Searching for waveform generator. Output impedance question  - Page 1
Forum section: electronics / testgear

author: Tomer
author_rank: Regular Contributor
timestamp: December 23, 2017, 11:15:11 pm
content: Hello to all.I want to setup a small/home lab for exploring and developing guitar effect pedals.Different guitar pickups outputs different impedance's and in general it's a High-Z and much higher than the standard 50Ohm output used in most signal generators.I saw that there are generators with "High-Z" option, and in some you can also specify a number.1) How can i match the generator signal for testing the pedals ?2) Can you please help me understand generator functions are a must for my attended use ?3) How can i find out what is a High-Z output ? is it auto changing according to the load ? is it a specific number but different from one manufacturer to another ?Thank you for your helpI am a beginner, hopping my questions are making senseThxTom

author: rstofer
author_rank: Super Contributor
timestamp: December 24, 2017, 12:07:45 am
content: I don't know about ALL signal generators but the Siglent SDG2000 series has an output impedance of 50 Ohm. It doesn't change.The Hi-Z setting vs 50 Ohm setting just determines how the output voltage is calculated.You need to think about a voltage divider with the top resistor as 50 Ohm (the output impedance) and some value for the lower resistor. If the lower resistor is given, we can figure out how to drive the circuit to achieve the voltage the user dialed in.The user expects the desired voltage to appear at the terminals for the circuit impedance specified.See page 6 (PDF page 16) here:https://mediacdn.eu/m/media/downloads/SDG2000X_UserManual.pdf

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2017, 03:12:54 am
content: At audio frequencies, the impedance has little effect unless the signal cables are very long as with telephone cables or the impedance level is very high. The impedance of a wound guitar pickup is not low but it is not all that high either.As rstofer says, high impedance and low impedance mode on modern signal generators is for calculating the output level. Internally they are matched to 50 ohms, usually with a series 50 ohm resistor, so the output level is halved when operating into a transmission line terminated into 50 ohms. With my old function generators, I have to do this calculation manually.600 ohms is a common impedance in balanced audio applications and some audio equipment is calibrated to this.

author: Tomer
author_rank: Regular Contributor
timestamp: December 24, 2017, 11:25:40 am
content: Thank you very much for the time answering my questions.Do you think that i will have to match the impedance in order to work with guitar pedals or 50Ohm will work fine with the pedals ?Also I would like to ask if you think i should go with modern (Rigol/Siglent) generator or to search for a much simpler and straight foreword older one ?Thanks a lotTom

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2017, 12:02:07 pm
content: Quote from: Tomer on December 24, 2017, 11:25:40 amEverything in the normal case is already mismatched so I think a 50 ohm test source will work fine. A series resistor can always be added at the source to raise the impedance sacrificing output level.QuoteI do not see how it would matter except for convenience; analog function generators often lack calibrated output levels but that is easily remedied with an AC voltmeter. Function generators do not usually have low distortion but it is low enough for this sort of audio application.My recommendation is to get an inexpensive function generator to see how it works and to get a better idea of your requirements before spending a lot of money. Or build or buy an inexpensive sine wave oscillator.

author: Tomer
author_rank: Regular Contributor
timestamp: December 24, 2017, 03:30:15 pm
content: Thank you!

author: TurboTom
author_rank: Super Contributor
timestamp: December 25, 2017, 07:38:36 pm
content: The Guitar pickup (at least as long as it's the "classic" configuration with a the magnetic system) contributes by a large degree to the "sound" of the guitar. The pickup represents a very high inductance that, together with the amp input impedance and cable capacitance forms an RLC resonance circuit which results in a predominant frequency within the audible range. The simulation circuit that would come closest to his would be to just wire a guitar pickup in series with your signal generator and then feed that signal through a typical guitar cable into your pedal. Since the pickups differ a lot (single coil / humbucker and so on), there is no "typical" configuration that you could use as an average. More modern guitars with active electronics may even provide a low impedance output, but my "active" guitar playing times (unfortuantely) aren't that recent...Hope this information my help a little.Merry Christmas,Thomas

author: 2N3055
author_rank: Super Contributor
timestamp: December 25, 2017, 08:19:54 pm
content: For this type of testing you might simply use tablet with a software or PC sound card..All you need to do is to calibrate level and you're good to go..And as TurboTom already said, impedance on guitar amplifier is a finicky thing..Passive guitar is a complex impedance, connected with cable that is complex impedance, to an amplifier input that is complex impedance.. They all together define sound, by creating all kinds of resonances and filtering of various frequency bands..You cannot trivially simulate guitar impedance, well you could do it, but you would need equivalent simulation network for every guitar you would like to test for...But that is not important. What you want to do is to characterise frequency characteristics of amplifiers (pedals) you like with your generator and then test your design with the same generator..That would, in theory, allow you to create similar characteristics... In practice it's not that easy..In this context, pedal input is preamp input you connect guitar to...