Emulating a fancy LCR meter

[Joe Gwinn]

I do not think that one need worry about transmission line effects when using a cable a few feet long at 20 kHz and below. Cable capacitance, maybe. But 250 pf at 20 KHz is a bit over 3e4 ohms. It does not seem likely that this would bother an amp that can operate into 600 ohms. I will try a very short cable tonight and see if that makes any difference.

By transmission-line effects I include the effect of a reactive load on the driver. The operate into 600 ohms part isn't really the issue, and the opamp output impedance will be far lower. As for the 30,000 ohm reactance at 20 KHz, we probably will want more headroom. More below.

[If matching did matter it could be handled at the load end only. A match there kills any reflection. If matching were required at both ends, rf transmitters could not necessarily be made efficient, but they can.]

If one will match only one end, it's better that it be the driver end, so the opamp output is isolated from the reactance of the line.

The vast majority of RF transmission lines are matched at both ends. One reason being that matching makes cable capacitance irrelevant.

The issue with [in-line] capacitors you mentioned would be one reason. The possibility that someone would come up with an impedance that would make it oscillate is another. I think the high resistance and capacitance of a pickup coil imply that there are no stability issues.

Given that we want to be able to track the pickup through and beyond resonance, all possible kinds of (non-negative) impedance are going to be explored. And not all pickups have high resistance. For instance an alumitone.

[Joe Gwinn]

The results of comparing a short cable (less than one foot) to a longer one (about five feet) from the output of the op amp to the sound card input (V1): There is a small difference (less than 1%) above 14 KHz. If the designed is changed so that both outputs use similar op amps, this effect should tend to cancel out.

Both attenuation and phase shift would need to be matched over the entire frequency band of interest, or computations of the complex impedance of the device under test will be affected. Phase is usually the harder one, especially at higher frequencies, especially if the device is complicated (meaning requires many circuit elements to model).

[Joe Gwinn]

An I-V circuit

Here is my I-V buffer circuit from October 2003. For driving long pieces of coax, I would probably add series 50-ohm resistors to the outputs of A1 and A2. In my case, the load was the inputs of a two-channel scope. I think I set the inputs to 50 ohms, but don't recall. In any event, I would clean that part of the design up, as there were annoying phase shifts at 20 KHz.

[Mike Sulzer]

By transmission-line effects I include the effect of a reactive load on the driver. The operate into 600 ohms part isn't really the issue, and the opamp output impedance will be far lower. As for the 30,000 ohm reactance at 20 KHz, we probably will want more headroom. More below.

Yes, the 600 ohm drive capability has little to do with the output impedance. But the ability to drive power does have an influence on how little the device will be affected by a reactive load of a given magnitude. The reactive part of the load is relatively high here, and probably not a big concern.

If one will match only one end, it's better that it be the driver end, so the opamp output is isolated from the reactance of the line.

I agree, that is, if matching is needed at all.

The vast majority of RF transmission lines are matched at both ends. One reason being that matching makes cable capacitance irrelevant.

If you have control over only one end of an rf system and power is not an issue, you certainly will match it to make sure your equipment snuffs reflections. And if power is a concern, you might want to go to a system using a hybrid. But cable capacitance is not an issue in a system which is end terminated only. As long as the load matches the cable, that is the impedance that the source sees.

Given that we want to be able to track the pickup through and beyond resonance, all possible kinds of (non-negative) impedance are going to be explored. And not all pickups have high resistance. For instance an alumitone.

If you want to look at an alumnatone without its transformer, you will need a circuit designed for very low impedances.

[Mike Sulzer]

Both attenuation and phase shift would need to be matched over the entire frequency band of interest, or computations of the complex impedance of the device under test will be affected. Phase is usually the harder one, especially at higher frequencies, especially if the device is complicated (meaning requires many circuit elements to model).

But small variations in amplitude or phase should not matter if the drivers for the two outputs and the cables are the same. The variations should drop out when the ratio is computed.

[Mike Sulzer]

Here is my I-V buffer circuit from October 2003. For driving long pieces of coax, I would probably add series 50-ohm resistors to the outputs of A1 and A2. In my case, the load was the inputs of a two-channel scope. I think I set the inputs to 50 ohms, but don't recall. In any event, I would clean that part of the design up, as there were annoying phase shifts at 20 KHz.

Good clean design, Joe. I think the 50 ohm resistors are a good idea. This is on my list of things to try. Why would you use an Rs as high as 1 Meg?

[Joe Gwinn]

Good clean design, Joe. I think the 50 ohm resistors are a good idea. This is on my list of things to try. Why would you use an Rs as high as 1 Meg?

Thanks. I stole it from Agilent et al.

Use a series resistance equal to the coax impedance, whatever that may be. Audio cable is not necessarily 50 ohms. It may be necessary to measure the cable impedance.

It may be useful to add a voltage follower stage between the output of the transimpedance amplifier A2 and the coax, with the series resistor between this new follower and the coax.

