Build an IM Distortion Analyzer.

Chapter 6   Calibration.

 6.1   Calibration to 0.316 volt = 100%.
6.1.1   Adjusting Trimmer in M 4.
6.1.2a   Method 1,Two HF Generators.
6.1.2b   Method 2, Fancy Schmancy Function Generator.
6.1.2c   Method 3, Post Detection DC Level.
6.2   Calibration to 1.00 volt = 100%.
6.2.1   Adjusting Trimmer in M 4.
6.2.2a   Method 1,Two HF Generators.
6.2.2b   Method 2, Fancy Schmancy Function Generator.
6.6.2c   Method 3, Post Detection DC Level.

Chapter 6 Calibration.

If you are performing this calibration for the first time the instrument will already be out of its case. If this is a recalibration, which I recommend you do annually, remove the instrument from the wooden frame by removing the 4 philips head screws in the 4 corners of the panel. Gently lift the panel out of the wooden frame and rest it on your workbench. Set the wooden frame in a safe place so it won't be accidently damaged.

Two separate calibration procedures are given. One is for 0.316 volts = 100%. This one will be presented in normal black text on a white background. The other will be 1 volt = 100%. To be sure you do not confuse the two the second one will be presented as white text on a dark blue background. Each procedure is completely stand alone and there will not be any references from one to the other.

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6.1 Calibration to 0.316 volt = 100%.

6.1.2a Method 1, two HF Generators.

It is assumed below that your readout device is a Harmonic distortion analyzer. The norm on these instruments is for 0.316 volts to equal 100% with the range switch marked off in ranges of 100%, 30%, 10% etc. The instructions will refer to the readout device as the HDA (harmonic distortion analyzer).

Three different methods of calibration are given. All three have in common the adjustment of the trimming potentiometer in module M 4. That procedure will be given only once. After completing this section move to one of the three alternate calibration methods.

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6.1.1 Adjusting Trimmer in M 4.

  1. Turn on all instruments to be used and let them warm up for at least half an hour. One hour is better.
  2. Connect the "METER OUTPUT" of the IM analyzer to the input of your HD analyzer.
  3. Use a short BNC to BNC cable to connect "TEST SIGNAL" to "TEST INPUT".
  4. Set the range switch on your HDA (Harmonic Distortion Analyzer) to the 0.3 volt / 100% range and the "FUNCTION" to "VOLTMETER".
  5. Set the "METER SELECT" on the IMA (Inter Modulation Analyzer) to "MON SIG".
  6. Set the "SIGNAL SELECT" on the IMA to "LF".
  7. Advance the "SIG LEVEL" clockwise while watching the meter in your HDA.
  8. Adjust the "SIG LEVEL" for a meter reading of 3 on the 0 to 3 scale. This represents 0.3 volts.
  9. Change the "SIGNAL SELECT" to "HF".
  10. Adjust the "HF LEVEL" for a reading of 0.075 volts. Switch to the 0.1 volt range to obtain a more accurate reading.
  11. Change the HDA back to the 100% range.
  12. Change the "METER SELECT" to "CAL".
  13. Set the "INPUT ATTENUATOR" to "HI Z" and the Vernier fully clockwise.
  14. Adjust the trimmer in M4 for a reading of 1 (100%) on the 0 to 1 meter scale.

I have devised three methods of calibration. I will present them in descending order of accuracy. The first method is the most difficult because it is rather fiddly as the British say. Don't bother looking at the second method unless you have one of those new digital function generators that will generate amplitude modulated waves. If you don't have one this is it. The third method should be used only if you don't have the equipment to accomplish 1 or 2.

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6.1.2a Method 1, two HF Generators.

Equipment required.

  • Function generator with sinewave output capable of being tuned to 7,000 Hz.
  • Frequency counter capable of measuring 7 kHz with 1 Hz resolution.
  • HDA (Harmonic Distortion Analyzer).
  • AC voltmeter, other than your HDA, capable of measuring 0 to 1 volt with 1% accuracy or better when the input frequency is 7,000 Hz. A DMM may be used here if it fits the specifications.
  • Test leads and cables.

