measuring resonant peak

[peskywinnets]

Money talks. VAT on the canadian CircuitGear doesn't look good.Congrats.

Well, there is one supplier in this country - but his price is coming in at about £185inc VAT delivered :-( (never a good thing when there's no competition!)

By the standard scope measure, that Hantek's 150 Msample/sec entitles them to claim 15MHz analog bandwidth. Since they don't open up the software interface (as do Velleman and CircuitGear), you may want to resell it.

They have opened it up (there's an SDK DSO-2150 USB_Hatek Electronic co.,Ltd._http://www.hantek.com.cn ) - but I may as well be reading the maintenance manual for the hubble space telescope for what it means to me! I'll see how the new scope stacks up against it - but since it'll only raise pennies, it'll probably be useful for scoping with high frequency clocks with the PICs I dabble with (also, it'll give me the option of 4 channels combined with the new scope - handy for when I develop that MRI scanner I've been mulling)

[salvarsan]

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, I spoke with the builder, an EE prof. He hand-rolled the DDS from an FPGA and uses a small fast AVR micro for USB communications. I think the idea was to provide an inexpensive lab tool for his students.

[Joe Gwinn]

Joe, I spoke with the builder, an EE prof. He hand-rolled the DDS from an FPGA and uses a small fast AVR micro for USB communications. I think the idea was to provide an inexpensive lab tool for his students.

OK, but that's not an answer to the question asked. For a cheap DDS, non-harmonic spurs can be a big problem, although one can evade the problem by careful choice of commanded frequency.

Anyway, if you know the bit widths of the DDS accumulator, tuning word, input to the phase to sine/cosine lookup table, and the input to the DAC, all can be deduced.

[salvarsan]

Anyway, if you know the bit widths of the DDS accumulator, tuning word, input to the phase to sine/cosine lookup table, and the input to the DAC, all can be deduced.

Joe, you are probably best qualified to ask that question of Dr. Peter Hiscocks at SysCompDesign.com.

He is an agreeable and genial sort, and I would like to think that you would enjoy chatting with him.

[-Elepro-]

i get pickup frequency response with white noise and A-B mode spectrum analyzer....

[Steve Conner]

Yes, this is the method I've always recommended too. You put the analyser in averaging mode and it gives you the frequency response in a few seconds. I don't make pickups, but I've used it to characterise tone stacks, EQs and the like.

All of the software is available for free. Back when I was using it, there was a thing called Wave Tools or such like. That is no more, the best free one I've found is Spectrum Lab. It has the White noise generator and A-B mode, but I can't remember if it does averaging. It works with any sound card and will use the ASIO driver if the card has one.

It was designed as a ham radio research tool, so has millions of confusing features. There are other, easier to use programs, but they're not free. Electroacoustics Toolbox for the Mac being an example. It looks beautiful and is very nicely thought out, but it'll cost you more than the Circuit Gear scope.

The interface electronics between soundcard and pickup are up to you.

[Mike Sulzer]

Electroacoustics Toolbox for the Mac being an example. It looks beautiful and is very nicely thought out, but it'll cost you more than the Circuit Gear scope.

Yes, that is what I use. For a random noise test signal, you want to select the dual fft mode where the software ratios the input and output results. Then the variations in power at different frequencies from the random nature of the signal are taken out. Use an I-V circuit in front and you have true impedance measurement. (For very accurate results, you still need to integrate because there is some noise in any system.)

[Joe Gwinn]

Joe, you are probably best qualified to ask that question of Dr. Peter Hiscocks at SysCompDesign.com.

He is an agreeable and genial sort, and I would like to think that you would enjoy chatting with him.

I have started by sending an email question to their tech support, after doing a bit of website research. Syscomp Electronic Design Ltd.

The users manuals are far more informative than the datasheets. What I have so far gleaned:

For the freestanding waveform generator WGM-201:

DDS clock frequency seems to be 134.208 MHz. The lookup table has 8 bits amplitude output, and 10 phase bits input. I don't know the accumulator width, but 32 bits is a common value, one that is well supported in FPGAs. (Hardware DDS chips more commonly use 48 bits.) The generator signal-to-noise level is quoted as 48 dB, but it isn't clear if this applies to the new all-in-one, and it isn't clear if spurs and harmonics are considered to be noise.

