Doubt about DC output on Waveform Generator - Page 1
Forum section: electronics / beginners

author: Syro
author_rank: Newbie
timestamp: September 17, 2022, 08:05:01 am
content: Hi everyone. This is my first post on this super useful forum.I'm an electronic musician and I started one year ago to study classic analog synth schematics with the goal to design my own instruments for my records. I always loved DIY and for the last ten years I build tons of DIY projects related to audio stuff and synthesizers, which gives me a little base on audio electronics.I started to protoboard an analog oscillator based on the classic CEM3340 IC. I followed the datasheet and all work as expected but I would like to improve the tuning process of the oscillator to do it faster. Here is where my question comes.To tune it properly I need to plug either 1V and 5V DC signals (as accurate as possible) on the exponential input of the oscillator. This input has a buffer followed by a 100k resistor. I have access to a waveform generator (Siglent 1032X) which could provide me both DC signals and also a Frequency counter which is also needed for the process.Can I connect the output of the Waveform Generator directly to the exponential input of my circuit with a regular 50 Ohm BNC cable? Should I add some termination to the cable?Sorry if is a dumb question!Thanks in advance!

author: RoGeorge
author_rank: Super Contributor
timestamp: September 17, 2022, 09:44:47 am
content: WelcomeAbout cable and 50 ohms termination: if you don't add the 50 ohms to ground termination, the voltage of the test signal will be double than what is it displayed on the generator. For example, if you set the generator for 1V and you don't have the 50 ohms terminator, then you'll measure there are in fact 2 volts applied to your circuit. This is not a bug, that's how it is supposed to be, here's the reason:https://edadocs.software.keysight.com/kkbopen/why-your-function-generator-outputs-twice-the-programmed-voltage-589745012.htmlKnowing that, you can always set the generator for half the voltage, and don't use any 50 ohms terminator. For example, if you want to test a circuit with 100mV (without 50 ohms terminator), then set the generator for 50mV.To make all even more confusing, some models of signal generators have a setting for High Z, and some have a setting to manually set the Z other than 50, so they will calculate the correct level. So, if you set the generator for high Z, nothing will change in the generator, the 50 ohms series internal resistor will still be there, and the internal impedance of the signal generator will still remain 50 ohms no matter what you set, just that when you set 1V in high Z mode you will see indeed 1V at the other end of the cable (without 50 ohms load) instead of the 2V you would see if instead of high Z you set 50 ohms in the generator.About the necessity of the 50 ohms terminator, in DC or audio frequency that doesn't matter (as long as you consider the voltage level is in fact double, as discussed before). In radio frequency and sharp pulsed signals however, the 50 ohms matching of cables and terminators are essential, otherwise it will appear unwanted reflections in the signal at every step in impedance. Again, not a problem in audio, but in RF any cable is a transmission line, and if the impedance is not properly matched (complex conjugate), then you will see reflections.Quote from: Syro on September 17, 2022, 08:05:01 amTL;DR yes, they can be connected directly. Better add a 50ohms terminator resistor at the circuit side end of the cable, so the voltage will be the one set from the generator. The terminator is not mandatory, but then you'll need to consider the explanation above.

author: Syro
author_rank: Newbie
timestamp: September 17, 2022, 11:58:46 am
content: Thanks a lot for the explanation! The generator has indeed the HiZ mode so I think I can use it this way.Big thanks again. It clarifies all my doubts!

author: radiolistener
author_rank: Super Contributor
timestamp: September 18, 2022, 10:41:37 am
content: Yes, you can use signal generator with selected DC waveform to provide 1V and 5V, but note that signal generator output is intended for 50 Ω load. So, if your circuit load is different than 50 Ω, then you will have different Voltage. But you can attach DMM in order to measure actual Voltage and tune it.Also note, that HiZ or 50 Ω mode in signal generator settings is just intended for proper Voltage indication. Signal generator output has always physical 50 Ω output impedance, despite the fact you change setting to HiZ or other value.Don't trust what Voltage is displayed on signal generator, because this Voltage is relevant for proper load impedance. So, I recommend you to just use DMM in order to see actual Voltage.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 18, 2022, 03:14:41 pm
content: There's no need to terminate the cable, at DC or AF. A few Ohms cable resistance is not going to add much to your ppm's. [Marco will no doubt be cringing out there]. Use the shortest cable you have to avoid ground loops and pick-up. You probably want to meaure the CV independently of the signal generator to calibrate your set up. The VCO ic's are not high precision devices so super accurate calibration voltages are not usually needed.I imagine you just want to set and forget the VCO scaling pots. In which case you might want a stable switchable 1V/ 5V reference. See untested idea attached. Operates from 6-12V, E96 resistors. No on off switch needed, it could last years on some coin cells. A battery powered reference also lets you ground the test CV at the VCO end. You might get it to output a a decent calibrated squarewave by repacing the mecahnical switch with a low voltage analog switch controlled by a signal generator.For a bench test with the generator set output to High-Z. The output voltage amplitude from the Siglent 1032X is not precise, say ±1% up or down from set value. Is that just the set point or general wander I wonder? The 50 Ohm output impedance of the generator is accurate to ±1%. Terminators are not precisely 50 Ohms, the tolerance is not usually in the spec. In fact, I've never seen it, even on the superexpensive 10GHz ones. So using one will just add to the uncertainty in an uncalibrated set up."50 Ohm" is not a precision specifcation. Its one of the "standard" impedances (not resistances) for RF cables which in reality are not tightly controlled. ±5% is good! A dB here and there who cares! This is one reason why RF aficionados are incorrigible tweakers.