It is assumed you have completed the adjustment of the trimmer in M 4 as instructed above and the "TEST SIG" and the "TEST INPUT" jacks are still connected together by a cable.

  1. Make sure the "SIGNAL SELECT" is still in the "HF" position.
  2. Set the "INPUT ATTENUATOR" to "X1" and the "VERNIER" fully counter clockwise.
  3. Connect both your AC voltmeter and frequency counter to the "HF OUT" connector.
  4. Adjust the "HF LEVEL" and "SIG LEVEL" to obtain a reading of 1 volt on the meter connected to the "HF OUT".
  5. Note the frequency of the "HF OUT" signal.
  6. Connect your function generator to the "CAL SIGNAL IN" connector.
  7. Move the connection of the voltmeter and counter to the "CAL SIGNAL OUT" connector.
  8. Adjust the amplitude of your function generator to match that noted for the HF output. Should be 1 volt.
  9. Adjust the frequency of your function generator to be 60 Hz higher than that noted for the HF output.
  10. Switch your voltmeter and counter back and forth between the "HF OUT" and "CAL SIGNAL OUT" to make sure that the amplitudes are identical and the frequency of the function generator is 60 Hz higher than the HF generator.
    You will likely observe some drift in both amplitude and frequency but get them as close as you can.
  11. Set the "METER SELECTOR" to the "CAL" position.
  12. Adjust the "VERNIER" for a reading of 1 (100%) on the HAD.
  13. Change the "METER SELECT" on the IMA to "READ".
  14. Change the range switch on the HDA to 10%.
  15. Adjust the trimmer potentiometer in M 5 for a reading on the HDA meter of 1 on the 0 to 1 scale. This represents 10%.
  16. Recheck the amplitude and frequency of your function generator one more time.
  17. If necessary retouch the trimmer a little to bring the pointer over the 1 on the meter scale.

The care with which you perform the above procedure will determine the accuracy of your IM Distortion Analyzer.

Install your meter in its case and enjoy.

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6.1.2b Method 2, Fancy Schmancy Function Generator.

Equipment Required.

  • Fancy Schmancy Function generator.
  • Your HDA
  • Connecting cables.

It is assumed you have completed the adjustment of the trimmer in M 4 as instructed above.

  1. Set up your FSFG (Fancy Schmancy Function Generator) as follows.
  2. Frequency to 7,000 Hz.
  3. Output level to 1 volt.
  4. Mode Amplitude Modulation (AM).
  5. Modulating frequency 60 Hz.
  6. Modulation depth (percent modulation) 10%.
  7. Connect the IMA to your HDA as is normal.
  8. Set the '"Vernier" of the input attenuator fully counter clockwise.
  9. Connect the output of your FSFG to the "TEST INPUT" of your IMA.
  10. There is nothing connected to the "TEST SIGNAL" connector.
  11. Set the "METER SELECT" on the IMA to "CAL".
  12. Set the range switch on the HDA to 100%.
  13. Use the input attenuator controls on the IMA to bring the pointer to 1 (100% reading).
  14. Change the "METER SELECT" to the "READ" position.
  15. Change the range switch on your HDA to 10%.
  16. Adjust the trimming potentiometer in M 5 for a reading of 1 (10%).

Using my equipment methods 1 and 2 agree within 1%.

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6.1.2c Method 3, Post Detection DC Level.

Equipment Required.

  • HDA.
  • DC Voltmeter. There is no problem with using a DMM.

  1. Using a short coaxial cable connect the "TEST SIGNAL" and "TEST INPUT" together.
  2. Set the "SIGNAL SELECT" to "HF".
  3. Set the "METER SELECT" to "MON SIG
  4. Set the range switch on the HDA to the 0.1 volt range.
  5. Adjust the "SIG LEVEL" and if necessary the "HF LEVEL" for a reading of 0.10 volts.
  6. Change the HDA to the 100% range.
  7. Change the "METER SELECT" to "CAL".
  8. Adjust the "INPUT ATTENUATOR VERNIER" for a reading of 100% on the HDA.
  9. Connect the "METER OUTPUT" to the input of your DC Voltmeter.
  10. Change the "METER SELECT" to "READ".
  11. Adjust the trimming potentiometer in M 5 for a reading of 0.460 volts DC.

