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Author Topic: I'm old-new guy to ham with question  (Read 1304 times)
KF6GUB
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Posts: 29




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« on: January 07, 2013, 09:52:53 AM »

I received my tech-no-code license 15 years ago.  Never owned a ham radio.  I'm reviewing tech knowledge base, which has changed in 15 years.  My question is concerns FM band.  Is not the electrical signal that influences the FM carrier affected by both amplitude and frequency?  I've read that the electrical signal only influences the amplitude of the FM frequency.  I find that quite confusing.   Clarification please.   thx  Jim
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W5FYI
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« Reply #1 on: January 07, 2013, 10:08:24 AM »

I'm not sure what you mean by "the electrical signal that influences the FM carrier...," but in most cases you are correct. The signal delivered by the microphone amplifier is typically an AM signal, with voltage peaks and valleys. This AM signal can be used to change the capacitance of a varicap diode in the RF oscillator circuit. The change in capacitance changes the oscillator's frequency in step with the AM audio input. The oscillator's output is very nearly linear in amplitude, but varies in frequency. Is this what you were asking?

The varicap method is just one of several ways to apply audio to an FM carrier; but it is one of the simplest to understand.

Welcome back to the hobby. Perhaps you'll get a radio this time around, and have some fun. GL
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KF6GUB
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« Reply #2 on: January 07, 2013, 10:22:52 AM »

W5FYI, thanx for getting back to me.  I've read that the FM carrier is only influenced by the amplitude of an electrical signal. When I speak into a microphone, don't I submit an electrical wave with varying amplitude and frequency properties?  Now I understand that an FM carrier only varies in apparent frequency, whereas an AM carrier is influenced by apparent amplitude.  Are not all carriers, FM and AM, experiencing both amplitude and frequency deviations?  thx again, Jim
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KA4POL
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« Reply #3 on: January 07, 2013, 11:05:10 AM »

Your observations are basically true. If you change the amplitude of a carrier you also change the steepness of the wave. This is similar to changing the phase angle in FM. However, the main change is the amplitude in AM and the frequency in FM. FM is usually not more than 5 kHz of change according to the audio amplitude. The audio frequency determines at what frequency this change occurs. So there are two ways of getting the audio signal on the RF signal.
There is a nice animation at http://en.wikipedia.org/wiki/Frequency_modulation
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KF6GUB
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« Reply #4 on: January 07, 2013, 11:19:22 AM »

KA4POL, thx for getting back to me.  Question on the tech test:   What determines the amount of deviation of an FM signal?
The answer that is considered correct is that it is the amplitude of the modulating signal.   I thought the answer should be that it is both the frequency and the amplitude of the modulating signal.  Thanks, Jim Grin
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AA4PB
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« Reply #5 on: January 07, 2013, 11:30:30 AM »

The output of a microphone is an AC signal. It is not an AM (Amplitude Modulated) signal because there is no modulation. The signal is a baseband AC signal that changes in both frequency and amplitude. In an FM (Frequency Modulated) transmitter that microphone's AC signal is used to change the frequency (only the frequency) of the transmiter in step with the amplitude change from the microphone. The higher the intantanious voltage from the microphone, the farther the transmitter frequency moves from the carrier (resting or center) frequency. The peak amplitude of the microphone signal then determines how far off the carrier frequency the transmitter moves and the frequency of the microphone signal determines how fast the transmitter frequency moves. For example, if you put a pure 1000Hz tone into the microphone then the transmit signal will move from carrier - to maximum - to carrier - to minimum - and back to carrier 1000 times per second. At no time does the amplitude of the FM signal change. The maximum distance that the transmit signal moves in each direction is called the "deviation". The deviation for most amateur FM work is +/-5KHz.



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KF6GUB
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« Reply #6 on: January 07, 2013, 12:00:30 PM »

AA4PB, thanks for getting back to me.  That question in my previous post is directly from the tech test for license qualification.  Go figure.  What you wrote sounds as if it contradicts the notions of the question I posted.  Does it not?   I'm thinking that the deviation of the FM carrier is influenced by both the amplitude and frequency of the baseband AC signal.   The FM carrier is generally of fixed amplitude, no?   The variations of phase and frequency of the wave of the FM carrier give both pitch (frequency) and volume (amplitude) of sound.  I'm a senior man of eroding intellectual skills. I just wanting enlightenment before it's too late.  thx, Jim  Grin
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N9BH
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« Reply #7 on: January 07, 2013, 12:59:05 PM »

I once sat in a seminar where it was said, the amplitude of an AM carrier does not vary, but the amplitude of an FM carrier does. They were talking about carrier, not band power. I may have gotten it wrong, but I think the idea is that with FM, the carrier swings in frequency with the modulation. That being the case, the carrier, or center frequency, is not always sitting where it would be with an modulated signal. If you are looking at that center carrier frequency, sometimes it is there and sometimes it is not there. That is a change in amplitude of the center carrier frequency. I admit I may have misunderstood, but this is what I got out of it. 
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VA7CPC
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« Reply #8 on: January 07, 2013, 01:07:56 PM »

AA4PB, thanks for getting back to me.  That question in my previous post is directly from the tech test for license qualification.  Go figure.  What you wrote sounds as if it contradicts the notions of the question I posted.  Does it not?   I'm thinking that the deviation of the FM carrier is influenced by both the amplitude and frequency of the baseband AC signal.   The FM carrier is generally of fixed amplitude, no?   The variations of phase and frequency of the wave of the FM carrier give both pitch (frequency) and volume (amplitude) of sound.  I'm a senior man of eroding intellectual skills. I just wanting enlightenment before it's too late.  thx, Jim  Grin

PMFJI --

>>>
 I'm thinking that the deviation of the FM carrier is influenced by both the amplitude and frequency of the baseband AC signal. 
<<<

That's your mistake.  The (instantaneous) frequency of the FM carrier is influenced by the _amplitude_ of the baseband AC signal, _not_ by its frequency.

