Superheterodyne Detector

What Heterodyning is

To heterodyne means to mix to frequencies together so as to produce a beat frequency, namely

the difference between the two. Amplitude modulation is a heterodyne process: the
information signal is mixed with the carrier to produce the side-bands. The side-bands occur at
precisely the sum and difference frequencies of the carrier and information. These are beat
frequencies (normally the beat frequency is associated with the lower side-band, the difference
between the two).

What Superheterodyning is

When you use the lower side-band (the difference between the two frequencies), you are
superheterodyning. Strictly speaking, the term superheterodyne refers to creating a beat
frequency that is lower than the original signal. Although we have used the example of
amplitude modulation side-bands as an example, we are not talking about encoding
information for transmission. What superheterodyning does is to purposely mix in another
frequency in the receiver, so as to reduce the signal frequency prior to processing. Why and
how this is done will be discussed below.

Advantages of Using Superheterodyning

Now, we easily see that this type of receiver can be constructed, but for what purpose? All we
have accomplished is to reduce the frequency to the IF value. We still must process the signal
as before. So why are so many receivers using the superheterodyne method? There are three
main advantages, depending on the application used for:

 It reduces the signal from very high frequency sources where ordinary components
wouldn't work (like in a radar receiver).
 It allows many components to operate at a fixed frequency (IF section) and therefore
they can be optimized or made more inexpensively.
 It can be used to improve signal isolation by arithmetic selectivity

Summary

 Superheterodyne receivers reduce the signal frequency be mixing in a signal from a local
oscillator to produce the intermediate frequency (IF).
 Superheterodyne receivers have better performance because the components can be
optimized to work a single intermediate frequency, and can take advantage of
arithmetic selectivity.
Superheterodyne receiver circuit blocks
There are some key circuit blocks that form the basic superheterodyne receiver. Although more
complicated receivers can be made, the basic circuit is widely used – further blocks can add
improved performance or additional functionality and their operation within the whole receiver
is normally easy to determine once the basic block diagram is understood.

 RF tuning & amplification:

This RF stage within the overall block diagram for the receiver provides initial tuning to
remove the image signal. It also provides some amplification. If noise performance for
the receiver is important, then this stage will be designed for optimum noise
performance. This RF amplifier circuit block will also increase the signal level so that the
noise introduced by later stages is at a lower level in comparison to the wanted signal.
 Local oscillator:
The local oscillator circuit block can take a variety of forms. Early receivers used free
running local oscillators. Today most receivers use frequency synthesizers, normally
based around phase locked loops. These provide much greater levels of stability and
enable frequencies to be programmed in a variety of ways.
 Mixer:
Both the local oscillator and incoming signal enter this block within the superheterodyne
receiver. The wanted signal is converted to the intermediate frequency.
 IF amplifier & filter:
This superheterodyne receiver block provides the majority of gain and selectivity. High
performance filters like crystal filters may be used, although LC or ceramic filters may be
used within domestic radios.
 Demodulator:
The superheterodyne receiver block diagram only shows one demodulator, but in reality
radios may have one or more demodulators dependent upon the type of signals being
receiver.
 Automatic Gain Control, AGC:
An automatic gain control is incorporated into most superhet radios. Its function is to
reduce the gain for strong signals so that the audio level is maintained for amplitude
sensitive forms of modulation, and also to prevent overloading.
 Audio amplifier:
Once demodulated, the recovered audio is applied to an audio amplifier block to be
amplified to the required level for loudspeakers or headphones. Alternatively the
recovered modulation may be used for other applications whereupon it is processed in
the required way by a specific circuit block.
Superheterodyne receiver block diagram explanation
Signals enter the receiver from the antenna and are applied to the RF amplifier where they are
tuned to remove the image signal and also reduce the general level of unwanted signals on
other frequencies that are not required.

