Post by aquin43 on Jan 29, 2019 17:05:53 GMT -5
Signal distribution within a pickup. A model of a sidewinder.
Theory:
A pickup produces output because the variations in the magnetic field within the pickup caused by the string movement create an electric field (Maxwell's 3rd equation) which, in turn, is sampled by the coils and produces the output voltage.
Even within a coil, the electric field may not be uniform so different parts of the windings may have different sensitivities.
It is possible to explore the variation in the electric field by measuring the voltage induced in a straight piece of wire immersed in the field. Such is the nature of magnetic induction, however, that special precautions need to be taken to achieve this.
The wire and the leads back to the meter will form a loop. Faraday's law states that the total flux variation throughout the loop will contribute to the measured voltage so it is necessary to shape the loop to minimise this effect. In the measurements that follow the loop was made large, so that the straight sensing wire could end in regions where the flux was low and the co-axial lead to the meter was connected a point on the neutral centre line of the pickup and about 400mm above it.
The model represents the string facing top of a sidewinder minus the coils. It consists of a flat rectangular metal plate with a magnet on the long centre line and a current driven exciter coil surrounding the magnet. The coils, if they were included, would have the long sides of the plate as their axes. An assumption is made that the one sided arrangement is sufficient to model the flux distribution to a reasonably close degree because the strings directly excite only one side and the baseplate would be expected to act as a shield. The measured results tend to validate this assumption.
The coil was driven by a current at 1kHz, and the output observed on a micro Volt meter patched through to an oscilloscope. The oscilloscope timebase was locked to the oscillator and a phase reference point was noted.
Measurements:
The straight wire was oriented in a direction parallel to the long axis of the magnet and extending beyond the ends of the plate.
Initially, the wire was maintained parallel to the long axis and moved outwards from the centre. On the centre line, no voltage was produced, which is what would be expected from symmetry and acts as a test of the arrangement of the leads to the wire. Just outside the exciter, a maximum voltage of 850uV was produced which gradually reduced to 300uV at the edge of the plate.
Passing under the plate, the voltage diminished to 150uV, without changing phase, and then gradually diminished on moving further under the plate to 25uV at the centre.
On the other side of the centre line, similar results were observed, but with 180 degree opposite phase.
A single turn around the exciter produced 1.85mV while a single turn on top close to the plate edge produced 900uV
A single turn around the plate thickness, close to the exciter produced 840uV, slightly less than the single wire close to the exciter, which is to be expected because it will be the initial 850uV of the single wire minus the small excitation from the wire under the plate.
A single turn around the plate thickness at the edge produced 150uV, rather less than the single wire above the plate at this position.
Conclusions:
These are the voltages that would be induced in individual sections of coil windings occupying the same locations. It is interesting that the phase of the field below the plate is the same as that above. This implies that a coil around the plate is not threaded by the signal flux in the normal way but in fact receives a cancelling signal from below the plate.
Considering the two coils of the pickup. The top and bottom long sides will be excited unequally but in phase, so that the bottom half turns will tend to oppose the upper.
The upper half turns of both coils are all connected in series via the pickup's internal connections and are wired so that they will sum, so these upper half turns are the main source of the pickup's output.
The lower half turns of the coils contribute much less to the output but they do provide the path that allows hum cancelling.
Perhaps it might be more sensible to convert the two upper half coils into one horizontal coil which would then have to be balanced by a lower horizontal coil for hum cancelling - a stacked humbucker.
Theory:
A pickup produces output because the variations in the magnetic field within the pickup caused by the string movement create an electric field (Maxwell's 3rd equation) which, in turn, is sampled by the coils and produces the output voltage.
Even within a coil, the electric field may not be uniform so different parts of the windings may have different sensitivities.
It is possible to explore the variation in the electric field by measuring the voltage induced in a straight piece of wire immersed in the field. Such is the nature of magnetic induction, however, that special precautions need to be taken to achieve this.
The wire and the leads back to the meter will form a loop. Faraday's law states that the total flux variation throughout the loop will contribute to the measured voltage so it is necessary to shape the loop to minimise this effect. In the measurements that follow the loop was made large, so that the straight sensing wire could end in regions where the flux was low and the co-axial lead to the meter was connected a point on the neutral centre line of the pickup and about 400mm above it.
Model:
The coil was driven by a current at 1kHz, and the output observed on a micro Volt meter patched through to an oscilloscope. The oscilloscope timebase was locked to the oscillator and a phase reference point was noted.
Measurements:
The straight wire was oriented in a direction parallel to the long axis of the magnet and extending beyond the ends of the plate.
Initially, the wire was maintained parallel to the long axis and moved outwards from the centre. On the centre line, no voltage was produced, which is what would be expected from symmetry and acts as a test of the arrangement of the leads to the wire. Just outside the exciter, a maximum voltage of 850uV was produced which gradually reduced to 300uV at the edge of the plate.
Passing under the plate, the voltage diminished to 150uV, without changing phase, and then gradually diminished on moving further under the plate to 25uV at the centre.
On the other side of the centre line, similar results were observed, but with 180 degree opposite phase.
A single turn around the exciter produced 1.85mV while a single turn on top close to the plate edge produced 900uV
A single turn around the plate thickness, close to the exciter produced 840uV, slightly less than the single wire close to the exciter, which is to be expected because it will be the initial 850uV of the single wire minus the small excitation from the wire under the plate.
A single turn around the plate thickness at the edge produced 150uV, rather less than the single wire above the plate at this position.
Conclusions:
These are the voltages that would be induced in individual sections of coil windings occupying the same locations. It is interesting that the phase of the field below the plate is the same as that above. This implies that a coil around the plate is not threaded by the signal flux in the normal way but in fact receives a cancelling signal from below the plate.
Considering the two coils of the pickup. The top and bottom long sides will be excited unequally but in phase, so that the bottom half turns will tend to oppose the upper.
The upper half turns of both coils are all connected in series via the pickup's internal connections and are wired so that they will sum, so these upper half turns are the main source of the pickup's output.
The lower half turns of the coils contribute much less to the output but they do provide the path that allows hum cancelling.
Perhaps it might be more sensible to convert the two upper half coils into one horizontal coil which would then have to be balanced by a lower horizontal coil for hum cancelling - a stacked humbucker.
Arthur
