Name of the exp:
1) Study of super heterodyne receiver.
2) Study of voltages of terminal of transistor used in Am super heterodyne
receiver
A superheterodyne receiver (often shortened to superhet) is a type of radio receiver that uses
frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be
more conveniently processed than the original carrier frequency. It was invented by US engineer
Edwin Armstrong in 1918 during World War I. Virtually all modern radio receivers use the
superheterodyne principle.
Having looked at the concepts behind the superheterodyne receiver it is helpful to look at a
block diagram of a basic superhet. The superheterodyne block diagram is relatively
straightforward and builds on the basic functional block used to convert the incoming
frequency down to a fixed intermediate frequency stage.
While there may be some simplified versions for a superheterodyne block diagram, each
receiver will be different as a result of the differing requirements for each receiver. However
the basic principles are the same, and many superheterodyne block diagrams are very
similar.
Components:
1) Ac power supply
2) KL-93062 AM
3) Multimeter
Basic superheterodyne block diagram and functionality
The basic block diagram of a basic superhet receiver is shown below. This details the most
basic form of the receiver and serves to illustrate the basic blocks and their function.
Block diagram of a basic superheterodyne radio receiver
The way in which the receiver works can be seen by following the signal as is passes through
the receiver.
Front end amplifier and tuning block: Signals enter the front end circuitry from the
antenna. This circuit block performs two main functions:
o Tuning: Broadband tuning is applied to the RF stage. The purpose of this is
to reject the signals on the image frequency and accept those on the wanted
frequency. It must also be able to track the local oscillator so that as the
receiver is tuned, so the RF tuning remains on the required frequency.
Typically the selectivity provided at this stage is not high. Its main purpose is
to reject signals on the image frequency which is at a frequency equal to
twice that of the IF away from the wanted frequency. As the tuning within
this block provides all the rejection for the image response, it must be at a
sufficiently sharp to reduce the image to an acceptable level. However the RF
tuning may also help in preventing strong off-channel signals from entering
the receiver and overloading elements of the receiver, in particular the mixer
or possibly even the RF amplifier.
o Amplification: In terms of amplification, the level is carefully chosen so that
it does not overload the mixer when strong signals are present, but enables
the signals to be amplified sufficiently to ensure a good signal to noise ratio is
achieved. The amplifier must also be a low noise design. Any noise
introduced in this block will be amplified later in the receiver.
Mixer / frequency translator block: The tuned and amplified signal then enters one
port of the mixer. The local oscillator signal enters the other port. The performance
of the mixer is crucial to many elements of the overall receiver performance. It
should eb as linear as possible. If not, then spurious signals will be generated and
these may appear as 'phantom' received signals.
Local oscillator: The local oscillator may consist of a variable frequency oscillator
that can be tuned by altering the setting on a variable capacitor. Alternatively it may
be a frequency synthesizer that will enable greater levels of stability and setting
accuracy.
Intermediate frequency amplifier, IF block : Once the signals leave the mixer they
enter the IF stages. These stages contain most of the amplification in the receiver as
well as the filtering that enables signals on one frequency to be separated from
those on the next. Filters may consist simply of LC tuned transformers providing
inter-stage coupling, or they may be much higher performance ceramic or even
crystal filters, dependent upon what is required.
Detector / demodulator stage: Once the signals have passed through the IF stages
of the superheterodyne receiver, they need to be demodulated. Different
demodulators are required for different types of transmission, and as a result some
receivers may have a variety of demodulators that can be switched in to
accommodate the different types of transmission that are to be encountered.
Different demodulators used may include:
o AM diode detector: This is the most basic form of detector and this circuit
block would simple consist of a diode and possibly a small capacitor to
remove any remaining RF. The detector is cheap and its performance is
adequate, requiring a sufficient voltage to overcome the diode forward drop.
It is also not particularly linear, and finally it is subject to the effects of
selective fading that can be apparent, especially on the HF bands.
o Synchronous AM detector: This form of AM detector block is used in where
improved performance is needed. It mixes the incoming AM signal with
another on the same frequency as the carrier. This second signal can be
developed by passing the whole signal through a squaring amplifier. The
advantages of the synchronous AM detector are that it provides a far more
linear demodulation performance and it is far less subject to the problems of
selective fading.
o SSB product detector: The SSB product detector block consists of a mixer
and a local oscillator, often termed a beat frequency oscillator, BFO or carrier
insertion oscillator, CIO. This form of detector is used for Morse code
transmissions where the BFO is used to create an audible tone in line with
the on-off keying of the transmitted carrier. Without this the carrier without
modulation is difficult to detect. For SSB, the CIO re-inserts the carrier to
make the modulation comprehensible.
o Basic FM detector: As an FM signal carries no amplitude variations a
demodulator block that senses frequency variations is required. It should also
be insensitive to amplitude variations as these could add extra noise. Simple
FM detectors such as the Foster Seeley or ratio detectors can be made from
discrete components although they do require the use of transformers.
o PLL FM detector: A phase locked loop can be used to make a very good FM
demodulator. The incoming FM signal can be fed into the reference input,
and the VCO drive voltage used to provide the detected audio output.
o Quadrature FM detector: This form of FM detector block is widely used
within ICs. IT is simple to implement and provides a good linear output.
Audio amplifier: The output from the demodulator is the recovered audio. This is
passed into the audio stages where they are amplified and presented to the
headphones or loudspeaker
Terminal voltages of the transistors,
DC value Ac value
TRANSISTOR
NUMBER B E C B E C
Q1 2.083 1.56 12 0.008 0.006 0.001
Q2 0.623 0.98 12.10 0.012 0.029 Not fixed
Q3 2.320 1.617 12.18 0.008 0.005 Not fixed
Q4 1.926 1.38 11.54 0.045 0.018 0.012
Q5 1.67 12.21 11.55 0.032 0.062 0.649
Q6 1.66 1.343 12.21 0.58 0.301 0.015
Q7 0.912 1.353 1.9 0.305 0.025 0.003
Discussion:
From this experiment it is clearly observed the operation of AM super heterodyne receiver as well as
its different parts.
