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audio frequency HP 334A Distortion Analyzer - MERENJA


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HP 334A Distortion Analyzer

An audio frequency distortion analyzer basically is a precise AC volt-meter with very flat frequency response from 10Hz to 1MHz ( +/- 0.05 db). Distortion measurement takes two voltage readings where the first reading includes fundamental and harmonics, the second reading includes only the harmonics. The percentage distortion is the ratio of these two readings. (2nd reading/1st reading). If the 1st reading being set to 1 unit, the 2nd reading is the answer.

The HP334A solid stated distortion analyzers were probably the most user-friendly harmonic distortion analyzers ever made. The automatic nulling design is helpful and efficient, which speeds up the time consuming portion of distortion measurements. This model includes two control loops that automatically phase lock the bridge circuit at fundamental frequency so that only harmonics can pass the bridge.


This machine can measure audio frequency (5Hz to 600KHz) THD (Total Harmonic Distortion) down to 0.1% full scale, very useful for audio amplifiers and tube gears.

tube334a1

 

 

Electrical features :

  • Voltage Measurement: 300uV to 300V RMS Full Scale
  • Residual noise : 25uV 600 OHM
  • Frequency Range: 5 Hz to 600 KHz.
  • Distortion Levels: 0.1% to 100% full scale.
  • Auto Nulling
  • AM detector (334A only)
  • Power – 115 or 230 VAC
  • Dimenions – 17 X 13 X 6 inchesMore information about HP 334A/333A

The HP333A/334A were produced from late 60’s to 70’s with very good build quality, such good build quality is hard to find nowadays.

I tested and sold many 333A/334A units. The most interesting case is that the machine displays 0.05% distortion while the actual signal distortion is 0.02%, this is because the rejection amplifier can not well clear the fundamental frequency, how to locate the 0.03% difference is a funny job but of course not simple as just contact cleaning.

Also a good 333A/334A machine should give same reading in two ranges for an overlap frequency, for example a 5.5KHz signal should be measured at both x100 or x1K range with SAME result.

The HEART of HP333A/334A is the auto nulling 90 degree phase spliter, it can accurately split the fundamental frequency into two components :- real axis and imaginary axis (also known as square root of negative one axis). These two voltage signal maintain the bridge balance to eliminate fundamental frequency.

Thank you for visiting!

Probacu ovih dana dal radi.jer neko ima iskustva sa merenjima ove vrste?

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Understanding, Calculating, and Measuring Total Harmonic Distortion (THD)

February 20, 2017 by David Williams


Total harmonic distortion (THD) is a measurement that tells you how much of the distortion of a voltage or current is due to harmonics in the signal. THD is an important aspect in audio, communications, and power systems and should typically, but not always, be as low as possible.

Introduction

Harmonics or harmonic frequencies of a periodic voltage or current are frequency components in the signal that are at integer multiples of the frequency of the main signal. This is the basic outcome that Fourier analysis of a periodic signal shows. Harmonic distortion is the distortion of the signal due to these harmonics.

A voltage or current that is purely sinusoidal has no harmonic distortion because it is a signal consisting of a single frequency. A voltage or current that is periodic but not purely sinusoidal will have higher frequency components in it contributing to the harmonic distortion of the signal. In general, the less that a periodic signal looks like a sine wave, the stronger the harmonic components are and the more harmonic distortion it will have.

So, a purely sinusoidal signal has no distortion while a square wave, which is periodic but does not look sinusoidal at all, will have lots of harmonic distortion. In the real world, of course, sinusoidal voltages and currents are not perfectly sinusoidal; some amount of harmonic distortion will be present. Figures 1 and 2 provide visual comparisons, in the time domain and the frequency domain, of a sinusoidal voltage and a square wave voltage.

 

Sine Wave

 

Square Wave

Figure 1. A sinusoidal voltage and a square wave voltage in the time domain.

 

Harmonics of Sine and Square Wave

Figure 2. A sinusoidal voltage and a square wave voltage in the frequency domain; only the square wave has peaks at the harmonic frequencies.

 

It is easy to see the harmonic distortion when examining the time domain and frequency domain representations of a square wave, but it is also important to be able to quantify harmonic distortion. The next section shows how to do that with the metric of total harmonic distortion.

