Palestine Polytechnic University
College Of Engineering
Analog Communication Lab
Exp. Name:
Frequency demodulation
Supervisor: Eng. Omar Abu-saif
Group Name:
Rawan Hassan Abu-Khreabeh
Yusra Na’el Arafeh
Israa alamlah
27/4/2014
Objectives:
Examine the operation of the foster –Seely FM detector .
Examine the operation of the ratio FM detector .
Measure the characteristic curve of the detector .
Equipment required:
Basis unit ( power supply mod.PSU/EV)
EXP. Module mod. MCM21/EV.
Dual trace oscilloscope .
Function generator .
Freq. meter .
Multimeter .
Theory:
FM demodulator :
 Travis discriminator .
 Foster –Seely discriminator .
 Ratio discriminator .
 Quadurat detector .
 PLC detector .
To demodulate a Freq. modulated signal a circuit is necessary which
supplies the output with a proportional voltage to the freq. deviation
of the input modulated signal .
To know ideal char. Of demodulator freq. we need to plot char.
Curve (the relationship between input and output) ,the input is freq.
and output is voltage .
The sensitivity is the char. Parameter of the freq. demodulator :
S= ∆𝑣 /∆𝑓
Results:
We changing in freq. to measure voltage output to plot char. Curve .
For Foster –Seely:
Scope_00
 Carrir signal from FG with 2vp-p and 450 kHz .
Scope_01
 Carrier signal from FG with 2vp-p and 500 kHz .
Scope_02
 Carrier signal from FG with 2vp-p and 400 kHz .
For Ratio discriminator :
Scope_03
 Carrier signal from FG with 2vp-p and 450 kHz .
Scope_04
 Carrier signal from FG with 2vp-p and 435 kHz .
Scope_05
 Carrier signal from FG with 2vp-p and 440 kHz .
Scope_06
 Message signal from FG with 200mVp-p and 500 Hz.
Scope_07
 Carrier signal from vco1 with 2Vp-p and 459 KHz.
Print -00
 The output of Freq. mod.
Scope_08
 The output of demodulated signal with foster –Seely discriminator
With 840 mVp-p and 498.8 Hz .
Scope_09
 FM mod. Signal .
Scope_10
 Message signal from FM with 600mVp-p and 450 Hz .
Scope_11
 The output of demod. When we change message signal .
Scope_12
 The output of demod. With ratio discriminator .
Scope_13
 The output of demod. When we change amp. Of freq. modulation
with disconnect capacitor .
Conclusions:
The only difference between foster- Seely and ratio discriminator is the
discharging time .
To plot char curve we take carrier signal from FG just for simplifying the
work .
When we change from FS to R we need to disconnect all the source and
circuit .
We use CV1 to calibrate demodulator center freq. at 450 KHz .
The ratio discriminator the output depends on the ratio of the tow voltages
detected by the diodes .
The foster –Seely discriminator is sensible to amp. variations of
the mod. signal