Note: When the voltage to the input of the HDA is at the 100% indication the input voltage is 0.3162 volts which is the square root of 0.1 or ten to the minus one. The theoretical DC value would be the square root of two times this value which is 0.4472. The discrepancy is caused by the fact that the 4 section low pass filter which follows the detector is slightly attenuating the 60 Hz signal but not the DC. The calibration compensates for this so the DC comes out a little higher than the theoretical value.

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6.2 Calibration to 1.00 volt = 100%.

It is assumed below that you are using an AC Voltmeter similar to one of the HP 400 series. The 1 volt range will be taken as 100% full scale, 0.3 V range as 30%, 0.1 V range as 10%, down to the 0.001 V (1 mV) range which will be 0.1% full scale. I'm sure it would be helpful to mark the range switch in some way. Preferably by making a label designed in a graphics program on your computer and printed out.

Three different methods of calibration are given. All three have in common the adjustment of the trimming potentiometer in module M 4. That procedure will be given only once. After completing this section move to one of the three alternate calibration methods.

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6.2.1 Adjusting Trimmer in M 4.

  1. Turn on all instruments to be used and let them warm up for at least half an hour. One hour is better.
  2. Connect the "METER OUTPUT" of the IM analyzer to the input of your AC Voltmeter.
  3. Use a short BNC to BNC cable to connect "TEST SIGNAL" to "TEST INPUT".
  4. Set the range switch on your AC Voltmeter to the 0.3 volt range.
  5. Set the "METER SELECT" on the IMA (Inter Modulation Analyzer) to "MON SIG".
  6. Set the "SIGNAL SELECT" on the IMA to "LF".
  7. Advance the "SIG LEVEL" clockwise while watching your AC Voltmeter.
  8. Adjust the "SIG LEVEL" for a meter reading of 3 on the 0 to 3 scale. This represents 0.3 volts.
  9. Change the "SIGNAL SELECT" to "HF".
  10. Adjust the "HF LEVEL" for a reading of 0.075 volts. Switch to the 0.1 volt range to obtain a more accurate reading.
  11. Change the range of your AC voltmeter to the 1 volt (100% range).
  12. Change the "METER SELECT" to "CAL".
  13. Set the "INPUT ATTENUATOR" to "HI Z" and the Vernier fully clockwise.
  14. Adjust the trimmer in M4 for a reading of 1.0 (100%) on the 0 to 1 meter scale.

I have devised three methods of calibration. I will present them in descending order of accuracy. The first method is the most difficult because it is rather fiddly as the British say. Don't bother looking at the second method unless you have one of those new digital function generators that will generate amplitude modulated waves. If you don't have one this is it. The third method should be used only if you don't have the equipment to accomplish 1 or 2.

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6.2.2a Method 1, two HF Generators.

Equipment required.

  • Function generator with sinewave output capable of being tuned to 7,000 Hz.
  • Frequency counter capable of measuring 7 kHz with 1 Hz resolution.
  • AC Voltmeter that is your readout device.
  • Second AC Voltmeter other than your readout device,, capable of measuring 0 to 1 volt with 1% accuracy or better when the input frequency is 7,000 Hz. A DMM may be used here if it meets the specifications.
  • Test leads and cables.

It is assumed you have completed the adjustment of the trimmer in M 4 as instructed above.