As for Rs, I was playing with it, to see the effect. High values of Rs limited bandwidth due to stray capacitance unless the device impedance was quite low, which then made noise more of a problem. While in theory Rs=0 ohms would be optimum, again it's best to isolate practical voltage sources from the vagaries of the device under test, and also to protect the voltage source against shorts. And to prevent overcurrent into the transimpedance amplifier input. But the exact value of Rs isn't critical, and I ended up with 2.2 Kohms.

[Joe Gwinn]

Yes, the 600 ohm drive capability has little to do with the output impedance. But the ability to drive power does have an influence on how little the device will be affected by a reactive load of a given magnitude. The reactive part of the load is relatively high here, and probably not a big concern.

There are two issues here. First, if the load impedance is too low, the amplifier will struggle. Second, assuming that the amplifier isn't struggling, a reactive load can provoke oscillation. It is this second issue I was alluding to. Either capacitance or inductance load on an opamp output can provoke oscillation. A series resistor pretty much abolishes the effect.

If you have control over only one end of an rf system and power is not an issue, you certainly will match it to make sure your equipment snuffs reflections. And if power is a concern, you might want to go to a system using a hybrid. But cable capacitance is not an issue in a system which is end terminated only. As long as the load matches the cable, that is the impedance that the source sees.

I guess by "if power is a concern" you mean DC power, as from a battery in a cell phone or the like. Cell phones are physically small and have control of all aspects of the line and what it is connected to, so many tricks are available.

In my day job, I (and a thousand of my closest friends) build megawatt radars. Believe me, everything is matched, both to ensure phase stability, and because at those power levels even a small mismatch can reflect enough power into someplace it doesn't belong to release smoke.

If you want to look at an alumnatone without its transformer, you will need a circuit designed for very low impedances.

Actually, the current circuit could do it, but with smaller resistor values. But I would keep the transformer. There are other pickup coils to consider, such as those in active pickups.


The point is to make our I-V Buffer as general as possible, so we only need to build one model. I think if one adds up the costs in time and money, the metal box, connectors, battery hardware, and circuit board are 90% of it, and the circuitry is almost free. The money is always in the boring stuff.

[Mike Sulzer]

There are two issues here. a reactive load can provoke oscillation. It is this second issue I was alluding to.

Yes, that is the issue I was thinking of.

I guess by "if power is a concern" you mean DC power...

No, I meant loss of rf power or efficiency. If you need to worry about power coming back down the transmission line from the antenna in a high power system, you want to divert it to a waster load rather than use a series resistor. You cannot always match the antenna to the line under all conditions, so this is a real concern.

In my day job, I (and a thousand of my closest friends) build megawatt radars. Believe me, everything is matched, both to ensure phase stability, and because at those power levels even a small mismatch can reflect enough power into someplace it doesn't belong to release smoke.

In my day job, I use a megawatt radar to do science. I am well aware of loud noises followed by smoke.

Actually, the current circuit could do it, but with smaller resistor values. But I would keep the transformer. There are other pickup coils to consider, such as those in active pickups.

And lower inductance coils used when you put a coil on each string. In either the classic I-V or the circuit I am using, one could use two or more resistors for different impedance ranges.

[Mike Sulzer]

Impedances for tele bridge

The impedance plot of a tele bridge type pickup is available here: http://www.naic.edu/~sulzer/tele2.png.

This is a 7K pickup wound with #43 wire using bobbin material and magnets from StewMac. The interesting thing about a tele bridge pickup is the effects of the back plate and bridge. The plot shows three curves of the unloaded pickup (No R or C across the pickup) and three curves with 500pf (to simulate a cable) and 110K (to simulate the loading with volume and tone on 10). The three curves for each set are for 1. no plate or bridge, 2. plate but no bridge, and 3. plate and bridge. The effect with no R and C is large, because the frequency of the resonance is high. (The effect of eddy currents generally increases with frequency.) Even loaded, the effect is quite noticeable.

The unloaded resonant frequency for this pickup is lower than that of the japanese humbucker (posted last week). However, the loaded resonant frequency of the tele is quite a bit higher than that of the humbucker. The humbucker, with two cols in series, has high inductance and low capacitance, and so is affected more by the cable loading than the single coil tele.

[Mike Sulzer]

Impedance of Strat Type Pickup

This post discusses the impedance curves of a strat type pickup. I put 5000 turns of #43 wire on a Guitar Parts USA plastic bobbin. This type of bobbin allows he cores to be removed, so one can compare the magnets to an air core and also use different types of material.

The two plots discussed in this post are here:
http://www.naic.edu/~sulzer/GPUSA2.png
http://www.naic.edu/~sulzer/GPUSA2Lows.png
The first shows the whole frequency range of the measurements; the second shows just the lower part.



