Syscomp Scope and application

[Jeff Callahan]

This comes from Peter at Syscomp, he's been very helpful to me figuring out all of this. Highly recommend the scope.

[salvarsan]

Looks like a definitive pickup test, Jeff.
Outstanding contribution from you and Peter.

[Peter Naglitsch]

Great Jeff. I have had the syscomp quite some time now but have not had time to play araound enough with it. Now this comes in very handy.

[mickmutante]

I am waiting for my Syscomp Scope... so I can "play" with it.
Thnks for the info Jeff.

[mickmutante]

I have a question... the driver coil described on the "Pickup Resonance.pdf", could be like the one described by Lemme: "The coil can consist of a pickup bobbin wound with about 50 turns of enamelled copper wire (approximately 0.5 mm, or 0.02 inches in diameter, no. 24)."
Or what do you use for driver coil.?

[Possum]

Its nice to see Syncomp finally made it easy for Mac users, I emailed them about using it on a Mac and never got an answer, their Mac instructions were totally confused.

For a driver coil use a humbucker bobbin and wind a coil with 38 AWG wire you can find in some electronics supply houses or off ebay, use a slug bobbin as this can fit nicely over pole screws on a bucker pickup. I wound mine to about 50 ohms. I don't think it matters as long as its low impedance so won't enfluence the results too much. I may actually buy one and hope it actually works on a Mac, I can't find any Mac reviews of this product....

[Possum]

What lead sets would I need to do pickup work with this thing? Is it as simple at hooking one set of leads to a driver coil and the other the leads of the pickup to input leads? If its more complex than this can someone post a photo of how to set it up, I'm more of a mechanic and suck at reading wiring diagrams.

[salvarsan]

Its nice to see Syncomp finally made it easy for Mac users, I emailed them about using it on a Mac and never got an answer, their Mac instructions were totally confused.

The software has always been cross-platform, looks the same whether on Windows, Linux, or Mac OS X.

For a driver coil use a humbucker bobbin and wind a coil with 38 AWG wire you can find in some electronics supply houses or off ebay, use a slug bobbin as this can fit nicely over pole screws on a bucker pickup. I wound mine to about 50 ohms. I don't think it matters as long as its low impedance so won't enfluence the results too much.

As I read the procedure, it doesn't matter much so long as your driver coil's resonance point is well out of the audio band.

[Possum]

Its always been cross platform but only computer geeks could figure out the Mac installation using the Terminal program. You can seriously damage the Mac OS by fooling around in Terminal if you don't know how to use it and not experienced in its use, which I'm not. I make my living on my Mac and can't afford to kill the machine, the installation instructions were confusing, Peter told me I'm not the only one who couldn't figure it out. Anyway, they've automated the installation and I'm going to buy one to identify resonant peak frequencies better than the multimater method, which never really worked very well.

Using a humbucker bobbin is a real good idea because almost all pickup coils are long coils, so you get better transfer between two coils if they are of the same type dimensions. Lemme did the same thing and thats where I got the idea.

[LtKojak]

Hey Possum!

Long time no see... I'm very happy to see you're around this forum.

Hoping things go really well for you, I'm happy to present my respects to you. (is this expression correct? I'm having a MAJOR brain fart as I'm typing this)

Yours very truly,

[Mike Sulzer]

This comes from Peter at Syscomp, he's been very helpful to me figuring out all of this. Highly recommend the scope.

Measuring the response of a pickup is complicated, and there are some factors not discussed in Peter's pdf file.

First, let's look at an idealization of Peter's measurement and show that it would lead to a measurement that is flat across frequency.

A magnetic field is cause by current, and so the first thing to look at is the current in the driver coil as a function of frequency. Peter's driver is an inductor, but it is not an ideal inductor. It has resistance, and this is important at the lower frequencies. But for the moment, let's assume that we have an ideal inductor. Let's also assume that Peter's generator is an ideal voltage source. If we connect the source across the inductor, we get a current that decreases with frequency. This is because the impedance of the inductor rises with frequency. Therefore the strength of the field produced by the source decreases with frequency.