 

Calculating Total Harmonic Distortion

THD is defined as the ratio of the equivalent root mean square (RMS) voltage of all the harmonic frequencies (from the 2nd harmonic on) over the RMS voltage of the fundamental frequency (the fundamental frequency is the main frequency of the signal, i.e., the frequency that you would identify if examining the signal with an oscilloscope). Equation 1 shows the mathematical definition of THD (note that voltage is used in this equation, but current could be used instead):

 

 

THD=n=2V2n_rmsVfund_rmsTHD=∑n=2∞Vn_rms2Vfund_rms
       Equation 1

 

  • Vn_rmsVn_rms
     is the RMS voltage of the nth harmonic
  • Vfund_rmsVfund_rms
     is the RMS voltage of the fundamental frequency

 

Since the amplitudes of the harmonics are needed to calculate the THD, Fourier analysis can be used to help determine THD. To see this application of Fourier analysis, let’s look at the simple example of a 50% duty cycle square wave. The Fourier series representation of a 50% duty cycle square wave is the following:

 

 

vsquare(t)=4πn=1,3,5...sin(2nπft)nvsquare(t)=4π∑n=1,3,5...∞sin(2nπft)n
       Equation 2

 

And in expanded form, this is:

 

 

vsquare(t)=4πsin(2πft)+43πsin(6πft)+45πsin(10πft)+...+4nπsin(2nπft)vsquare(t)=4πsin(2πft)+43πsin(6πft)+45πsin(10πft)+...+4nπsin(2nπft)
       Equation 3

 

The expanded form is useful to look at because it highlights the peak voltage (Vpk) of each frequency component, and the THD can be calculated by determining the RMS value (i.e., 

Vpk2Vpk2
) of each frequency component and plugging them all in to Equation 1:

 

 

THDsquare=(432π)2+(452π)2+(472π)2+...+(4n2π)242πTHDsquare=(432π)2+(452π)2+(472π)2+...+(4n2π)242π
       Equation 4

 

This equation is starting to get unwieldy, but one thing to notice is that every term in the expression has a 

42π42π
 component. This component can be factored out, and since it appears in both the numerator and denominator, it actually cancels out, which leaves the expression for THD of a square wave as follows:

 

 

THDsquare=n=3,5,...1n21=132+152+172+...+1(n)2THDsquare=∑n=3,5,...∞1n21=132+152+172+...+1(n)2
       Equation 5

 

To calculate the THD from this expression requires a tricky little bit of mathematics. If the summation under the square root in Equation 5 started at n=1, then it would be a convergent series that adds up to 

π28π28
:

 

n=1,3,5...1n2=π28∑n=1,3,5...∞1n2=π28
       Equation 6

 

The only difference between the expression in Equation 6 and the one in the THD calculation of Equation 5 

(n=3,5,...1n2)(∑n=3,5,...∞1n2)
 is the value of 
1n21n2
 when n is 1. Since this value is 1, the summation in the THD expression can be re-written as:

 

 

n=3,5,...1n2=n=1,3,5...1n21=π281∑n=3,5,...∞1n2=∑n=1,3,5...∞1n2−1=π28−1
       Equation 7

 

Finally, plugging this equation back into the THD equation for the square wave (Equation 5) gives:

 

 

THDsquare=π2810.483THDsquare=π28−1≈0.483
       Equation 8

 

Our assumption at the beginning that a square wave has a lot of harmonic distortion was based on visually examining the square wave in the time and the frequency domain. The calculations that we just went through confirm our assumption. A square wave actually has about 48.3% total harmonic distortion meaning that the RMS of the harmonics is about 48.3% of the RMS of the fundamental frequency.

 

Measuring Total Harmonic Distortion

Calculating theoretical THD can be a good exercise, but it can be a lot of work, and in practice, you aren’t going to get an ideal signal (e.g., a perfect square wave) anyway. The outcome of these calculations can therefore only give an approximation for the THD that you might get for a given signal type. In practice, THD must be measured to obtain the RMS value of the fundamental frequency and all of the harmonics. This measurement can be done in a couple of ways.

In the first method, filters can be used to split the signal into two parts: a signal with all of the harmonics filtered out leaving just the fundamental frequency, and a signal with the fundamental frequency filtered out leaving all of the harmonics. Then the RMS value of each of those two parts can be measured and the THD calculated:

 

 

THD=VRMS_Without_FundamentalVRMS_FundamentalTHD=VRMS_Without_FundamentalVRMS_Fundamental

 

The upside of this method is that it is easy to perform these measurements. The downside is that noise will also be included in the measurement so you actually get a measurement of THD plus noise (although in audio systems THD+noise is actually an important measurement too).

The second method for measuring THD is to measure the amplitude of the fundamental frequency and each harmonic and then use those measurements to calculate THD using Equation 1. This measurement can easily be done using a spectrum analyzer or a THD analyzer which will execute Equation 1 automatically. An alternative measurement technique is to capture voltage or current data and then perform a Fourier transform on the data collected. The example below outlines how this second method is used.

 

Example THD Measurement

The example block diagram in figure 3 shows a 1 kHz sine wave passing through an amplifier to create a new 1kHz sine wave that has some crossover distortion. This new wave is fed in to a spectrum analyzer which gives a graphical display of the amplitude of a number of the harmonics. 

 

System Under Test

Figure 3. A system that introduces crossover distortion into a signal.

 

Zooming in on the frequency spectrum of the distorted sine wave output, we can see the amplitudes at several of the harmonic frequencies:

 

FrequencySpectrum.jpg

Figure 4. Frequency spectrum of sinusoidal voltage with crossover distortion.

