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#1
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Measurements of ampllifiers
Hi the
Can you guys help me to create a test bench setup in order to perform measurements for amplifier frequency response (-3dB bandwidth). In other words what test equipment do I need to test and generate a Amplitude vs. Frequency plot of a given amplifier. Yet it sounds expensive to me. Does one 'sweep' an amplifier from 20Hz to 20KHz to obtain an amplitude response curve with an Audio Sweep Generator? Or does one test the amplifier at selected frequencies and then generate a table of data points of Frequency vs. Amplitude? Maybe a PC solution exists for such an interface on the market today. I would like to plot the frequency response of my home-brew amps and also rebuilt classics. If it's a manual thing then some ideas on how to go about calibrating the input levels and measuring the gain and output levels would be helpful. My test gear at the moment is an EICO Audio Sine Wave Signal Generator, an 40 MHz Dual Trace Oscilloscope and also a Stabilock Model 4032 RF Communications Analyzer. Plus the usual Volt-Ohm meters, power test resistors and small coaxial BNC Cables, etc. Thanks, Rich Sherman |
#2
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"Rich Sherman" wrote
Can you guys help me to create a test bench setup in order to perform measurements for amplifier frequency response (-3dB bandwidth). In other words what test equipment do I need to test and generate a Amplitude vs. Frequency plot of a given amplifier. Yet it sounds expensive to me. Does one 'sweep' an amplifier from 20Hz to 20KHz to obtain an amplitude response curve with an Audio Sweep Generator? Or does one test the amplifier at selected frequencies and then generate a table of data points of Frequency vs. Amplitude? Maybe a PC solution exists for such an interface on the market today. I would like to plot the frequency response of my home-brew amps and also rebuilt classics. If it's a manual thing then some ideas on how to go about calibrating the input levels and measuring the gain and output levels would be helpful. My test gear at the moment is an EICO Audio Sine Wave Signal Generator, an 40 MHz Dual Trace Oscilloscope and also a Stabilock Model 4032 RF Communications Analyzer. Plus the usual Volt-Ohm meters, power test resistors and small coaxial BNC Cables, etc. You can use the dual trace scope for a quick and easy plot. Use one channel to monitor the generator, and one to monitor the output of the amp. Set the generator level at 1V (or whatever) at 1k and the scope to show this at a convenient near-full-screen vertical scale. Set other channel to appear exactly the same, so it looks like one trace. Then scan through the frequency range with the generator, noting the proportional difference in amplitude between the traces. It will be obvious where you need to note many points, and where just a few will be enough. You should need to go well beyond the audio range to find the top end limit. You can also note the phase shift, but soon you will notice that this can be entirely predicted from the amplitude variation so you won't need to bother. This method takes into account variations in generator output, and it is simple to follow what you are doing. It assumes some frequency standard...either a frequency-accurate generator or scope, or a separate counter. Alternatively, you can simply use the generator and a voltmeter. A true rms meter is preferable but not really necessary as long as there isn't lots of variation in distortion level. Of course you can use a frequency scan output if you have one from the generator, but not so easy to measure. cheers, Ian |
#3
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OK Ian:
I'll give iot a go and let you know how the Excel plot end up looking. My signal generator has discrete setting so it makes it easy to set the data points for frequency. Thanks for the method. I'll also look at the phase differences. Bye, Rich "Ian Iveson" wrote in message k... "Rich Sherman" wrote Can you guys help me to create a test bench setup in order to perform measurements for amplifier frequency response (-3dB bandwidth). In other words what test equipment do I need to test and generate a Amplitude vs. Frequency plot of a given amplifier. Yet it sounds expensive to me. Does one 'sweep' an amplifier from 20Hz to 20KHz to obtain an amplitude response curve with an Audio Sweep Generator? Or does one test the amplifier at selected frequencies and then generate a table of data points of Frequency vs. Amplitude? Maybe a PC solution exists for such an interface on the market today. I would like to plot the frequency response of my home-brew amps and also rebuilt classics. If it's a manual thing then some ideas on how to go about calibrating the input levels and measuring the gain and output levels would be helpful. My test gear at the moment is an EICO Audio Sine Wave Signal Generator, an 40 MHz Dual Trace Oscilloscope and also a Stabilock Model 4032 RF Communications Analyzer. Plus the usual Volt-Ohm meters, power test resistors and small coaxial BNC Cables, etc. You can use the dual trace scope for a quick and easy plot. Use one channel to monitor the generator, and one to monitor the output of the amp. Set the generator level at 1V (or whatever) at 1k and the scope to show this at a convenient near-full-screen vertical scale. Set other channel to appear exactly the same, so it looks