Palestine Polytechnic University
College Of Engineering & Technology
Analog Communication Lab
Unit 3 .
Exp. Name:
Mixer,IF filter ,and Envelop Detector
Supervisor: Eng. Omar Abu-saif
Group Name:
Rawan Hassan Abu-Khreabeh
Yusra Na’el Arafeh
7/4/2014
Objectives:
Explain the operation of the mixer
To describe the function of the filter
To describe how the envelop detector converts a 455 kHz AM
signal to the message signal
Equipment required:
F.A.C.E.T base unit.
Analog Communications circuit bored .
Oscilloscope “dual trace “
Generator ,sine wave .
Theory:
Rf stage
Mixer
Block diagram of Exp .
If filter
Envelop
detector

Local oscillator(LO) : an oscillator in a piece of electronic equipment that is
usually used as a source of electromagnetic wave freq’s for mixing with
other freq’s that the equipment handles

Intermediate frequency (IF) stage : the section
The role of the mixer is to join the RF stage to the IF stage ,the mixer combines
the 1000 kHz AM signal from the RF stage with a 1455 kHz LO signal to
produce a 455 kHz difference signal for the If stage .besides the 455 KHz
difference freq,
The sum freq. 2455 kHz of tha AM signal and the local oscillator is also the
output of the mixer .
The IF filter in IF stage is a ceramic filter ,which has a higher attenuation outside
of the filter’s bandwidth than LC filters do .
IF filter designed to pass a 455kHz signal with a bandwidth that includes the AM
sidebands ,LSB 445 kHz and USB 465kHz with bandwidth = 20 kHz , it has
higher attenuated outside
For envelop detector , t=RC is time constant and it the most important parameter .
Mixer ,there are two inputs of mixer ,M(message) and C( carrier ),the mixer’s RF
signal input(M) is from RF amplifier output ,the mixer’s local oscillator input (C)
is 1455 KHz signal from the VCO-HI circuit block .
The mixer converts the AM signal freq. (1000KHz ) to the fixed IF signal freq.
(455KHz)
Bec. The RF input signal to the mixer contains three freq. components , the
balanced mixer output signal will contain the sum and difference freq . for
three RF components ,the RF input freq. don’t have significant amplitude in
the output .
Results:
Scope_00
 By F.G we generate Message signal with 100mVp-p ,2KHz
Scope_01
 By using VCO-LO we generate a carrier signal with 100mVp-p ,1MHz
Scope_02
 Modulated signal (output after modulation ) with MI = 1
Scope_03
 Carrier from VCO-HI with 100mVp-p and 1.455MHz (1455KHz) .
Scope_04
 Output of mixer .
Scope_05
 The output of filter .
Scope_06
 The signal that :ch1 on input of mixer with freq.= 2KHz
Ch2 on output of mixer with freq.= 76.92KHz
Scope_07
 The output of mixer ( complex signal) with 200mVp-p ,475.1KHz
F= 1/T1 = 1/(2.5*10^-6)
= 400KHz
Scope_08
 F2=1/T2= 1/.5
= 2000KHz
Scope_09
 The output of filter
Scope_10
 The output of envelop detector .
Scope_11
 The signal with CM .
D.E = Vin /Vout *100%
=
Conclusions:
The mixer combines the 1000KHz AM signal from the RF stage with a
1455KHz LO signal to produce a 455KHz difference signal and a
2455KHz sum frequency.
A diode detector is a nonlinear charging circuit formed by a diode in
series with a parallel RC network .
IF filter is designed to pass 455KHz signal with BW 20KHz that includes
AM sidebands :LSA at 445khZ and USB at 465KHz .
The diode conducts only during +ve portion of the AM signal .
For envelop detector :
1_ if T >>>1/fc ,then we have distortion by diagonal cutting .
2_ if T<<<1/fc ,then we have a ripple problem
On software we have a CM such that it can change in hardware and
varied the value of T.