  1. Make sure the "SIGNAL SELECT" is still in the "HF" position.
  2. Set the "INPUT ATTENUATOR" to "X1" and the "VERNIER" fully counter clockwise.
  3. Connect both your second AC voltmeter and frequency counter to the "HF OUT" connector.
  4. Adjust the "HF LEVEL" and "SIG LEVEL" to obtain a reading of 1 volt on the meter connected to the "HF OUT".
  5. Note the frequency of the "HF OUT" signal.
  6. Connect your function generator to the "CAL SIGNAL IN" connector.
  7. Move the connection of the voltmeter and counter to the "CAL SIGNAL OUT" connector.
  8. Adjust the amplitude of your function generator to match that noted for the HF output. Should be 1 volt.
  9. Adjust the frequency of your function generator to be 60 Hz higher than that noted for the HF output.
  10. Switch your voltmeter and counter back and forth between the "HF OUT" and "CAL SIGNAL OUT" to
    make sure that the amplitudes are identical and the frequency of the function generator is 60 Hz higher than the
    HF generator. You will likely observe some drift in both amplitude and frequency but get them as close as you can.
  11. Set the "METER SELECTOR" to the "CAL" position.
  12. Adjust the "VERNIER" for a reading of 1 volt (100%) on the readout voltmeter.
  13. Change the "METER SELECT" on the IMA to "READ".
  14. Change the range switch on the readout voltmeter to 10% (0.1 volt range).
  15. Adjust the trimmer potentiometer in M 5 for a reading on the readout voltmeter of 1 on the 0 to 1 scale. This represents 10%.
  16. Recheck the amplitude and frequency of your function generator one more time.
  17. If necessary retouch the trimmer a little to bring the pointer over the 1 on the meter scale.

The care with which you perform the above procedure will determine the accuracy of your IM Distortion Analyzer.

Install your meter in its case and enjoy.

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6.2.2b Method 2, Fancy Schmancy Function Generator.

Equipment Required.

  • Fancy Schmancy Function generator.
  • Your readout AC Voltmeter.
  • Connecting cables.

It is assumed you have completed the adjustment of the trimmer in M 4 as instructed above.

  1. Set up your FSFG (Fancy Schmancy Function Generator) as follows.
  2. Frequency to 7,000 Hz.
  3. Output level to 1 volt.
  4. Mode Amplitude Modulation (AM).
  5. Modulating frequency 60 Hz.
  6. Modulation depth (percent modulation) 10%.
  7. Connect the READOUT VOLTMETER to your IMA as is normal.
  8. Set the '"Vernier" of the input attenuator fully counter clockwise.
  9. Connect the output of your FSFG to the "TEST INPUT" of your IMA.
  10. There is nothing connected to the "TEST SIGNAL" connector.
  11. Set the "METER SELECT" on the IMA to "CAL".
  12. Set the range switch on the READOUT VOLTMETER to 100%.
  13. Use the input attenuator controls on the IMA to bring the pointer to 1 (100% reading).
  14. Change the "METER SELECT" to the "READ" position.
  15. Change the range switch on your READOUT VOLTMETER to 10%.
  16. Adjust the trimming potentiometer in M 5 for a reading of 1 (10%).

Using my equipment methods 1 and 2 agree within 1%.

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6.2.2c Method 3, Post Detection DC Level.

Equipment Required.

  • Readout AC Voltmeter..
  • DC Voltmeter. There is no problem using a DMM.

  1. Using a short coaxial cable connect the "TEST SIGNAL" and "TEST INPUT" together.
  2. Set the "SIGNAL SELECT" to "HF".
  3. Set the "METER SELECT" to "MON SIG
  4. Adjust the "SIG LEVEL" and if necessary the "HF LEVEL" for a reading of 0.10 volts.
  5. Change the "METER SELECT" to "CAL".
  6. Adjust the "INPUT ATTENUATOR VERNIER" for a reading of 100% on the readout voltmeter.
  7. Connect the "METER OUTPUT" to the input of your DC Voltmeter.
  8. Change the "METER SELECT" to "READ".
  9. Adjust the trimming potentiometer in M 5 for a reading of 1.455 volts DC.

Note: When the voltage to the input of the readout voltmeter is at the 100% indication the input voltage is 1.000 volt. The theoretical DC value would be the square root of two times this value which is 1.41421. The discrepancy is caused by the fact that the 4 section low pass filter which follows the detector is slightly attenuating the 60 Hz signal but not the DC. The calibration compensates for this so the DC comes out a little higher than the theoretical value.