measuring resonant peak

[StarryNight]

I'm following Joe's diagram but I'm not getting too far. I've got the 1M pot, I'm using my Ipod with a signal generator/sweep app. My multi meter has two AC settings 200 and 750. Do I need a more sensative meter? Joe says to "measure the frequency of the signal generator" I'm not sure what that means. I can see the value of the frequency on the screen - is that it? I should have sat at the front of the class dammit!

[peskywinnets]

Edit: (original post deleted - bad info is worse than no info!)

Now I'm no expert on measuring the resonant frequency of pickups/coils, but as I (thought) I understood it, you need to slowly sweep through a frequency range with your sig gen say 80Hz thru 15khz) & at some point during that sweep, the AC waveform generator signal voltage as seen/measured across the 1M series resistor will change significantly - this phenomena occuring at the pickup's resonant frequency.

Now at resonance, I thought the pickup coil becomes essentially purely resistive (as opposed to resistive and reactive when not at resonance)...if this is the case I'd have thought the waveform across the 1M resistor would increase when the pickup's resonant frequency is hit?!! My line of thinking here is 'outside' of resonance, the pickup would have a higher AC resistance due to the summing of reactance & DC resistance, if the 'reactance aspect' drops out at resonance, then this increases the 1Meg resistor's 'play' in the test circuit's resistive voltage divider network).

The notes accompanying the diagram that Possum linked to above, suggests the AC signal across the resistor will be at it's smallest when the pickup is at resonance - so can someone explain the techie bit wrt why this occurs?

I've just tried the method referred to - I've got a nice sig gen & a decent USB scope - I've just wired up a generic humbucker as per Joe's diagram (again, linked to by Possum further up this page)...I was expecting to see a notable change in the signal level across the 1M resistor in or around resonance (which for a no name generic humbucker, I'd expect that to be in the 3Khz-7khz range).

What I saw, was no discernible difference in the signal amplitude as measured across the resistor while slowly moving through that frequency band. This is with a 25Vpp signal being applied across the pickup coil & series resistor.

So....are we sure the theory is right? And bearing in mind the 25V seen across the resistor, what kind of divergence could I expect to see at resonance? (are we saying it should drop significantly?)

[Possum]

....

iPods don't have enough power.

[Peter Naglitsch]

I'm seriously considering getting me one of these:
Syscomp Electronic Design Ltd.
There is a Network analysis mode that seems to be exactly whats needed:

and it isn't to costly at 189$. I have e-mailed their support with a few questions and if I get the "right" answers I'm getting one of these

[Joe Gwinn]

I'm seriously considering getting me one of these: [image deleted]
There is a Network analysis mode that seems to be exactly whats needed: [image deleted]
and it isn't to costly at 189$. I have e-mailed their support with a few questions and if I get the "right" answers I'm getting one of these

Wideband (2 MHz) noise isn't so much of a problem, but I would ask about the peak and average total harmonic distortion of the output for the worst frequencies (whatever they are in this product). I assume that the generator is based on a DDS (Direct Digital Synthesis) chip. If you know the chip number, you can read the chip's datasheet.

[Joe Gwinn]

Edit: (original post deleted - bad info is worse than no info!)

Now I'm no expert on measuring the resonant frequency of pickups/coils, but as I (thought) I understood it, you need to slowly sweep through a frequency range with your sig gen say 80Hz thru 15khz) & at some point during that sweep, the AC waveform generator signal voltage as seen/measured across the 1M series resistor will change significantly - this phenomena occurring at the pickup's resonant frequency.

True.

Now at resonance, I thought the pickup coil becomes essentially purely resistive (as opposed to resistive and reactive when not at resonance)...

This too is true.

if this is the case I'd have thought the waveform across the 1M resistor would increase when the pickup's resonant frequency is hit?!! My line of thinking here is 'outside' of resonance, the pickup would have a higher AC resistance due to the summing of reactance & DC resistance, if the 'reactance aspect' drops out at resonance, then this increases the 1Meg resistor's 'play' in the test circuit's resistive voltage divider network).

The thing to remember is that reactance is electrically orthogonal to resistance, so ordinary addition (which ignores phase) doesn't work.

In the Space Orthogonal to Real, one must use vector addition.

The notes accompanying the diagram that Possum linked to above, suggests the AC signal across the resistor will be at it's smallest when the pickup is at resonance - so can someone explain the techie bit wrt why this occurs?

It's a bit complex to prove without complex variables and circuit theory, but with a parallel LC circuit (pickup coil inductance in parallel with its own self-capacitance), as one slews the test frequency to resonance, as the reactance drops towards zero the AC resistance grows to a maximum (not a minimum). This causes the AC current through the 1 Mohm resistor to drop, so resonance is where the voltage across the 1 M resistor is minimum.

Why a maximum you may ask. The simple answer is that at resonance a fair amount of energy is stored in the LC circuit, bouncing endlessly between the L and C components. Resistance is the ratio of voltage divided by current. The voltage across the LC circuit is that induced by the bouncing energy, while the current entering the LC circuit is that required to replace the energy lost in the inevitable parasitic R component of the inductor. The bouncing energy is maximum at resonance. This is discussed under the rubric of "LCR circuits" (or some permutation of LCR).

RLC circuit - Wikipedia, the free encyclopedia


By contrast, in series LC resonance (inductor and capacitor in series), the reactance is zero and the AC resistance is minimum at resonance.

