metal detector part 3, Darlington emitter follower

One thing I suddenly realized when trying to figure out what frequencies we have in the oscillator nodes, the probes will affect the circuit. The Tektronix probe has 10M\(\Omega\) and 10pF. Our nodes have about 2 pF and between 20k and 1.5\(\Omega\), so they will change the effective values. What we need is the Darlington emitter follower, which will introduce about the same impedance and capacitance as the probe, but with the difference that the follower will always be a part of the circuit.
The circuit now looks like this:

 The output from Q4's emitter is seen here:

The two outputs show the mixed down frequencies when the local oscillator is tuned to its two extremes.
The lowest frequency is about 13kHz, which is probably a bit high. But let's wait until we can listen to the signal until we fine tune it some more.
The next step will be to define a push/pull amplifier for the loudspeaker. By the way, a loudspeaker might draw a bit to much power, so I'll see if I can use a summer from a PC motherboard instead. I'll need to measure its impedance first, to make the calculations for the amplifier.