Download Total Harmonic Distortion Test

FPGA Controlled High Performance Amplifier
May06--14
Total Harmonic Distortion Test
Overview:
The Total Harmonic Distortion Test is a LabVIEW automated test designed to verify the amount of
distortion of the FPGA controlled amplifier using HP 8662A Function Generator, HP 3585A Spectrum
Analyzer and HP 8563E Spectrum Analyzer.
As in the Frequency Response Flatness Test, extra caution should be taken to avoid inputting DC
voltage into the analyzer. Prior to testing, a LabVIEW program should be developed to automate the
test. Proper driver should be downloaded from the Internet.
The computer controls the HP 8662A Function Generator through a GPIB bus to output a signal at
desired frequency. When the signal is below 40MHz, HP 3585A Spectrum Analyzer should be used to
measure the frequency spectrum and the HP 8563E Spectrum Analyzer should be used to measure the
distortion for frequency between 40MHz-100MHz.
Since we are measuring the third, fifth…. Harmonics of the input signal, proper LabVIEW program
should be set up to find peaks at multiples of input frequency f. Then we shall take the log of the
following equation result and multiple it by 10 to obtain the total harmonic distortion:
where terms H2…..Hn are the power levels of the harmonics and term H1 is the power
level of the fundamental (the pure tone).
FPGA Controlled High Performance Amplifier
May06--14
Total Harmonic Distortion Test
Equipment:
Function Generator: HP 8662A Function Generator
Specs: http://www.spectratest.com/8662A.asp
Spectrum Analyzer: HP 8563E Spectrum Analyzer (For 40MHz-100MHz, but this
one will burn up with more than 200mV DC. Must be very careful.)
Specs: http://www.valuetronics.com/vt/assets/pdfs/hp_8560eseries.PDF
Spectrum Analyzer: HP 3585A Spectrum Analyzer
Specs: http://www.testbuyer.com/c-52045A080AD5-asd.htm
Terms & Definitions:
GPIB – General Purpose Interface Board. Same as IEEE 488
IEEE 488 – A communication bus, standard in the test industry. Same as GPIB
TEK – Tektronix
THD –
Total Harmonic Distortion
Fundamental frequency—The harmonic component of a complex wave that has the lowest
frequency and commonly the greatest amplitude.
Harmonics -- Signals that are integer multiples of the fundamental frequency.
FPGA Controlled High Performance Amplifier
May06--14
Total Harmonic Distortion Test
Explanation of tests:
By changing resistor values in the circuit, the team should be able to adjust the gain
setting of the circuit. There are 4 available gain settings and the frequencies
corresponding to these settings are up to 100MHz. The amount of total harmonic
distortion generated by an analog circuit is an important measure of the performance of
that circuit. There should be as little harmonic as possible generated.
10 log (THD) = 10 log(√ (E2f2 + E3f2 + … + Enf2)/√ (Ef2 +
THD is calculated using
E2f2 + E3f2 … + Enf2))
THD = percentage of total harmonic distortion
Ef = amplitude of fundamental voltage
E2f = amplitude of second harmonic
Enf = amplitude of nth harmonic voltage
Test Setup:
FPGA Controlled Amplifier
Spectrum Analyzer
Signal Input
Output Observation
GPIB Control
LabView Program
FPGA Controlled High Performance Amplifier
May06--14
Test Specifications:
Input
Frequency
Range
Total
Harmonic
Distortion
(dB)
DC – 1kHz
> 1kHz - 20 kHz
> 20kHz – 100kHz
> 100kHz – 1MHz
> 1MHz – 10MHz
> 10MHz – 20MHz
> 20MHz – 50MHz
> 50MHz – 100MHz
< - 105 dB
< - 95 dB
< -85 dB
< - 80 dB
< - 70 dB
< -65 dB
< -50 dB
< -40 dB
Reference:
http://seniord.ece.iastate.edu/may0614/