I've just tried the method referred to - I've got a nice sig gen & a decent USB scope - I've just wired up a generic humbucker as per Joe's diagram (again, linked to by Possum further up this page)...I was expecting to see a notable change in the signal level across the 1M resistor in or around resonance (which for a no name generic humbucker, I'd expect that to be in the 3Khz-7khz range).

What I saw, was no discernible difference in the signal amplitude as measured across the resistor while slowly moving through that frequency band. This is with a 25Vpp signal being applied across the pickup coil & series resistor.

I would search above 7 KHz.

I would also verify that the USB oscilloscope isn't overloading the test circuit, as mentioned below.

What is the make and model of the USB scope?

So....are we sure the theory is right? And bearing in mind the 25V seen across the resistor, what kind of divergence could I expect to see at resonance? (are we saying it should drop significantly?)

The theory is right, but there may be a silly fumble in the test setup. What is the input resistance and capacitance of the USB scope? If you have a digital multimeter that claims to work to at least 20 KHz, I would try that as well.

[salvarsan]

I'm seriously considering getting me one of these:
Syscomp Electronic Design Ltd.
and it isn't to costly at 189$. I have e-mailed their support with a few questions and if I get the "right" answers I'm getting one of these

What right answers do you need?

Got one meselfs. *Works. *Add in a couple of ~60MHz 10:1 probes and a BNC-to-clip-leads cable for a total under $250.

Since the network analysis display is dB vs. frequency, I drove the pickup through an equivalent DC resistance, e.g., *~9k P90 pickup is driven through a ~9k resistor, a got this -6dB plot:

[Possum]

...

they didn't reply to my questions, don't get one if you're using a Mac, they are clueless about Macs.

[Mike Sulzer]

they didn't reply to my questions, don't get one if you're using a Mac, they are clueless about Macs.

While we are on the subject of Macs, consider that a mac laptop has the hardware that you need to make these measurements. All you need is software.

Is a sine wave generated with software using the mac hardware good enough? Probably not, so do not use a sine wave; use random noise. Sweeping is no longer necessary. Nor are the details of the random noise important if one uses software that measures the transfer function. That is, at each frequency, it measure the ratio of the output to input. So the random noise need not be perfectly flat in its frequency response; there are no requirements on the linearity of DAC. The A to D converters must be good, but they are.

Since it is the O/I ratio that is involved, no integration is required to get a pretty good answer. Of couse, there is always noise in any system, and so integration is required to get really accurate results.

[salvarsan]

they didn't reply to my questions, don't get one if you're using a Mac, they are clueless about Macs.

No disrespect Dave, but you may have come off as someone who wasn't comfortable with oscilloscopes and they didn't know how to handle you. *It's probably just as well that you didn't get a digital scope from SysComp or anyone else.


Not everyone wants to, or even should, mess with an oscilloscope. *It's kinda like Weber carburetors and cars -- it's a way of life.



-drh

[salvarsan]

Windoze, Linux, and Macs

CGRscope relies upon having a standard Tcl/Tk interpreter. Tcl/Tk has been ported to Windoze, MacOS, and every active flavor of Unix.

* I've run the Syscomp CGR software on several Linux flavors, and on Windoze XP and Vista without incident. *It looks the same on the screen, too.

No Mac tests, yet, but since the Mac migrated to a BSD Unix variant a few years back, I see no reason why it can't run off a Mac that has a USB 2.0 jack.


-drh

[jbforrer]

Greetings,

I've been using the "STEPS" program with a home-made, low-Z driver coil and FET buffer amplifier. An impressive piece of software that will do pickups, speakers, mikes, and other acoustic work as well. Works well under XP with a decent sound card; I use a Delta 44.

Please see "STEPS" --- ARTA Home 03012010

Please note that I have no affiliation with the author of this software --- been playing with the demo version of the software and found it very useful for my pickup R&D.

HTH.

JBF.

[Possum]

....

All I asked them was if I couldn't figure out how to INSTALL it could I get a refund? Their instructions for Mac installation aren't even written by them, I've been using Macs since the Plus and I couldn't make head or tails out of those instructions and suspect there are problems with it. They obviously are uncomfortable with selling to Mac users, anyway I was glad I didn't buy it.

[salvarsan]

All I asked them was if I couldn't figure out how to INSTALL it could I get a refund? Their instructions for Mac installation aren't even written by them, I've been using Macs since the Plus and I couldn't make head or tails out of those instructions and suspect there are problems with it. They obviously are uncomfortable with selling to Mac users, anyway I was glad I didn't buy it.

They figured you were not clear on the process.
If you are comfortable with the Mac .DMG file installation process,
then you can do it. Their CGR application is free for download
from the site.

You'll also need Tcl/Tk for the Mac.
There are plenty of folks to help you with this,
so all you need is a little patience.


-drh

[Possum]

...

Of course I know what a DMG file is, sheesh, I made my living on Macs for 33 years, and work on a latest generaton iMac and have a very powerful Mac graphics desktop tower still, duh. They didn't reply so how can they help me? I told them their instructions were confused and the instructions are not even from their own company, some volunteer geek wrote a really confusing set of instructions written for geeks. If its too intimidating for them to know how to explain how to install their hardware/software to a Mac user then obviously there is very poor support there, they can't even be bothered to answer an email that would have made the sale if they had but I guess they don't know how to support their products on Mac; I definitely don't want to buy something where they tell you figure it out yourself :-) I figured if I bought I'd be stuck and since they don't answer email, may as well flush a couple hundreds bucks down the toilet. In the end you can use Mac software written for Macs that does a better job anyway for free, so thats what I did. Its fun to play with for awhile but it won't really help you make better pickups.