Will all this effort to create a good sine wave for use as Component Tester, and all the testing on it, I might as well put the CT to the test which started it all, i.e. the one built into my Hameg HMO2024 oscilloscope.

Nothing easier than that – just hook up a probe to the signal it generates on the front panel, and do an FFT on it:

(yeah, a green trace for a change – the other channels were tied up for another experiment…)

Ok, but not stellar: 1.6% harmonic distortion on the 3rd harmonic – more than with the Phase Shift Oscillator.

The frequency is also about 10% high, although that’s usually not so important for a Component Tester.

PS. The ∆L units should be “dB”, not “dBm” – Hameg says they’ll fix this oversight in the next firmware update.

One last experiment I wanted to do after the recent sine wave circuits, was to compare a few different op-amps.

I’m including the original one here as well – the LM358, running at ±13.6V:

Here’s the LT1413, running at ±14.4V:

And here’s the NE5532ANG, running at ±15.3V:

In each case, the supply voltage was adjusted until the output sine wave was ±10 V, with all other components identical. Note the slight difference in oscillation frequency.

What’s also interesting, is the mean output voltage: it should be 0V with an ideal circuit. Looks like the NE5532ANG performs best – within 1%. It’s described as being an “Internally Compensated Dual Low Noise Operational Amplifier”. Second harmonic is at -51 dBm, i.e. 0.28% harmonic distortion – an excellent signal!

As a quick test with that last op-amp, I reduced the supply voltage to ±2.5V – the effect was a slightly higher frequency of 522 Hz, a much lower output of 2.14 Vpp, i.e. ±1.07V, but relatively far off-center: 240 mV. Harmonic distortion rises to 3.5% in this case. But that’s not surprising: the NE5532ANG is only specified down to ±3V, and it’s not a “rail-to-rail” op-amp, which means it cannot generate an output voltage too close to its supply voltage (with a ±5V supply, distortion drops back to 1.25%).

Lots of op-amps. Lots of trade-offs.

Ok enough op-amp chit-chat for now, I’ll stop :)

Let’s move on, now that we have a clean sine wave. The goal is to produce a ±10V sine wave to use for constructing a Component Tester. The sine wave produced so far was merely ±65 mV.

I re-used the same circuit as yesterday, but with slightly different settings. First of all, I replaced the op-amp by an LM358, which can handle higher voltages. Next, I halved all the R’s to 5 kΩ and doubled all the C’s to 0.2 µF. This reduces the loading of the feedback loop – it shouldn’t really affect the frequency.

To increase the output voltage, I connected the oscillator output signal to a non-inverting op-amp circuit:

In a nutshell: this circuit tries to keep Ve as close to zero as possible at all times. IOW, the op-amp will constantly adjust its output so that the tap on the Rf:Rg voltage divider tracks the Vin voltage on the “+” input pin.

Using Rf = 10 kΩ, and Rg = 470 Ω, its gain will be about 22x. The nice property of this circuit is that it has a very high input impedance, so there is virtually no current draw from the oscillator.

And sure enough, the output of this op-amp is a sine wave with many volts of output swing. Now it’s simply a matter of cranking up the supply voltages to ±13.6V and bingo, a ±10V sine wave:

Very clean. Better even than the original circuit – the 2nd harmonic is now -49 dB w.r.t. to the base frequency. That’s just 0.35% of harmonic distortion – excellent!

That second op-amp came for free, since an LM358 DIP-8 package has two of them anyway. So the first op-amp is oscillating (at about 470 Hz) and the second op-amp brings the output level to ±10V.

It’s quite a mess on the mini-breadboard I used, but who cares – that’s what prototypes are for:

One last check is needed to make sure that the LM358 is suitable. A component tester measures the effects of an unknown component in series with a 1 kΩ resistor. So in the worst case, with a simple wire as “unknown component”, the maximum current through that resistor will be ±10 mA. Luckily, according to the specs, an LM358 can supply at least 10 mA, and typically up to 20 mA on its output.

So that’s it: our Component Tester will need a ±13.6V supply, an LM358, and a few R’s and C’s. That supply voltage is not critical, as long as it’s stable – the output level could be adjusted to ±10V via a trimpot.

Welcome to the world of analog electronics!

After the pretty bad sine wave trial of the last two days, it’s time to try another circuit:

This is a “Phase Shift Oscillator” from the same op-amp book as the other one. I used half a TLV2472.

