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 Post subject: Spectrum Analyzer
PostPosted: Sun Jun 13, 2010 2:10 pm 
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(PCSU1000)
I would like to use the spectrum analyzer of the PCSU1000 to determine harmonics
generated by my 40 meter CW low power transmitter, in terms of harmonic frequency
and the amount of db's below the fundamental freq. The output voltage of the transmitter is 50 volts peak, operating frequency is 7.035 MHZ. I don't know what freq range/log freq scale to use and how to interpret the display. Any help will be greatly appreciate.


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 Post subject: Re: Spectrum Analyzer
PostPosted: Sun Jun 13, 2010 5:23 pm 
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Joined: Mon Dec 11, 2006 12:03 pm
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You need to use x10 probe setting.
Select "Big Screen" mode.
Here a screenshot of the settings.
Image

To measure very low harmonics you have to use the vector average mode in the spectrum analyzer. The trigger must be on when this average mode is used.
Image

Then use the markers to measure the harmonics relative to the fundamental carrier (dBc).
Image

Here an example screenshot.
Image
If you use the normal screen, you need to use the horizontal slider to see all the harmonics.


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 Post subject: Re: Spectrum Analyzer
PostPosted: Sun Jun 13, 2010 6:53 pm 
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Thanks for your quick reply, I set the view and options as you indicated and
performed all the other steps. I connected a rf signal generator to the PSCU1000, freq set to 7 Mhz, set level to 500 mv peak and I see see many peaks to the right of the fundamental, I am assuming that the first peak is the second harmonic, and if it is, it's freq should be about 14 Mhz. and all the other peak to the right will be harmonically related. From your screen shot, you marked the second peak from the fundamental and if I am reading it right, it is 62.81 db down from the fundamental peak level. Is there a reason why you did not use the first peak instead of the second? I have a couple of more questions to asked, what does f21.10 Mhz at the base line represent?, how do I determine the freq's of the fundamental and harmonics by looking at the display?, and finally what does setting the freq range to 25 Mhz accomplish? I am going through a learning process, I never used a spectrum analyzer. Appreciate your help and time.
abeza


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 Post subject: Re: Spectrum Analyzer
PostPosted: Sun Jun 13, 2010 8:02 pm 
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Quote:
I connected a rf signal generator to the PSCU1000, freq set to 7 Mhz, set level to 500 mv peak and I see see many peaks to the right of the fundamental, I am assuming that the first peak is the second harmonic, and if it is, it's freq should be about 14 Mhz.
You must use the oscilloscope display to see that there is no clipping of the waveform. The waveform should look nice sine wave on the oscilloscope screen.

Quote:
From your screen shot, you marked the second peak from the fundamental and if I am reading it right, it is 62.81 db down from the fundamental peak level. Is there a reason why you did not use the first peak instead of the second?
I selected the second peak just as an example to show how you can use the markers (cursors) on the screen to make measurements. You can freely move the marker lines by the mouse and measure the other peaks displayed, e.g. the first or the second harmonic.

Quote:
I have a couple of more questions to asked, what does f21.10 Mhz at the base line represent?
It is the frequency of the position of the vertical marker on the screen.

Quote:
how do I determine the freq's of the fundamental and harmonics by looking at the display?
Just move the vertical marker using the mouse. You'll see the frequency at the base line.

Quote:
and finally what does setting the freq range to 25 Mhz accomplish?
It is the displayed frequency range on the screen.


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 Post subject: Re: Spectrum Analyzer
PostPosted: Sun Jun 13, 2010 8:11 pm 
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Thanks again VEL255, you answered all my questions, I now understand how
to use the spectrum function of my PSCU1000 to measure frequency and db difference
between the fundamental and related harmonics. Your explanation and screen shots
explained it clearly.

abeza


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