 

From this frequency spectrum, I manually measured the amplitude of each of the harmonic frequencies and recorded the data in the table below:

 

Amplitudes of Harmonics of Distorted Sine Wave
Harmonic Amplitude
1 3.08V
3 0.308V
5 0.159V
7 0.090V
9 0.0487V
11 0.0253V
13 0.0164V
15 0.010V

 

The amplitudes of even-numbered harmonics and harmonics above the 15th are nearly 0, so I didn’t include them in my calculation.

The measured amplitudes are plugged in to the THD equation:

 

 

THD=0.3082+0.1592+0.0902+0.04872+0.02532+0.01642+0.01023.08THD=0.3082+0.1592+0.0902+0.04872+0.02532+0.01642+0.01023.08

 

(note that I am able to use the voltage amplitudes instead of RMS voltage because 

VRMS=Vp2VRMS=Vp2
 and since the 
22
 occurs in all terms, it can be factored out and cancelled).

This calculation gives a THD of 0.118 or 11.8%.

Of course a THD analyzer would automate the process of calculating THD from the amplitudes of the harmonics. Using a THD analyzer for this signal gives a value of 11.9%, which confirms the accuracy of the manual method that I just went through.

 

Final Words

This article has provided some background about THD and how to determine it, both theoretically and in a real (simulated) system. But it has not discussed the kinds of systems where THD is an important measurement.

THD is important in several types of systems, including power systems, where a low THD means higher power factor, lower peak currents, and higher efficiency; audio systems, where low THD means that the audio signal is a more faithful reproduction of the original recording; and communication systems, where low THD means less interference with other devices and higher transmit power for the signal of interest.

Look for future articles where I will go into more detail about these specific types of systems.

 

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To je pesadijska furije analiza, s tim sto je za ocekivati odredjen visok nivo greske. Mislim, visok za audiofilske zelje.

Savremeni digitalni spektralni analizatori to sve sami odrade, ili mogu da posalju podatke do racunara, u npr Matlab ili Labvju, i signal se moze obradjivati po zelji..iscrtavati grafikoni po zelji...i to sa nekoliko stotina mernih tacaka

 

Uzgred, jesu te sprave radjene temeljno i detaljno, robusno i inzenjerski, ali ih treba povremeno bazdariti..

Naravno, upotreba je tek nakon zagrevanja od bar pola sata, za neke sprave(iz tih vremena) su preporucivali i 2,5 sata zagrevanja, da se sve lepo ujednaci i termostatira..

 

Ali mislim da moze da se koristi, da stvori neku sliku o uredjajima koje moze da analizira, i napravi iskustvo merenja..

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treba mi samo za lampe, i valjda ce da radi-jos ga nisam upalio,u nekoj sam guzvi.Ne tezim da se bavim bas vrhunskim merenjima.imam i uputstvo za njega na ingleskom,jbg,al sta je tuje.ima i nesto na youtube pa cu polakoo..

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Malo je zez sto je to samo plolovina potrebne instrumentacije za merenje THD-a.

Potreban ti je i low THD generator i to sa manjim THD-om bar za red velicene od tog analizatora.

 

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imam vise signal generatora low distortion ali nijedan nije sa prefiksom ultra,evo  na primer ovaj:

HP / Agilent 3314A
20MHz Function Generator, Refurbished

  • Lin/Log sweeps
  • AM/FM/VCO
  • Phase lock XN and ÷N
  • Gate and counted burst
  • 1/2 cycle mode
  • Arbitrary waveform generator
  • Available Options, Below

3314Athumb.jpg

Hewlett Packard 3314A Shown

 

 

The 3314A is a Function/Waveform Generator with the precision and versatility to produce numerous waveforms.  Its feature set includes accurate sine, square, and triangle waves, with ramps and pulses available using variable symmetry.  Additional features include counted bursts, gate, lin/log sweeps, AM, FM/VCO, dc offset, and phase lock.  For increased versatility, the Arbitrary waveform mode allows a countless number of user defined waveforms.  Since complete programmability is provided, all of these capabilities are available for ATE systems, as well as bench applications.

Precise Functions

The 3314A provides sine, square and triangle waveforms from 0.001 Hz to 19.99 MHz with an amplitude range of 0.01 mV to 10 Vp-p into 50 Ω, with optional 30 Vp-p into > 500 Ω.

Continuous waveforms are provided with high accuracy and low distortion, with frequency accuracy on the upper ranges of 0.01% and sine disortion < -55 dBc to 50 kHz.

 

ili ovaj hp

 The Model 654A is a stable, low distortion sine-wave signal source 

 

HP654A large.jpg

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17 hours ago, dunavko said:

 

imam vise signal generatora low distortion ali nijedan nije sa prefiksom ultra,evo  na primer ovaj:

 

Pa to je sasvim dovoljno za testiranje lampasa. 

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