The waveform generator built into the all-in-one unit is much cruder. Clock is 20 MHz, and max output frequency is 2 MHz (versus 10 MHz). I would expect far more phase truncation error, which causes "tones" (spurs at various frequencies having no obvious harmonic relationship to anything). Tones that are too close to the fundamental (the commanded frequency) or too strong will interfere with measurements.

It would also be useful if the generated waveforms were clean enough to be used with a Maxwell-Wein Impedance Bridge, used to measure inductance of low Q inductors like pickup coils.


It turns out Dr Hiscocks has a US patent application, 20060077085, that describes his waveform generator. I would guess that this describes the free-standing arbitrary waveform generator.

[peskywinnets]

Well, my new Velleman scope arrived today....got home tonigkt...kids were duly thrown into bed way earlier than normal (hehehe!) & have now had a quick dabble with it - very pleased with it....the GUI has a nice vibe to it & the overall package seems of good quality (even the trace vs my chinese scope seems to have a much nicer look/response)

I've just plotted the phase response of a circuit that I'd hoped had a flat phase response (it did!)

Just this minute pulled out a low impedance pickup (about 100 Ohms DCR), to run an amplitude/phase bode plot which clearly shows its resonant peak to be about 30k. The phase plot is the dotted line



(I used a 4.7k resistor - not sure how much that affects the actual height of the 'peak', but it was just a quick 'n dirty test)

Recommended.

[freefrog]

Hi everybody,

I've been watching this great forum for a while and have finally decided to register as a member...

My first post here will be to say that I often "measure" the resonant peak of guitar pickups.

Method: the one used by Helmuth Lemme.

Gear: a calibrated soundcard with output/input and a low impedance coil - whose own resonant frequency is therefore really high and doesn't interfere with the resonance of the tested pickups.

Software: currently RightMarkAudioAnalyzer but I've used other RTA's in the past.

I don't pretend my "measurements" to be professional: I'm just an old French hobbyist, self taught in electronics (although an engineer of my family helps me to master all this stuff) and I'm specialized in nothing but crappo cheapo DIY (LOL).

That said, I obtain pretty consistent results, regardless of the "coiled exciter", soundcard and software used, and my graphs generally agree with my ears as well as with my 5spice simulations. :-)

Below is an example:

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The overall shape of these curves, with their tight bass, is due to the 100pf input cap of the guitar input used (=a 1M input).

The green and blue lines show the response of a Gbison Tim Shaw HB plugged through a 370pf cable and a single 470k pot. There's almost no other hardware involved as the PU is not mounted in a guitar and has no cover.

The white and pink lines show the response of a Seymour Duncan SH55 mounted in a LesPaul: 1 500k vol pot, 1 500k tone pot... same 370pf cable.

One can clearly see how the response of the SH55 is softened by...
-its cover;
-the lower total resistance of its pot network;
-the stray capacitance of its own structure + the whole guitar wiring...

The pink line translates the response of the coil with slugs. These slugs being "lower in their coil" than the screw poles and being below the cover, they deliver less high frequencies and the graph shows it.

IOW, although my measurement is not pro at all, it's still informative, at least IMHO.

I hope this 1st post to be correctly wrote (English is not my mother tongue) AND to be useful for other members: this first post has been done to show them how easy and interesting (or not) it can be to "measure frequency peaks". :-))

Have a nice day!

FOOTNOTE: I have forgotten to precise the specs of the PU's tested above:
Tim Shaw: 7.5k, 4.15H.
SH55: 8.1k, 4.2H.
Their inductance is almost the same hence the possible "surprise" to see a gap of 1khz between their resonant peaks (gap due to parasitic capacitance, AFAIK).

[mickmutante]

I couldn't resist getting one of these, so I bought one I think I will have it next week.
So.... My native tongue is Spanish... I speak English too, but trying to understand all the technical (electronic, physics, etc.) language, sometimes I get confused... so a have a question about using the oscilloscope....
Salvarsan post a schematic about how to connect it. Is it necessary to make a transmitting coil as Lemme describe it on Measuring Frequency Response?
Depending on the numbers of turns, DC, pickup specs, etc... Is it necessary to change the value of the 1.5KΩ that Salvarsan described on the schematics?
Or is there another way to connect the pickup to the oscilloscope to analyze it?
Maybe these are dumb questions but I don't have experience with oscilloscopes...