Four of the five lines on the plot show both measurements and model results, that is, impedance curves resulting from calculations. (The fifth shows the impedance with the alnico magnets and the pickup loaded with cable and volume and tone.) The model consists of a C, an L, and two resistors, one in series (Rs) with the L, and the other (Rq) in parallel with the pickup. The pickup capacitance and the series resistance are the same for all the core types:
___C____L___Rs___Rq__core type
146e-12 .76 4800 5.5e6 aircore
146e-12 .99 4800 .59e6 alnico
146e-12 1.24 4800 .46e6 slugs 1 per hole
146e-12 1.4 4800 .33e6 slugs 2 per hole

The inductance varies with the core type because the permeability of the material is different. An air core gives the lowest inductance, but also the highest Rq, indicating the lowest loss. (The loss would be from eddy currents flowing in a metal core.) Why is Rq not infinity? Two possible reasons: 1. the pickup capacitance might be somewhat lossy; it is the result of electrostatic coupling between the turns of wire, and there is a significant total resistance. 2. The measuring instrument might not be accurate for high Q/high frequencies, due to the finite high frequency gain of the op amp.

Notice that in addition to matching at the peak, the model for the air core case matches very well at low frequencies. This is expected when there are no eddy currents as in a metal core. The good match between measurement and model indicates that the pickupMeter is working well.

The measurement and model using the alnico magnets, as intended in the design of this pickup, shows significantly higher inductance, due to the permeability of the material, and much higher losses (lower value of Rq). But both the permeability and the conductivity of the alnico are lower than the material used in humbucker slugs. A humbucker slug is both shorter and thinner than than an alnico magnet, so one in each hole is less magnetic material, but two (with one sticking out quite a bit) is probably too much, so a good comparison might be in between.

Since the inductance has been matched at the peak, it is too low at the lower frequencies since the effect eddy currents in decreasing the inductance is more pronounced at high frequencies. On the low frequency plot, the orange dots goes with the yellow curve, and the blue dots go with the green curve.

[David Schwab]

In my day job, I use a megawatt radar to do science. I am well aware of loud noises followed by smoke.

LOL!

And lower inductance coils used when you put a coil on each string.

Like one of these suckers:

[Mike Sulzer]

LOL!



Like one of these suckers:

Exactly! I am working on a six coil pickup system for guitar with separate preamp and distortion for each string in the guitar. Will discuss it on this forum when ready. (It will take some time.)

[David Schwab]

Exactly! I am working on a six coil pickup system for guitar with separate preamp and distortion for each string in the guitar. Will discuss it on this forum when ready. (It will take some time.)

That's a Wal bass pickup. I'm going to be making my own version of it soon. I have all the parts made and just have to wind it.

Hex fuzz is one of the best things ever. I used to have an ARP Avatar guitar synth, and it had that feature. Even with the simplest gnarly sounding fuzz, you get this very clear tone when you play chords due to the lack of intermodulation.

[bbsailor]

Exactly! I am working on a six coil pickup system for guitar with separate preamp and distortion for each string in the guitar. Will discuss it on this forum when ready. (It will take some time.)

Mike,

You will need to use some pretty thin wire to get enough turns on each magnet core to get an output in the 100mv to 200mv range. You can get around this by using AWG 32 to AWG 36 wire connected to a transformer to boost, by a high transformer turns ratio (TR), back to a high impedance. Miniature output transformers (inexpensive) 3.2 ohms or 8 ohms to 20K or 50K would work.

You can also use 1/8" neo magnets to obtain a little more winding area on the individual string bobbins. Used as a bridge pickup, the 1/8" magnets would work where the string movement is less. Use a 3/8" teak plug cutter, available at marine stores to make your own 3/8" plastic or fiber washer that can be directly glued at the ends of each neo magnet to make the individual string bobbins. With a bobbin about .5" tall, you can probably get 4 to 8 ohms of wire on each coil. Burns, in his pickup patent, only uses 1 ohm DC per individual coil but needs a transformer with a 1:50 TR. With the 1/8" magnet core, you will be in the range of 200 to 400 turns and would need a TR of around 1:20 or higher to get the output in the typical pickup range. Using a 3/16" magnet only gives you 3/32" of space to wind over the magnet core. If you angle the pickup or alternate the strings in two rows (coil 0.75" OD each string to fit inside a humbucker cover), you can obtain a some more space to fit the coils between the strings.

There are many sources of commercially made solenoid coils larger than 3/8" diameter. The pickings are slim to non existant looking for a commercially made stock solenoid 3/8" OD with an ID of 0.125".

Hint: Go to Wal-Mart and pick up 2 four packs of plastic Singer Class 15 sewing bobbins PN/30023. They can use .25" magnets X .5" tall and 6 will fit inside a humbucking cover in two rows. You should get about 6000 turns of AWG 42 on each one. The wire winding area is .0375" high by 0.125" wide.

Keep us posted about your progress.

Joseph Rogowski