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Author Topic: Another General Exam question  (Read 1831 times)
KC8OYE
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Posts: 297




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« on: September 17, 2010, 11:09:09 PM »

What is the frequency deviation for a 12.21-MHz reactance-modulated oscillator in a 5-kHz deviation, 146.52-MHz FM-phone transmitter?
  A. 101.75 Hz
 B. 416.7 Hz
 C. 5 kHz
 D. 60 kHz

I know the answer is (B) - 416.7hz... but how do you arrive at that number?  I prefer to know WHY an answer is.. not just memorize the answer to a particular test question..
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NJ2E
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Posts: 35




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« Reply #1 on: September 18, 2010, 12:15:06 AM »

12.21 mhz is multiplied 12 times to get to the 146.52 mhz frequency. So to get the 5 khz
deviation at 146.52 mhz, you only modulate the oscillator by 416.7 hz (5000 hz / 12 = 416.7 hz), since the amount of deviation is likewise multiplied. Regards, Don
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KC8OYE
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« Reply #2 on: September 18, 2010, 01:48:33 AM »

ahhhhh.. I see.. said the blind man.. to his deaf dog..


lol. j/k.. strangely enough.. that actually makes sense..
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VK2FXXX
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Posts: 102




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« Reply #3 on: September 18, 2010, 03:03:28 AM »

I learnt something
thankyou both.

73 Brendan
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N2EY
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Posts: 3834




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« Reply #4 on: September 18, 2010, 04:54:19 AM »

how do you arrive at that number? 

The question assumes you know some things about the hypothetical FM transmitter being described.

In that transmitter, the oscillator generates the FM signal at a low frequency, which is then passed through frequency-multiplying stages to get to the output frequency. (This is a common design approach since the beginning of FM).

The important point is that not only is the carrier frequency multiplied, but so is the deviation caused by the modulating signal - and by the same factor (12 times).

73 de Jim, N2EY
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AB2T
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Posts: 246




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« Reply #5 on: September 18, 2010, 05:16:32 AM »

Gosh, I don't remember the General being this hard.  So much for the "dumbing down" argument!  All I remember was that the Advanced had trigonometry on it.  I had to memorize the equations the summer before actually taking high school trig.  Not hard, but life out of sequence.

73, Jordan
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