like one trace. Then scan through the frequency range with the generator, noting the proportional difference in amplitude between the traces. It will be obvious where you need to note many points, and where just a few will be enough. You should need to go well beyond the audio range to find the top end limit. You can also note the phase shift, but soon you will notice that this can be entirely predicted from the amplitude variation so you won't need to bother. This method takes into account variations in generator output, and it is simple to follow what you are doing. It assumes some frequency standard...either a frequency-accurate generator or scope, or a separate counter. Alternatively, you can simply use the generator and a voltmeter. A true rms meter is preferable but not really necessary as long as there isn't lots of variation in distortion level. Of course you can use a frequency scan output if you have one from the generator, but not so easy to measure. cheers, Ian |
#4
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Velleman (www.velleman.be) makes a very inexpensive dual scope and signal
generator controlled by a PC that will do the job. See PC-500. "Rich Sherman" wrote in message ... Hi the Can you guys help me to create a test bench setup in order to perform measurements for amplifier frequency response (-3dB bandwidth). In other words what test equipment do I need to test and generate a Amplitude vs. Frequency plot of a given amplifier. Yet it sounds expensive to me. Does one 'sweep' an amplifier from 20Hz to 20KHz to obtain an amplitude response curve with an Audio Sweep Generator? Or does one test the amplifier at selected frequencies and then generate a table of data points of Frequency vs. Amplitude? Maybe a PC solution exists for such an interface on the market today. I would like to plot the frequency response of my home-brew amps and also rebuilt classics. If it's a manual thing then some ideas on how to go about calibrating the input levels and measuring the gain and output levels would be helpful. My test gear at the moment is an EICO Audio Sine Wave Signal Generator, an 40 MHz Dual Trace Oscilloscope and also a Stabilock Model 4032 RF Communications Analyzer. Plus the usual Volt-Ohm meters, power test resistors and small coaxial BNC Cables, etc. Thanks, Rich Sherman |
#5
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tubesforall wrote: Velleman (www.velleman.be) makes a very inexpensive dual scope and signal generator controlled by a PC that will do the job. See PC-500. I would stick to legitimate test equipment manufacturers, which needn't be that expensive as many are available very cheaply at hamfests today. The exception would be a serious oscilloscope-"simple scopes" can be had for five to twenty dollars all day and repaired by jackleg methods, but modern, calibrated, DC-coupled triggered sweep scopes with attenuators and calibrated sweep are probably best bought _new_. Or late model(still supported) refurb from legitimate test houses (i.e. not *ucker.) Fifteen years ago one was safe in saying "buy nothing but a Tek scope" but Tek does not build actual oscilloscopes, per se, today. Iwatsu in Japan is one of the last respoectable builders of scopes. Hameg in Germany may still be at it as well, but I don't know. People who "can't afford test equipment" are usually full of ****. And if they really aren't they need to get a job, or a better one at any rate. As the True Hams said in the old days,"If you have time to ham, you have time to build". (There are very, very few true hams left.) |
#6
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I would agree--you want real test equipment. I have a cal'd 466 dual trace
Tek scope (1980's vintage) that does everything I need except give digtal sampling, FFT, and storage capability. You can get those for about $150-$200 used. The Velleman complement costs about $800 for both pieces--and is very respectable quality using a self calibrated front end that is optically isolated. It is, however, a little fragile. Unlike the Tek scope you can't hit the AC coupled front end with 1000VDC--it will zap it for sure. If you can live with those restrictions, the combination of rugged Tek, and cheaper DSO make a nice bench setup. The Velleman has good software for running BODE plots with phase, and multiple stacked traces. Again, it's not top end--but does the job for audio signals. wrote in message oups.com... tubesforall wrote: Velleman (www.velleman.be) makes a very inexpensive dual scope and signal generator controlled by a PC that will do the job. See PC-500. I would stick to legitimate test equipment manufacturers, which needn't be that expensive as many are available very cheaply at hamfests today. The exception would be a serious oscilloscope-"simple scopes" can be had for five to twenty dollars all day and repaired by jackleg methods, but modern, calibrated, DC-coupled triggered sweep scopes with attenuators and calibrated sweep are probably best bought _new_. Or late model(still supported) refurb from legitimate test houses (i.e. not *ucker.) Fifteen years ago one was safe in saying "buy nothing but a Tek scope" but Tek does not build actual oscilloscopes, per se, today. Iwatsu in Japan is one of the last respoectable builders of scopes. Hameg in Germany may still be at it as well, but I don't know. People who "can't afford test equipment" are usually full of ****. And if they really aren't they need to get a job, or a better one at any rate. As the True Hams said in the old days,"If you have time to ham, you have time to build". (There are very, very few true hams left.) |
