# What is conversion factor?

Discussion in 'Electronic Basics' started by MRW, Dec 13, 2005.

1. ### MRWGuest

Merry Christmas and a Happy New Year!

I was reading a datasheet from Philips and I encountered this:

"The demodulator is also fully integrated. This has the advantage that
no alignment is necessary. The demodulator has a conversion factor of
75mV at 22.5kHz deviation."

What does the last sentence mean? I can't seem to comprehend it.

Thanks!

2. ### Dan HollandsGuest

Without knowing any background, it sounds to me like an FM demodulator the
puts out 75mV when the input freq is 22.5kHz off the center freq

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606

www.QuickScoreRace.com

3. ### PeteSGuest

That's exactly what it is. Perhaps the OP doesn't realise that FM
converts (at the modulation stage) an amplitude to a frequency
deviation around the carrier. This leads to the inescapable conclusion
that the demodulation process must convert a frequency difference [from
a carrier, which at this is probably not the same one] to an amplitude

Dear OP: Some basics of FM might help (google is your friend).

Cheers

PeteS

4. ### MRWGuest

Thanks all! Happy Holidays! I am familiarizing myself with FM.

If I gathered it right, the demodulated signal (in amplitude) would
"sketch" out the audio waveform, no? Does this indicate that at the
zero deviation the modulator would put at 0 V and the value would go up
as high as 75 mV at the higher limit of the audio range?

Thanks again!

5. ### Rich Grise, but drunkGuest

No. Not with frequency modulation.
No, the "audio range" is an entirely different animal. If I read Dan
Hollands' interpretation correctly, it means that when the carrier is
just at the carrier frequency, the output of the discriminator is at
0 volts. But when the carrier frequency deviates from center, the
discriminator puts out a voltage that is proportinal to the
instantaneous difference between the FM modulated carrier _right_
_now_ and the center freq.

This can happen at such a fast rate that the resulting instantaneous
changes can be processed as audio, recovering the information from
the station. Another side benefit is "automatic frequency control,"
which uses this 75 mv/22.5khz or whatever it was, after filtering,
to feed back to the local oscillator and kind of "lock in" to the
signal's frequency.

Hope This Helps!
Rich

6. ### PeteSGuest

As Rich said, it depends on your point of view. Instaneously, you will
have a DC voltage proportional to the difference between the carrier
and the *current* frequency* (this is somewhat simplified, as there
will usually be a low pass filter involved, so what you will have is
the integral of past history DC levels)

If the original carrier was modulated with audio, then the output of
the demodulator should be that audio, although not necessarily at the
same amplitude.

In pure infornation terms, the 'information' in the signal is S(fc -
fi) where fc is the carrier and fi is the instaneous frequency and S is
the integral over some specified time. Depending on the point of view,
one may take the instaneous difference [not taking the integral], or
the integral (as in the case of filtered audio).

At zero deviation, there is no 'information' signal, so the output
should be 0 offset from some arbitrary level, but not necessarily 0V.

The 75mV is simply a spec showing the transfer curve of the
demodulator. The actual output may be higher or lower depending on the
actual deviation of the carrier (there are a number of different specs,
each with their own deviation limits).

Cheers

PeteS