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Author Topic: Yaesu FT990 Downward modulation in AM  (Read 1517 times)
VE3TMT
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Posts: 387




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« on: May 11, 2014, 07:39:13 AM »

Hi,

My FT990 puts out 25W AM carrier. If I whistle in to the mic the output drops according to my watt meter but shows increase on the radios RF meter. Even more pronounced when the amp is in line.

Something out of adjustment in the radio?

Max
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VE3TMT
Member

Posts: 387




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« Reply #1 on: May 11, 2014, 07:48:19 AM »

I should add my scope shows a positive power increase, so it would appear only the analog meters are showing the downward drop.
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KB4QAA
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Posts: 2329




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« Reply #2 on: May 11, 2014, 10:08:09 AM »

Downward drop in power is one sign of over modulation.  This is caused by 'Flat topping" or limiting of the signal.

Whistling is about the worst way to test a transmitter.

Looking at page 33 in the FT-990 manual:
-RF Carrier is limited to 25w max.  You may set a lower power if desired.
-Modulation is controlled with the MIC control.
-While speaking adjust MIC gain so the Po Meter just moves (on peaks-my comment).  Setting any higher will result in over modulation.

AM modulation and Linearity is best measured with an RF Sampler installed inline after the transmitter (and can be moved to after any amplifier for checking it as well).  The rectified output is then fed to the X,Y plates of an Oscilloscope where a triangular pattern in is formed.  A standard way to evaluate and adjust the radio is to input a tone, e.g. 1000khz into the MIC input.

An RF Spectrum Analyzer is the best way to check for resulting "splatter" that over modulation causes.

Clean RF Inc, has some simple tutorials.
http://www.cleanrf.com/applications.html#app_2

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VE3TMT
Member

Posts: 387




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« Reply #3 on: May 11, 2014, 10:25:26 AM »

Thank you,
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KB4QAA
Member

Posts: 2329




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« Reply #4 on: May 11, 2014, 10:32:09 AM »

http://www.icycolors.com/nu9n/scope_your_audio.html
NU9N has another one page description

http://home.comcast.net/~msed01/RFpickup.html
Several home brew sampler circuits

http://amfone.net/Amforum/index.php?topic=15743.0
One of many good discussions at www.AMforum.com

W2AEW has numerous outstanding tutorial videos:
https://www.youtube.com/watch?v=0Kk_N_TpDeo
Samplers

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K7KBN
Member

Posts: 2782




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« Reply #5 on: May 12, 2014, 10:02:38 PM »

Downward drop in power is one sign of over modulation.  This is caused by 'Flat topping" or limiting of the signal.

Whistling is about the worst way to test a transmitter.

Looking at page 33 in the FT-990 manual:
-RF Carrier is limited to 25w max.  You may set a lower power if desired.
-Modulation is controlled with the MIC control.
-While speaking adjust MIC gain so the Po Meter just moves (on peaks-my comment).  Setting any higher will result in over modulation.

AM modulation and Linearity is best measured with an RF Sampler installed inline after the transmitter (and can be moved to after any amplifier for checking it as well).  The rectified output is then fed to the X,Y plates of an Oscilloscope where a triangular pattern in is formed.  A standard way to evaluate and adjust the radio is to input a tone, e.g. 1000khz into the MIC input.

An RF Spectrum Analyzer is the best way to check for resulting "splatter" that over modulation causes.

Clean RF Inc, has some simple tutorials.
http://www.cleanrf.com/applications.html#app_2



Not many MIKE inputs will respond well to that tone.
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73
Pat K7KBN
CWO4 USNR Ret.
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