Educational function generator kit

[c4757p]

Anybody have any clever ideas for capturing a modulated waveform on an old DSO (TDS-380)? All my attempts look like poo. Even in peak detect mode, it seems incapable of not screwing up the carrier. I wanted some data for more MATLAB analysis...

[sync]

What do you want to measure?
Perhaps a demodulator (probe) helps?
 

[c4757p]

Yes, I wanted to demodulate and measure the distortion of the modulating signal. I was hoping not to spend the time that would be required to build a demodulator probe that distorts very minimally

[sync]

Can't you just measure the distortion of the demodulator with your 3325A and compensate it in software?

[c4757p]

Doubtful - a "simple" demodulator probe is pretty much just a diode peak detector, and there's not much compensation I can do for the fact that anything under the threshold voltage is lost. Lost is lost. Even a "lower-distortion" one will have more distortion than this circuit, which has proven to be at least somewhat linear.

I think if I wanted to make a precision demodulator, I'd differentiate the carrier, trigger a fast sample-hold off the zero crossings of the derivative, then LP filter that around half the carrier frequency. It'd be some work to make sure it distorts minimally. I'd rather do it digitally.

[adnewhouse]

there's nothing quite as fun as stepping the signal generator through 100 combinations of modulation amplitude and carrier frequency and writing down each output amplitude... Except perhaps knowing it's got a nice little port on the back that could automate it all...

This may be the most aggravating thing I've ever heard. I feel your pain. Last year I had to calculate the standard deviation of a huge data set for school, but since it was a learning exercise we had to do it manually. The worst part? I had a calculator with stat functions and a computer less than three feet away. Yet doing it manually was required. Automation is your friend...

[c4757p]

Last year I had to calculate the standard deviation of a huge data set for school, but since it was a learning exercise we had to do it manually.

Yep, a "learning exercise", because nothing teaches you how to do something in the real world quite like wasting a large amount of time doing it how you will never, ever do it again in the real world.

[sync]

Doubtful - a "simple" demodulator probe is pretty much just a diode peak detector, and there's not much compensation I can do for the fact that anything under the threshold voltage is lost. Lost is lost.

DC bias the diode. This is used in the HP 810C 410C VTVM. It uses a diode tube with about 7V forward voltage. The lowest value/tick on the scale is 50mV.

[c4757p]

Yeah, that probably should have been obvious. I might play around with that a bit.

[GK]

A diode detector (even biased) will have very poor linearity. Since you have the unmodulated carrier signal avaliable, synchronous demodulation/AM detection with a product detector would be the best way to make a linear detector.

A linear analogue multiplier made with the '1496 will have a temperature dependant scaling factor, independent from internal device matching and external offset trims (log amps have the exact same issue). In any critical/precision application, this has to be externally compensated for, typically with a thermistor. Blow your '1496 with a heat gun and watch the amplitude of the modulated output signal increase.

[c4757p]

Good point - I'll investigate that. I don't think it will be an issue - just like most function generators, it's not a "precision" modulator. But I can look into thermistor compensation if necessary. (Now that I have this working, I'm back on the homework wagon now for a couple days, so things are back to moving slowly...)

(Or were you just referring to my comment about building an analog multiplier with it?)

[GK]

Now that I think of it, also look up "clamping amplifiers" from AD - makes it easy to implement a MHz-input, precision half wave rectifier suitable for AM detection.

[GK]

(Or were you just referring to my comment about building an analog multiplier with it?)

Yes.

[c4757p]

Quick question for anybody with a bit more experience with IC manufacturing - am I correct to assume that pin 4 (V-) on the LM7171 op amp goes to the bulk? That chip gets pretty hot even just with its own quiescent current, never mind if it's actually doing something, and I have the room to stick a copper pour under and around it to soak up some of that heat. I'm trying to get the noise down a bit, so probably not a bad idea.

Now that I think of it, also look up "clamping amplifiers" from AD - makes it easy to implement a MHz-input, precision half wave rectifier suitable for AM detection.

Every time someone suggests an AD part, I start to salivate, and my wallet runs and hides.

[c4757p]

Not much has happened in a few days - waiting on PCBs, working on other stuff, schoolwork.......

But here is a draft of the "how BJTs work" lesson section. Anyone brave enough to dig through eight pages and help me proofread/fact-check? Note that this was written from a "how BJTs are used" perspective, not an "underlying physics" perspective, and has in many places simplified the math. I started with a simplified (forward active region only) Ebers-Moll equation, and derived the hybrid-pi equation from this. The other operating regions are explained in terms of general function, not in their full, mathematically modeled forms.

Note that I have not introduced any of the usual transistor circuits here, with the exception of the emitter follower. This is because the majority of those circuits don't require a BJT, but just a generic transconductance amplifier, and I have chosen to introduce them from that direction. (The text is designed to allow the reader to skip sections he already knows about, and doing that made it more modular.)

Warning: large file. Also, there should be figure and section hyperlinks, but those were stripped when I extracted the section from the full PDF.

bjt.pdf (2.33 MB)

[c4757p]

Tentative schematic for the AM option module.

Edit: And a draft of the PCB. Quickest way I could export it.... it's almost 3AM and I'm going the hell to sleep now

Edit: Removed files to make room for Dave.

