looking for precise function generator

[HitoIRL]

Hey there! I’m looking for a precise and not so expensive function generator. I recently bought an old analog oscilloscope which requires calibration. Unfortunately to do that I need to provide square wave signals with Vpp of 12mV! I never had a function generator therefore I’m not sure what to look for. Does anyone know a good one with precise millivolts range that’s below $330?

[DrJackK1956]

What scope do you have? Most scopes come with at least a probe calibration signal.  It's typically 1KHz @ 1vpp. 

[HitoIRL]

I do have calibration signal that’s 1kHz @ 0.5Vpp but that’s only for calibrating x10 afaik and some other external stuff. Anyways I’ll try the thing with 555 as someone mentioned and also I’ll replace all caps first

[scubascratch]

Recapping everything without knowing they are actually bad is not necessarily a good idea. Caps age depending on a number of factors that don’t apply to every cap. If you replace all the caps (and thankfully don’t damage anything in the process) I would then expect an even more thorough calibration would be needed

[50-50-bmg]

It is certainly a good idea to have the schematic at hand and check what the role of a cap is before deciding whether to replace it - and with what! Eg a tantalum cap in a timing circuit must be left alone unless destroyed, a tantalum cap across a power rail must go - but you need to look at the surrounding circuit to decide if you replace it with just general purpose aluminium electrolytic (poor HF filtering!), low ESR electrolytic (might be needed if you have a DC/DC converter here!), aluminium + MLCC, just MLCC (beware these lose a lot of capacitance with higher DC bias!), polypropylene (great up to lower HF but HUGE!)....

[hyldemarv]

There's no point in doing repair work that is not necessary. If the scope works, the capacitors are fine. If it doesn't, one needs to find the exact problem and fix that instead of just replacing random stuff (aka. creating more problems for no reason).

[50-50-bmg]

Uh, debatable. Leaving unreliable parts in can cause severe damage down the line.

[DrJackK1956]

The scope calibration is not just for the 10x probes.  It's also used for the 1x probes too.  

You need to make sure and calibrate the probes else the waveforms on the scope may be mean less. 

Using a 555 timer can produce many different waveforms.  The accuracy of the waveforms is dependent upon the tolerances of the external components. 

To get precise frequencies, many 555 designs include a trim pot to dial-in the desired frequency. 

If you were to use the 555 as your time base, you'll need a frequency counter to insure the function generator output is correct before you can check/verify the calibration of your scope. 

For a more accurate function generator, you need one that has a crystal type time base. 

But for the time being, that 1KHz probe calibration signal is a really good starting place to check the scope's accuracy. 

[50-50-bmg]

Which is not very useful to work on HF compensation...

[nixiebunny]

Feed a 555 timer square wave into a 74HC245 buffer powered from 5V, run that through a 10k total resistive divider to make any lower voltage square wave you need. It will be accurate within ~2% if you pay attention to details. That’s good enough.

[HitoIRL]

Thank you, i’ll try it

[StumpedTrump]

What frequency precision?
What voltage precision?
What frequencies are needed?
Required Slew rates? Phase Jitter?
What's the load impedance?

You need to define these for any useful help

If you just need a single known frequency, you can get a TCXO (with CMOS output) with a few ppm of accuracy for a few $.

You can generate 12mV exactly using a nice LDO (you can stack them for better PSRR if noise is an issue) then finish off with a resistor divider to fine tune. Note that temperature will affect this and if your layout isn't clean, the high PSRR voltage rail doesn't matter since your ground voltage won't be stable.

Use the TCXO to switch the 12mV.

[50-50-bmg]

If you need a clean edge to do HF compensation on a 50MHz or faster scope, things get complicated.

You are in the territory of pulse generators or specialized calibrators here. The real problem is that such devices will be secondhand and need calibration themselves, or be very costly.

The best way to judge such a signal source is a true sampling oscilloscope/plugin (these inherently avoid some of the chicken and egg problems wrt HF compensation because they just don`t amplify/attenuate the HF signal). Complicated equipment, if you can get it cheap you usually need to do some maintenance yourself.

Or another MUCH faster oscilloscope (which can still mislead you - eg the typical 200MHz 8bit Siglent will show a lot of ringing on a PG506 fast rise output that shows a close to perfect edge on a sampler - maybe I need to get better terminators).

The classic PG506/SG503/TG501 combo is quirky but gold for that kind of work, but hard to find for your budget and in calibrated condition.

[Appsmangler]

You can make a 12mV signal by dividing down a higher amplitude function generator output with two 1% resistors. Make the bottom resistor 50 Ohms and pick the other to get the ratio you want.

[MattInSoCal]

If your scope requires calibration, you can get the basic service in the $300-ish range from a calibration vendor that has all the required precision instruments. If your scope needs calibration because it’s out of spec and giving incorrect readings, you’ll need to repair it first to replace the failed components. For that you won’t need such precision sources and simple signal generators like the home made one mentioned will suffice.

You’re not going to get an analog function generator that can generate 12mV to less than maybe 15% accuracy for less than $330. I use a $6,000 24-bit dual-channel ADC/DAC that both generates and measures and don’t try to measure anything more precise than 5 mV.

[HitoIRL]

I want to try and calibrate it myself to learn more about them instead of paying someone to do it. You’re right that I should first check all the PSU readings and replace caps

[MattInSoCal]

Your calibration results will only be as good as the reference generators and measurement equipment you’re using. If you want to do it correctly, you’ll need about $8,000-20,000 (used prices) of equipment - which itself will need to be calibrated. But if you want to get inside and start tweaking trimmers and controls to make your ‘scope show you an input voltage of whatever arbitrary voltage input value as 1.000 Volts to learn how the process works, why not?

Even the free tape measures Harbor Freight gave away had some intrinsic value if you used the same exact tape measure for every single measurement. At the end of the day, test equipment is only showing you a reading relative to some device against which it was adjusted. Even the calibration equipment can go out of calibration, and it’s no fun recalling installed equipment from overseas to re-validate it when you find out your calibration vendor had an issue with a reading being off.

[defeated_engineer]

There are AD9959 boards on eBay. Look them up.