The signals are then applied to the mixer along with the local oscillator where the wanted signal
is converted down to the intermediate frequency. Here significant levels of amplification are
applied and the signals are filtered. This filtering selects signals on one channel against those on
the next. It is much larger than that employed in the front end. The advantage of the IF filter as
opposed to RF filtering is that the filter can be designed for a fixed frequency. This allows for
much better tuning. Variable filters are never able to provide the same level of selectivity that
can be provided by fixed frequency ones.

Once filtered the next block in the superheterodyne receiver is the demodulator. This could be
for amplitude modulation, single sideband, frequency modulation, or indeed any form of
modulation. It is also possible to switch different demodulators in according to the mode being
received.

The final element in the superheterodyne receiver block diagram is shown as an audio
amplifier, although this could be any form of circuit block that is used to process or amplified
the demodulated signal.

Block diagram summary

The diagram above shows a very basic version of the superhet or superheterodyne receiver.
Many sets these days are far more complicated. Some superhet radios have more than one
frequency conversion, and other areas of additional circuitry to provide the required levels of
performance.
However the basic superheterodyne concept remains the same, using the idea of mixing the
incoming signal with a locally generated oscillation to convert the signals to a new frequency.

Automatic Volume Control

What AVC Does
An automatic volume control (AVC) automatically adjusts the volume, or loudness, of an audio
signal, usually to compensate for ambient noise in an effort to make the audio signal better
heard and understood above the noise. An AVC is primarily used to enhance intelligibility of
speech or appreciation of music heard by the user in noisy environments. Most conventional
AVCs attempt to keep the signal-to-noise ratio (S/N) constant for the user.

Where AVC is Needed

AVC is needed wherever a changing noise background can make it difficult to hear or
understand an audio signal:

 In cars, to compensate for road noise, wind, traffic, open windows, fans, radios, and
occupants talking
 In public places, such as airports, malls, stadiums, meeting halls, stores, markets,
lobbies, to compensate for what is mostly human-generated noise
 Outdoors, to compensate for wind, traffic noise, and passers-by
Potential AVC Applications
AVC has potential applications in any audio devices or platforms that are sometimes used in
noisy environments:

 Mobile phones, including smartphones

 All other handheld audio devices
 Headsets, such as Bluetooth-enabled earpieces
 Personal media players, such as MP3 players
 Tablets
 Car radios and sound systems
Conventional AVC
Many users of phones with conventional AVC believe that sound clarity is diminished when the
AVC is turned on. In fact, in many phones the default setting for the AVC feature is ‘Off.’
Conventional AVC has not progressed beyond the first generation that focuses on loudness of
the noise, rather than intelligibility of the signal.

AMD (Advanced Micro Devices) described the first modern digital AVC in U.S. Patent No.
5,666,426 in 1996. The patent proposed a supplementary microphone on a car radio to
measure total ambient noise.
Nokia modified this invention in 1999 with U.S. Patent No. 5,907,823 by measuring not total
noise, but rather the A-weighted noise level, which is a measure of how loud noise sounds to
the human ear. The invention keeps constant the ratio of signal to A-weighted noise (S/NA).

Conventional AVC can be found today, with default setting ‘Off,’ on many mobile phone models
from Nokia, LG, Motorola, Sony, RIM, and others and on Bluetooth headsets from Jabra,
Samsung, and others.

AVC Benefits
AVC tackles a universal problem with mobile phones and other portable communications
devices – the user can’t consistently hear and understand clearly. A survey reported at an AARP
conference that 57% of baby boomers with mobile phones had trouble using them because of
their hearing loss, and 30% of those with hearing loss said the problem is mostly their hearing.
Significantly, 40% said they would use their mobile phones more if they could hear better. The
benefits of AVC, all of which could lead to an expanded customer base, are:

 Increased customer satisfaction

 Easier hearing
 Improved intelligibility
 Enhanced user experience

Reference
https://fas.org/man/dod-101/navy/docs/es310/superhet.htm

https://www.electronics-notes.com/articles/radio/superheterodyne-receiver/block-
diagram.php

http://www.starmarktechnologies.com/Automatic-Volume-Control/index.html