Now let's consider an ideal pickup; this would be one without the effect of resonances or eddy currents. A pickup works by the law of magnetic induction, which says that the voltage induced is dependent on the strength of the magnetic field and how fast it changes. (If you want to think of it as a variable reluctance magnetic circuit, that is OK, and you will come to the same conclusion. Working with the induction law is less confusing.) Since a high frequency sine wave changes faster than a low frequency one, the pickup has a built in response that rises with frequency for a magnetic field that is constant in strength with frequency.

So if we make the measurement with the ideal inductor driven by a voltage source, we see a response that is flat at all frequencies. The decrease with rising frequency of the current in the coil is compensated by the rising sensitivity of the pickup.

In Peter's measurements we see this behavior approximately, not exactly, in the middle of the frequency range. This is because Peter's driver coil is inductive in this frequency range, and the pickup resonance is higher in frequency and not a factor.

What about the falling response at lower frequencies? Look at figure 4, the impedance of the driver coil. It is resistive at the lower frequencies. This means that the current does not keep increasing as the frequency goes down as it would with an inductor. Constant current with frequency means constant magnetic field with frequency, and so the pickup response rises with frequency at the lower frequencies.

So that leveling off of the response with rising frequency in Peter's plot has more to do with the properties of the driver coil, not the pickup. All pickups would show this because all pickups have a rising frequency response (excited with a magnetic field that is constant with frequency). The base response of all pickups is essentially the same, because this is below the effects of the resonance and eddy currents.)

Of course, the pickups do have different frequency responses, and this shows up. The dip in the KHz range in #1 is characteristic of eddy current losses. It is possible that #1 is a humbucker with steel cores, but it could be something else as well. Jeff could tell us if he likes.

Pickups #2 and #3 lack this dip, and so they appear to have lower eddy current losses. This could be typical of a single coil pickup using alnico rod magnets, but of course there are other possibilities.

[salvarsan]

Excellent analysis, Mike.
I think it's time to settle on a relatively standard driver coil and get empirical data.

[Mike Sulzer]

Excellent analysis, Mike.
I think it's time to settle on a relatively standard driver coil and get empirical data.

Some ideas:

Peter's coil uses the right approach, that is, we want a coil that has the current decrease with frequency. This is to minimize the dynamic range of the measurement. The Syscomp hardware has only ten bits.

So ideally the coil would appear inductive down to the lowest frequency of interest, but have a resonance above the audio range. This might not be practical because of the 150 ohm output impedance of the generator. So it might be necessary to accept a more limited performance and make a correction to the apparent response after the measurement. There is nothing wrong with that as long as the coil is well-characterized.

It is also a good idea to think about what one really wants to do. What is one accomplishing with the coupling coil? It is not necessary for measuring the frequency response; you can get that from a good impedance measurement and some computation. On the other hand, assuming one has the right coil, this is probably the easiest way to get the frequency response.

Also, using a coil is the easiest way to get information about the relative sensitivities of pickups. With some careful calibration, that could even be absolute sensitivity. But it is unclear how much the coil measurement really represents what happens with a vibrating string. Maybe the best way to go is a small coil with a high permeability ferrite core that can be placed right over one pole piece of the pickup.

[Possum]

Howdy Kojak, same niceties to you. Well, the Syncomp goody brought me out of the woodworks, I don't stop by here much but saw that post, and then corresponded with Peter, so I bought one for my Mac. Looks like it could be useful, but we'll see, price was right for what it does and the pdf paper helps left brain challenged guys like me make sense of some things.... All these methods are personal, there's obviously no "right" way or there would be an industry standard, which there isn't. Lemme's driver coil works for the experiments I did in the past and is what I'll use for this one, don't forget to tip it up 90 degrees and place in the middle of the 2 coils for humbucker readings......

[Mike Sulzer]

Lemme's driver coil works for the experiments I did in the past and is what I'll use for this one, don't forget to tip it up 90 degrees and place in the middle of the 2 coils for humbucker readings......

I wonder a bit about inaccuracies that introduces if you are making sensitivity comparsons between humbuckers and single coils. I think two small coils, one over a slug and the other over the corresponding screw pole piece, would allow better comparisons. You would use just one coil with the single coil. But I have not tried this, just speculating.