This one is actually a bit simpler to explain: the op-amp is set up with 25..50x amplification, i.e almost a comparator (with 50x amplification, a 50 mV input above or below the 2.5V will drive the output to its limit). And indeed, the output signal of the op-amp looks somewhat like a heavily clipped sine wave:

The 3 resistors and 3 capacitors create 3 RC “low-pass” filters in series, removing all the higher frequencies, i.e. harmonics. A fairly clean sine wave comes out at the end, as you can see here:

The only problem is that the signal level has been reduced from a ±2.5 V level to ≈ ±65 mV, a 40-fold reduction!

So the op-amp itself has to amplify that level back up to produce the clipped ±2.5V signal again.

The frequency is determined by “phase shifts”. Each RC filter changes the phase of its input signal, and it will be by 60° at a certain frequency, so that 3 of them in series will then shift it by 180°. Since the signal is fed back to the “-” pin of the op-amp, that’s exactly the proper signal to generate the opposite output, i.e. shifted 180° out of 360°. This analog stuff gets complicated – don’t worry too much about it: just pick R and C values to get the right frequency, and make all of them the same.

I used 0.1 µF caps i.s.o. 10 nF caps, i.e. 10x larger than the original circuit. With these values, the oscillation in my setup turned out to occur at just about 440 Hz, i.e. a pure musical “A” tone!

I did have to increase the gain (1.5 MΩ / 55.2 kΩ = 27 in the above setup) to force oscillation. I changed RF to 1 MΩ and RG to 22 kΩ, for a gain of 47. This RG value is a bit low, it loads down the last RC section quite a bit.

What you’re seeing here is a classical example of a negative feedback loop, which ends up in a very stable state of oscillation. It oscillates because we’re delaying the feedback signal by about 2.27 ms through the RC chain. So the op-amp constantly overshoots around its mid-point (the 2.5V applied to the “+” input), but does so in a very controlled way. The amplitude can’t increase any further, since the op-amp is clipping at its limits already, and the amplification factor is large enough to keep boosting the swing up to that limit. You can see the startup ramp and stabilization when powering up:

Here’s the FFT spectrum analysis of the generated sine wave:

A clean signal compared to the previous experiment. The 2nd harmonic is ≈ 42 dB below the fundamental wave, the rest is even lower. Using this calculator, we can see that this represents about 0.8% harmonic distortion.

The only issue is that the signal is much weaker than the ±10V needed for a standard Component Tester.

But hey, let’s declare success for now – we’ve got a clean sine wave!

After yesterday’s failed attempt to generate a clean since wave, I started experimenting a bit further. How could the Op-amp book be so wrong about the quadrature oscillator circuit?

The nice thing about op-amps in DIP-8 packages, is that most of them use the same pinout, so it’s very easy to swap them out and test different brands and types. The TLV2472 only supports up to 6V as power supply, most of the other ones go much higher – usually above 30V, i.e. ±15V.

Here’s the list of op-amp chips I tried (yeah, got quite a bunch of them in my lab, for various reasons):

• TLV2474
• LM358N
• LM833N
• NE5532ANG
• OP2340
• NJM14558D
• MCP6023
• LT1413

All of them had similar behavior, i.e. clipping at both limits of the voltage range, except for the LT1413:

Still nowhere near a sine wave, BTW. But what’s more interesting, is the the voltage swing of this signal was just 4.5 Vpp, while the op-amp was being driven from a ±15V power supply in this particular case. So for some reason, it was “oscillating” at 1.25 KHz (about 8x higher than the other mode).

I have no idea what was going on. When trying to reproduce this a second time, I couldn’t get this behavior back. I suspect a loose connection, or perhaps some odd interaction due to the breadboard.

I’m not really interested in tracking down this issue, since it looks like this quadrature oscillator circuit is not suitable for a Component Tester – not without some sort of amplitude regulation anyway.

So there you have it – analog circuits also need to be debugged, as you can see!

Update – this issue has now been resolved, see the comments on yesterday’s weblog post.

After the recent pretty disappointing results with a transformer-based Component Tester, I’d like to try and generate a ± 10 V sine wave at approximately 50 Hz in some other way. Using as few components as possible.

This is where we enter, eh, squarely into the analog electronics domain. Yes, we could generate it with an ATmega, but frankly that sounds like a bit of overkill, would require a fair amount of filtering to remove residual switching effects, and besides we’d still have to amplify it up to 10 Vpp.

