# Studying for general, found a brain burner...

Discussion in 'Becoming an Amateur Radio Operator/Upgrading Privi' started by N0WUE, Apr 14, 2010.

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1. ### N0WUEHam MemberQRZ Page

I decided I should really actually study for this test, using the arrl guide.

I couldn't help but take a practice test and I encountered a question I didn't even have an educated guess for. I'll eventually get to this, but this will burn in my brain until I do. Here is the question...

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 guessed D, didn't expect to get it right.

anybody know the math, or have a link handy?

2. ### KL7AJHam MemberQRZ Page

You divide the final deviation by the carrier frequency multiplication factor, in this case x12. The deviation of an FM transmitter is increased in proportion to the frequency multiplication.

Eric

3. ### KB3TZKHam MemberQRZ Page

KL7AJ has it right. Now, some questions are written in such a way that you need to really pay attention to the language. I think if the question above were formulated as:

"What is the frequency deviation of a 12.21-MHz reactance-modulated oscillator in transmitter emitting a 146.52-MHz FM-phone signal showing a 5-kHz deviation?"

it would be clearer.

4. ### AI3VHam MemberQRZ Page

To expand on the above:

This is describing a "Old School" method of generating a 2M FM signal.

The general idea was that you use a quartz crystal oscillator for frequency stability.

But quartz crystals don't work at 146Mhz.

So we use a lower frequency crystal, And multiply the frequency several times in the transmitter to get to 146.

Since we need a total of 12 times, we can use any series of multipliers that add up to X12.

For example:

2 X 2 X 3

12.21mhz X 2 = 24.42 mhz, 24.42mhz X 2 =48.84mhz, 48.84 X 3 = 146.52mhz

Or;

2 X 3 X 2

12.21mhz, 24.42mhz, 73.26 mhz, 146.52

Or,

3 X 2 X 2

12.21mhz, 36.63mhz,73.26mhz,146.52mhz

Usually, the signal is only doubled or tripled at any one time, but this is not a hard rule, sometimes microwave radios use much larger multipliers.

Rege

EDIT:

The question is worded typical of government tests, It makes ASSUMPTIONS that you are using the multiplier method of generating a FM signal.

ASSUMING that instead of using a multiplier system, we instead use a HETERODYNE system, than the Q is bull.

We could just as easily generate a 146.52 mhz signal by mixing (heterodyning) a 12.21 mhz signal with a 134.31 mhz (or 158.73 mhz !), in this case, we would need a deviation of 5 khz.

Now, Nobody would pick 12.21 in the heterodyne system, because the multiplier system is much cheaper to build to meet harmonic/spurious regulations, but that is a COST issue.

Rege

Last edited: Apr 14, 2010
5. ### K7JEMHam MemberQRZ Page

Actually, Motorola did use either an 11.7 or 16.7 MHz "exciter" in their UHF mobile Micor. That was mixed with the RX injection frequency to provide either simplex or +5MHz offset for repeater access.

In this case, the 11.7 or 16.7 MHz signal actually had +-5KHz of deviation. The mixer was followed by a very sharp BP filter to allow only a small range of UHF freqs to pass. At 5MHz away, the filter was down 50 db or so. At 11.7 MHz or greater, it was down more than 80 dB.

You had to be careful to get the filter tuned properly, since it was possible to tune it up on the injection freq, where the radio would make plenty of power (unmodulated), but be 11.7 or 16.7 MHz lower than it should have been.

Joe

6. ### N0WUEHam MemberQRZ Page

I think I got it, after reading folks explanations I felt like I could only see half the picture. I understood the multiplier aspect, but it wasn't after seeking additional explanation did I actually make sense of the question.

Let me throw this out in crude terms, and see if I am putting it together right

What is the frequency deviation for a 12.21-MHz reactance-modulated oscillator in a 5-kHz deviation, 146.52-MHz FM-phone transmitter?

ok, so what it is really saying is...

How much deviation of the 12.21 MHz crystal is needed to achieve 5kHz after running through the multipliers needed to get to 145.52MHz?

I basically reversed the 5kHz down the same way I multiplied 12.21 to 145.52... I know this sounds freakishly simplistic now, but at the time I didn't realize the frequency was deviated before the multiplier.

The more I read that, the more stupid I feel for saying all that. When actually, I understand more than I did 12 hours ago. I feel my 4th grade math teacher kicking me in the shins right now.

7. ### AI3VHam MemberQRZ Page

Tnx for the info Joe.

The shop I worked at was a GE/Ericsson dealer, and old RCA equipment left over from the days RCA had a plant here at the Meadowlands. we only did a little of the Moto gear .

I gotta learn to never say never on this inteweb thingy

********************

You got it!

Don't be too hard on yourself, I was thinking this thread over, this question, IMHO, should be re-written.

How does one go about suggesting such a change?

Rege

Last edited: Apr 16, 2010
8. ### N0WUEHam MemberQRZ Page

sure enough, this question was on the test... funny enough, every question I had wrong from practice tests... was on the test

9. ### W0ISHam MemberQRZ Page

If that's the case, then can we safely assume that congratulations are in order?

If so, congratulations!

10. ### WA6DJMHam MemberQRZ Page

So to clarify the Math

You multiply the 12.21Mhz by the 5Khz and Divide by the frequency of 146.52MHz to get the value of 416Hz correct?

KJ6GJB
David also studying for my General