[c4757p]

Final PCB for the AM module, to be sent off after the "oh shit!" moment I'm sure I'll have in six or seven hours.

Schematic has not changed, see above.

Since all the headers are towards the top, I may fit one screw/standoff to the bottom side to help fasten it in.

Edit: Removed files to make room for Dave

[BravoV]

4 mounting holes at corners ? 

[c4757p]

No room, really. That board's 50x50mm, that would be a lot of space to lose to mounting holes. Considering it's lightweight, the headers at the upper right and near-upper left should be enough for horizontal support, and for vertical support at the back, and one standoff at the bottom will keep it up and keep it from lifting off.

I don't want to push the components any closer to make more space. I originally had them very tight, with no room for reference designators and barely room to avoid solder bridges. Then I remembered the target audience and spread them out more. That's as dense as it needs to be.

[c4757p]

OK, I've got two new PCBs here. I redesigned the comparator PCB layout from scratch - it has a couple problems:

- The input and output are single-wire, ungrounded connections. Stupid, stupid stupid stupid. Let's see a 4ns rise square wave have to snake back through the massive loop to the one ground connection that it had......  I have no idea what I was thinking, I know better than that!

- 0603 is unnecessarily small IMO, I'm backing the size out to 0805, and I also replaced the PMSS3904/6 (SC-70) with MMBT3904/6. They're cheaper, too! I will still use the occasional SC-70 where necessary (BFS17W are much cheaper than BFS17, for instance), and TSSOP for chips with more than eight pins. Those are pretty easy.

I am considering one further small change. The 4151 diodes are a bit hard to find, and depending on the phase of the moon and what the DigiKey and Mouser stockists had for breakfast, they can be available in different footprints. I may see if I can make up a "combo" footprint that will take a SOD-80, MicroMELF, or SOD-123 (at least they're all pretty much the same shape...).

Also, the final final AM module (no big changes, but I tidied it up a bit, it looked a bit crazy).

I'll send these out Sunday night, so hopefully we catch the mistakes that I'm sure they still have...

Nobody seems to bother looking at the 3D renders, so I'll just stop posting them and filling up Dave's server...

[echen1024]

I would like to have a sample of these for testing...
LAck a decent function gen right now.

[c4757p]

These are the only two boards that will be plug-in modules, everything else goes on the single, main PCB (the remaining few options are simple enough to be just "populate optional component U6" or "omit U5 and bridge jumper JP2"), and it'll be a while before there's a sample of that, unfortunately. I tend to get in about five or six hours of design work a week between the other things I'm doing

[AndersAnd]

Nice project, and a very useful tool to most.

Will the source code, schematics- and PCB CAD-files be available too? Or just the firmware, gerber and and PDF-schematics?

*Naturally, when I changed out the buffer circuit, I swapped the NPN and PNP transistors, and managed to blow up half the damn VCO, including half the diode bridge, one current source, at least half a TL072, and contributed to the extinction of the obsolete 2N5950 JFET...

Wouldn't it be better to base the design on a non obsolete part, or is the obsolete 2N5950 just for prototyping?
Fairchild recommends MMBF4416A as Replacement Part for 2N5950 in their  Obsolescence Notice:
2N5950: http://www.fairchildsemi.com/pf/2N/2N5950.html
MMBF4416A: http://www.fairchildsemi.com/pf/MM/MMBF4416A.html
Pretty similar specs by the looks of it, although MMBF4416A is SMD and 2N5950 is leaded.

[c4757p]

The final version will have a BF545. It's just a source follower - pretty easy to find replacements as the things inevitably become declared "obsolete"... I just tested with 2N5950 because I have a pile of them (I bought 100 when production stopped and Mouser was selling them off for peanuts)

Nice project, and a very useful tool to most.

Will the source code, schematics- and PCB CAD-files be available too? Or just the firmware, gerber and and PDF-schematics?

I will make it all available when finished. (Hell, I might toss it into GitHub before then, it's all in a git repo anyway)

[AndersAnd]

It's looking like the AVR I was initially going to use is a better fit. Specifically, the ADC on the NXP chip doesn't appear to take an external reference. It is also cheaper - I can probably fit everything into a mega48 or mega88 easily. It also has the benefit of being familiar to a lot of people.

I agree that using an Atmel AVR is a good idea. A very common MCU and cheaper and easier to find locally than many ARM based MCUs. A lot of hobbyists are already familiar with AVR and you can also download a free tool-chain including GCC-compiler. Will you be programming it in C?
And since Arduino became very popular, even more hobbyists has become familiar with AVR.

A lot will already have an ISP-programmer for AVR. But otherwise clones of Thomas Fischl's USBasp (USB programmer for Atmel AVR controllers) can be bought on places like eBay or AliExpress for less than 3.5 USD with free shipping.

Or if you are willing to spend a bit more you can buy a clones Atmel's own AVRISP mkII for less than 13 USD.

And an AVR ISP programmer will be a very useful tool for many other electronics projects too.

Programming the AVR could be part you tutorial too, and something useful to learn for those not already familiar with it. Or you could just link to someone else's online tutorial.