Time to introduce some new circuitry!

One of the most incredible electronic building blocks invented in the second world-war era was the Operational Amplifier, or “op-amp” in short.

There’s way too much to say about this amazingly universal circuit, which even has its own schematic symbol:

A positive and negative power supply pin, a positive and a negative input, and an output pin. That’s it.

I’ve only just started exploring op-amps, really – one superb resource on the web comes in the form of a free eBook from 2002 on the Texas Instruments site, titled “Op Amps For Everyone”, by Ron Mancini.

In his chapter on Sine Wave Oscillator, he mentions a “Quadrature Oscillator” built from two op-amps:

It uses very few components. This one was dimensioned for about 1.6 KHz, so I started with capacitors ten times as large, i.e. 0.1 µF, to lower the oscillation frequency. Here’s the result, using a TLV2472 dual op-amp:

Powered by a supply of ±2.5V (i.e. 0 / 2.5V / 5V), I see this result on the scope, when attached to the sine output:

Yeah, right. Clipping like crazy, i.e. overshooting into the limiting 0V and 5V power lines. The FFT shows it’s not anywhere near a pure sine wave, even though the shape vaguely resembles one:

A pure sine wave would have a single peak at the oscillating frequency.

Here’s the cosine output, again showing that it’s running way outside its linear range:

So yeah, we’re generating a 160 Hz signal, but it’s no sine wave and it would be useless as Component Tester.

Oh well, it was still an interesting first trial!

Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.

(this is again a bit of a side excursion, about checking the quality of a measuring instrument)

I recently visited a friend who had to get his frequency meter’s calibration verified to a fairly high precision. Thinking of the Rubidium clock I got from eBay, he came up with the idea of using a transfer standard.

The thing with accuracy, is that you have to have an absolute reference to compare against. One option is to go to a “calibration lab” and have them test, adjust, and certify that your instrument has a certain accuracy. Awkward, expensive, and usually a bit over the top for “simple” hobby uses.

So the other way to do things, is to “transfer” the calibration in some way. Buy or build a device which can keep the desired property stable, calibrate it to some standard, move it to where the instrument needing calibration is located, and compare the two. Or vice versa: match to instrument, then compare with a standard.

The latter is exactly what we ended up doing. First we created a little Arduino daughter board with a “VC-TCXO” on it: that’s a “Temperature Compensated Xtal Oscillator” which can be fine-tuned through a voltage. Here’s the setup, created and built by Rohan Judd:

On the left, an SPI-controlled digital potmeter, on the right a VC-TCXO running at 10 MHz.

Via a sketch, the VC-TCXO was fine-tuned to produce exactly 10.000,000 MHz readout on the frequency counter we wanted to verify. This was done at about 18°C, but a quick test showed that this VC-TCXO was indeed accurately keeping its frequency, even when cooled down by more than 10°C.

I took this device back home with me, and set up my frequency generator to use the Rubidium clock as reference. So now I had two devices on my workbench at JeeLabs, both claiming to run at 10 MHz …

Evidently, they are bound to differ to some degree – the question was simply: by how much?

Remember Lissajous? By hooking up both signals to the oscilloscope, you can compare them in X-Y mode:

Channel 1 (yellow) is the VC-TCXO signal, some sort of odd square wave – I didn’t pay any attention to proper HF wiring. Channel 2 (blue) is the sine wave generated by the frequency generator.

The resulting image is a bit messy, but the key is that when both frequencies match up exactly, then that image will stay the same. If they differ, then it will appear to rotate in 3D. It’s very easy to observe.

The last trick needed to make this work is simply to adjust the frequency generator until the image does indeed stop rotating. This is extra-ordinarily sensitive – the hard part is actually first finding the approximate setting!

After a bit of searching and tweaking, and after having let everything warm up for over an hour, I got this:

IOW, the frequency I transferred back to JeeLabs with me was 9.999,999,963 MHz. We’re done!

To put it all into perspective: that highlighted digit is 0.1 ppb (billion!). So the frequency counter appears to be 3.7 ppb slow. Assuming that the transfer standard did not lose accuracy during the trip, and that my Rubidium clock is 100% accurate. Which it isn’t of course, but since its frequency is based on atomic resonance properties, I’m pretty confident that it’s indeed accurate to more than 0.1 ppb.

The real story here, though, is that such breath-taking accuracy is now within reach of any hobbyist!