#7
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wrote in message oups.com... tubesforall wrote: Velleman (www.velleman.be) makes a very inexpensive dual scope and signal generator controlled by a PC that will do the job. See PC-500. I would stick to legitimate test equipment manufacturers, which needn't be that expensive as many are available very cheaply at hamfests today. The exception would be a serious oscilloscope-"simple scopes" can be had for five to twenty dollars all day and repaired by jackleg methods, but modern, calibrated, DC-coupled triggered sweep scopes with attenuators and calibrated sweep are probably best bought _new_. Or late model(still supported) refurb from legitimate test houses (i.e. not *ucker.) Fifteen years ago one was safe in saying "buy nothing but a Tek scope" but Tek does not build actual oscilloscopes, per se, today. Iwatsu in Japan is one of the last respoectable builders of scopes. Hameg in Germany may still be at it as well, but I don't know. People who "can't afford test equipment" are usually full of ****. And if they really aren't they need to get a job, or a better one at any rate. As the True Hams said in the old days,"If you have time to ham, you have time to build". (There are very, very few true hams left.) Cal, Tek's high end scopes are still made here. The ones assembled in China are reported to have very good mechanical construction, and I'm sure they didn't give the Chinese the responsibility of making the chips. The best way to acquire a scope is to use eBay to purchase a used Tek scope. The 7000 series are exemplary instruments, representing the finest in American craftsmanship. The economics of maintenance are actually superior to when these scopes were in production, as parts are plentiful through cannibalization. I personally have purchased a 7904 with a set of plugins for $170 off of eBay, and it has worked flawlessly out of the box ever since. Many specialized plugins are available for virtually any measurement need, from low frequency differential to microwave sampling. As for the odds of getting a bad one, I've bought around ten that way, and never got bit. The scopes are well supported by the listserver TekScopes , who represent a wealth of expertise and generous knowledge sharing. A Tek 7000 scope equipped with the appropriate 7A21 differential plugin can measure into the microvolt range, something a Hameg cannot. And when it breaks, the superb documentation enables self-repair, or simply calibration. |
#8
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The old Teks are lovely. My reluctance to invest money in them is due
to the lack of full support-there are chips and assemblies, plus the CRT, that just are no longer available. For years I've hoped someone would make replacement CRTs but it's not going to happen. The new Tek scopes are all digitizing scopes with LCD displays. These have many advantages but they do not, even with the "digital phosphor technology", fully replace the analog CRT. There are many fine Tek scopes out there from about a 30-year period that are still good to use, and I don't mean to put off their buffs. But they are just not supported by Tek anymore, and I don't want to have to look at them when the jug, attenuators, or what have you fails and it has to go to its final resting place. It's interesting to note there is a company that-at ridiculous cost-makes an aftermarket "mainframe" to allow 7000 series plugins to be used with modern (or any other) scopes. |
#9
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wrote in message oups.com... The old Teks are lovely. My reluctance to invest money in them is due to the lack of full support-there are chips and assemblies, plus the CRT, that just are no longer available. For years I've hoped someone would make replacement CRTs but it's not going to happen. The new Tek scopes are all digitizing scopes with LCD displays. These have many advantages but they do not, even with the "digital phosphor technology", fully replace the analog CRT. There are many fine Tek scopes out there from about a 30-year period that are still good to use, and I don't mean to put off their buffs. But they are just not supported by Tek anymore, and I don't want to have to look at them when the jug, attenuators, or what have you fails and it has to go to its final resting place. It's interesting to note there is a company that-at ridiculous cost-makes an aftermarket "mainframe" to allow 7000 series plugins to be used with modern (or any other) scopes. The parts are actually more available. How can you say a jug is not available when every 7904 takes the same CRT as a 7704A, a mainframe that is plentiful for $50? Contrast this with the $500 (conservative) bill for a minor Iwatsu repair, or the complete nonrepairability of any digital scope after the warranty runs out. Certain later Tek scopes should be avoided: the 2200 series and the 2400 series. But the classic 7000 & 5000 series, and even the 500 series, are fine. I have a Tek 555, built in 1961 or so, that I bought surplus from GE. It was originally part of the Saturn V support project in Huntsville, AL. Every time I turn it on, it works, unless it needs a tube. It has never needed more. If you really get stuck, Sphere in Canada has lots of Tek parts. |
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