Let's say the AC signal is a 1000 Hz square wave, at maximum permitted amplitude (voltage), just to make life simple.  

And let's say the maximum deviation permitted on the FM output is 5 kHz, and the FM channel frequency is 5000 kHz.

Then the FM signal will be at 4995 kHz when the AC modulating signal is maximum negative, and it will switch to 5005 kHz when the AC signal changes to maximum positive.

The FM signal will make that round-trip ( 4995 kHz --> 5005 kHz --> 4995 kHz) 1000 times each second.  It will _not_ change amplitude.  

If the AC signal were a 200 Hz square wave, at maximum amplitude:

. . . The FM signal would go from 4995 --> 5005 --> 4995 (the same deviation as before,
. . . . . because the maximum and minimum voltage of the AC modulating signal is the same as before),
. . .     but it would make the round-trip 200 times per second.

.              Charles

PS -- there are subtleties here that we're all ignoring, and that we should continue to ignore.  You have to understand the main point, first.
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AA4PB
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« Reply #9 on: January 07, 2013, 01:13:19 PM »

I'm thinking that the deviation of the FM carrier is influenced by both the amplitude and frequency of the baseband AC signal.   

No, the deviation of the FM signal is influenced only by the amplitude of the baseband signal. The frequency of the baseband signal determines how quickly the FM signal moves between minimum and maximum deviation. Deviation is defined only as the maximum amount that the FM signal moves off center. Increasing the amplitude of the baseband signal will increase the deviation but increasing the frequency of the baseband signal will not change the deviation.

That said, if you look at the spectrum on a typical FM transmitter you will see the deviation increase with an increase in frequency but that is because a pre-emphasis circuit in the transmitter automatically increases the amplitude of the baseband signal as its frequency rises. This is done to improve the signal to noise ratio at the receiver. That aspect is not a function of frequency modulation, but rather the pre-emphasis circuit that is placed ahead of the modulator.
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KF6GUB
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« Reply #10 on: January 07, 2013, 01:59:43 PM »

Thx, folks, I think that did it.  I think I get what you're saying. I shant repeat it, though.  Summarized:  FM deviation is not what I thought.  It's about amplitude. 
Just now I received in the mail the ARRL Guide to Antenna Tuners.  Just something to read: illumination.   Like I said, I've yet to get on the air.  Maybe UHF/VHF.  Appears to me to be a good way to start.  Thanks again, Jim
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N4CR
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« Reply #11 on: January 07, 2013, 02:36:59 PM »

Summarized:  FM deviation is not what I thought.  It's about amplitude. 

Not quite.

The audio that drives the FM modulator has two characteristics, amplitude and frequency.

Let's say a 1 volt signal changes the frequency by 1000 Hz.

If would change 1000 Hz if the 1 volt signal was 10 Hz or 100 Hz.

At 10 Hz, there would be 10 1000hz deviations per second.

At 100 Hz, there would be 100 1000hz deviations per second.

During modulation, input voltage becomes frequency shift and input frequency becomes rate of shift.

Question on the tech test:   What determines the amount of deviation of an FM signal?

The amount of deviation is determined by the amplitude of the signal driving the modulator. The questions says nothing about rate.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W6EM
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Posts: 787




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« Reply #12 on: January 07, 2013, 07:46:15 PM »

To be fair, there has been no explanation so far about bandwidth of an FM signal.  It is not just the deviation rate.  An FM signal has a bandwidth that is a function of deviation and modulation frequency.  The bandwidth is determined by the modulation index, which is the deviation divided by the audio modulation frequency.  The actual bandwidth and amplitude of the sidebands are then determined by application of a table of Bessel functions.

For example, a 5kHz deviation and a 5kHz audio modulation signal produces an index of 1, which has three equally spaced function values every 5 kHz from the center frequency, or /-15kHz.  Similarly, an audio signal of 1kHz produces an index of 5, which has 8 equally spaced 1kHz vectors, or a +/- 8kHz bandwidth.

AM, on the other hand, is simply the product of carrier and audio amplitudes times the cosine of carrier times the cosine of the audio.  Simple trig and you can observe the power and bandwidth.

Now, this hasn't changed in more than 50 years, so ARRL's logic in not allowing lifetime credit for passing theory elements is a tad skewed.......toward more VEC fee revenue.

Hope this helps,

Lee
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G3RZP
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« Reply #13 on: January 08, 2013, 02:28:50 AM »

Just to complicate matters, the carrier does vary in amplitude, and when the modulation index is 2.4, it disappears. That is an easy way to test deviation. To see this effect, you need a narrow bandwidth rx or a spectrum analyser.
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