A guitar pickup is uncommon in that it handles two distinct electrical tasks with one component: the coil serves as both an inductor and a current source. It rare in electronics that you have both tasks occur at the same location within the circuit, and this is one of several things that makes a guitar pickup unusual.
The guitar pickup acquires current by way of Faraday's Law of Induction. First, the pole pieces under the guitar strings endow the strings with a magnetic polarity. Now the six guitar strings then contain their own magnetic fields that each extend down into the coils of the pickup. When you pluck the strings, the amount of magnetic field with the coils increased and decreases rapidly, as the string moves up and down, nearer and further from the pickups and it's coil(s), and per Faraday's Law of Induction, a current is generated in the coil. The current is alternating. When the guitar strings approach the pickup, the current flows one way, and when the string move away, the current flows the other way.
If current is analogous to water, then voltage is analogous to water pressure. You can have a lot of water, or current, without water pressure, or voltage. In order to get water pressure, you need resistance, and the same is true of electricity. The way a guitar pickup produces resistance, and hence a voltage, is by containing an inductor. The same coil which generates the current by way of Faraday's Law of Induction, happens to have so many winds (anywhere from 5,000 to 10,000), that it not only receives the magnetic fiend from the guitar strings, but as current courses though the wire, it generates a new magnetic field of its own. This magnetic field pushes back against the alternating current produced by the guitar string, and thus you have resistance (technically referred to as "reactance" in this context), you have voltage.
The primary reason there are "hot pickups" and "vintage" or "cool pickups" is on account of the inductor, or inductance of the pickup. An inductor is never "just an inductor". To some extent, it is also a capacitor, because electric charge attempts to attract along the windings of the coil. As you know from measuring guitar pickups with a multimeter, pickups also have some resistance; usually between 5k and 10k, on account of the resistance in the copper wire. So you have resistance "R", inductance "L" and some capacitance "C", therefore evey inductor can be said to be a "parallel RLC circuit". A parallel RLC circuit will form a "resonant peak", and at the frequency of the "resonant peak" the resistance (referred to as impedance in this context) reaches a maximum, and therefore causes the pickup to produce the most voltage at that particular frequency. The lower the inductance L and capacitance C, the higher the resonant peak will be, while higher values of L and C will result in a lower resonant frequency.This is the underlying reason why there are "hot pickup" and "cool pickups". When a pickup is "hot", it has more winds on the coil, thus it has higher inductance and a higher capacitance, and so it has a lower resonant peak, producing more voltage at low frequencies rather than high ones.
The job of the resistance "R" in the parallel RLC circuit is to decide what the Q factor of the resonance will be, but I'll get into that later.
What do we want to know about guitar pickups?
There are five major metrics to be known about a guitar pickup, and in the order I'd rank them in importance, they are
- Resonant Peak
- DC Resistance
- Flux Density
If you know the resonant peak of a pickup, it is easy to make an educated guess as to how that pickup will compare to others like it. The same can be said of inductance. The main reason it is good to know the capacitance is that it offers clues as to how the pickup was produced. Generally, if the capacitance is high, the coil was wound tighter, and vice versa.
The capacitance also tells you how the pickup might react to the use of a short versus long guitar cable. The guitar cable add a lot of capacitance, and therefore lowers the resonant peak of the pickup. A pickup with lower capacitance can likely afford to be paired with a long guitar cable without becoming overly dark as a result. A typical Strat pickup has about 100pF capacitance, while a guitar cable typically adds 200pF to 400pF on top of that
The DC resistance has a minimal effect on the Q factor. It's ore useful in determining roughly how many winds are on the coil, but in order to make that determination, you must also know, or be able to intuit the size of wire used. 42 AWG is standard, but 43 AWG will show a higher resistance for the same number of winds. If the wire is overly stretched during the winding process, then the wire will become smaller, and read a higher resistance than it would otherwise. It's worth noting that the inductance relates to the number of turns, and not the size of the wire or it's DC resistance. 1000 turns of 42 AWG will result in nearly the same inductance as 1000 turns of 43 AWG wire.
Finding the "Resonant Peak"
Everyone knows you can find the DC resistance with a cheap multimeter. Finding the resonant peak requires the use of a frequency generator and an oscilloscope. Fortunately, we live in the future, and both of these things can be had in a single USB device that only costs about $160
The basic idea is to impose a changing magnetic field directly upon the pickup at every frequency between 100Hz and 30kHz, and then measure the output voltage at multiple frequencies in order to plot the pickup's frequency response. This is not unlike how a magnetized guitar string works; as the string moves back and forth, it too imposes a changing magnetic field upon the pickup, at multiple frequencies.
Step 1) Create a driver coil using the Strat pickup bobbin and the magnet wire.
Wrap the magnet wire around the bobbin about two hundred times, or enough to fill the bobbin. This will give the coil an inductance of a several millihenries, and a resonant peak that is beyond the audible range, which makes it suitable for this purpose. Either solder lead wires to the coil as seen in the picture, or use the ends of the magnet wire as the leads. You will have to sand off the insulation coat from the wire in order to get to bare copper for soldering, or for making electrical contact.
Step 2) Install the USB oscilloscope and install the software. Besides the Velleman PCSGU250, I know that the Syscomp CGR-101 can be used, but I don't recommend it, as it uses more crudely written software.
Connect the included probe to "Channel 2" output, and connect the included alligator clips to the function generator output.
Step 3) Connect the grounds of the function generator alligator clips and the "Channel 2" probe to one of the leads of the driver coil, then connect the positive leads of the probe and the function generator alligator clips to the other lead of the driver coil, as seen in the picture.
Step 4) Connect the extra probe to "Channel 1", and then connect it's lead and ground to the pickup being tested.
This is how the full setup will look:
Setting Up the Software
I'll presume you're using the Velleman PCSGU250. Any USB oscilloscope will work as long as it provides built in bode plotting against a function generator, and it's settings will be similar.
Looking at the main screen:
- Make sure both probes are set to "x10"
- Make note of the voltage slider on the far right. This is where you can adjust the amount of voltage that is sent to the driver coil.
- Click "Circuit Analyzer"
Looking at the Circuit Analyzer screen:
- Click "Options", select "Show Multiple Traces"
- Click "View", select "Markers f & V", and "Audio Range 30kHz"
- Under "Vertical Scale", select "5db/div"
- Under "Frequency Range", select 30khz
- Under "Frequency Start", select 100Hz
- Uncheck "Log. Freq. Steps"
Finally, place the driver coil over the pickup and attempt to create a bode plot. Click the "Start" button to fire off the process.
When you measure a humbucking pickup (even single-coil sized rails humbuckers), you must position the driver coil on it's side like this: