Receiver Types:

The component technology, and in particular semiconductor technology has surged forwards enabling much higher levels of performance to be achieved in a much smaller space.Many of the different radio receiver types have been around for many years. Some of them are

1. Tuned radio frequency, TRF:
This type of radio receiver was one of the first that was used. The very first radio
receivers of this type simply consisted of a tuned circuit and a detector. Crystal sets were early forms of TRF radios. Later amplifiers were added to boost the signal level, both at the radio frequencies and audio frequencies. There were several problems with this form of receiver. The main one was the lack of selectivity. Gain / sensitivity were also a use. 

2.Regenerative receiver:
The regenerative radio receiver significantly improved the levels of gain and selectivity
obtainable. It used positive feedback and ran at the point just before oscillation occurred. In this way a significant multiplication in the level of "Q" of the tuned circuit was gained. Also major improvements in gain were obtained this way. 

3. Super regenerative receiver:
The super regenerative radio receiver takes the concept of regeneration a stage further. Using a second lower frequency oscillation within the same stage, this second oscillation quenches or interrupts the oscillation of the main regeneration – typically at frequencies of around 25 kHz or so above the audio range. In this way the main regeneration can be run so that the stage is effectively in oscillation where it provides very much higher levels of gain. Using the second quench oscillation, the effects of running the stage in oscillation are not apparent to the listener, although it does emit spurious signals which can cause interference locally.

4. Super heterodyne receiver:
The super heterodyne form of radio receiver was developed to provide additional levels of selectivity. It uses the heterodyne or mixing process to convert signals done to a fixed intermediate frequency. Changing the frequency of the local oscillator effectively tunes the radio. 

5. Direct conversion receiver:
This type of radio format converts the signal directly down to the baseband frequency.
Initially it was used for AM, Morse (CW) and SSB transmissions. Now it is widely used for digital communications where IQ demodulators are used to take advantage of the variety of phase shift keying, PSK, and quadrature amplitude modulation, QAM signals. 

Tuned Radio Frequency Receiver(TRF)

The tuned radio frequency receiver is one in which the tuning or selectivity is provided at the radio frequency stages.Tuning is provided by a tuned coil / capacitor combination, and then the signal is presented to a simple crystal or diode detector where the amplitude modulated signal is recovered. This is then passed straight to the headphones.
The tuned radio frequency receiver was used in the early days of wireless technology but it is rarely used today as other techniques offering much better performance are available. Many of these different types of radio receiver are in widespread use today. Each type of radio has its own characteristics that lend its use to particular applications.

Block diagram of Tuned radio frequency receiver

Operation:

TRF receiver consists of two or three stages of RF amplifiers, detector, audio amplifier and power amplifier. The RF amplifier stages placed between the antenna and detector are used to increase the strength of the received signal before it is applied to the detector. These RF amplifiers are tuned to fix frequency, amplify the desired band of frequencies. Therefore they provide amplification for selected band of frequencies and rejection for all others. As selection and amplification process is carried out in two or three stages and each stage must amplify the same band of frequencies, the ganged tuning is provided.
The amplified signal is then demodulated using detector to recover the modulating signal. The recovered signal is amplified further by the audio amplifier followed by power amplifier which provides sufficient gain to operate a loud speaker. The TRF receivers suffered from number of annoying problems.
The tuned radio frequency receiver was popular in the 1920s as it provided sufficient gain and selectivity for the receiving the broadcast stations of the day. However tuning is difficult in which as each stage in the early radios needed to be adjusted separately. The TRF receiver has largely been disregarded in recent years. Other receiver topologies offer far better levels of performance, and with integrated circuit technology, the additional circuitry of other types of receiver is not an issue.Later ganged tuning capacitors were introduced, but by this time the superheterodyne receiver was becoming more widespread.

Disadvantages of TRF receiver

• Poor selectivity and low sensitivity in proportion to the number of tuned amplifiers used.
• Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections.
• An additional problem for the TRF receiver is tuning different frequencies. All the tuned
circuits need to tune together to the same frequency or track very closely. Another problem is to
keep the narrow bandwidth tuning. Keeping several tuned circuits aligned is difficult.
• The bandwidth of a tuned circuit doesn’t remain constant and increases with the frequency increase.
• The need to have all RF stages track one another
• Instability due to large number of RF stages.

Superheterodyne Receiver:

To solve basic problem of TRF receivers, first all the incoming RF frequencies are
converted to fix lower frequency called Intermediate Frequency (IF).Then this fix intermediate frequency is amplified and detected to reproduce the original information. Since the characteristics of the IF amplifier are independent of the frequency to which the receiver is tuned, the selectivity and sensitivity of superheterodyne receivers are fairly uniform throughout its tuning range. The basic concept and theory behind the superheterodyne radio involves the process of mixing. This enables signals to be translated from one frequency to another. The input frequency is often referred to as the RF input, whilst the locally generated oscillator signal is referred to as the local oscillator, and the output frequency is called the intermediate frequency as it is between the RF and audio frequencies.

Block diagram of a Superheterodyne receiver

Operation:

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.
Another important circuit in the superheterodyne receiver is AGC and AFC circuit. AGC is used to maintain a constant output voltage level over a wide range of RF input signal levels.
It derives the dc bias voltage from the output of detector which is proportional to the
amplitude of the received signal.This dc bias voltage is feedback to the IF amplifiers to control the gain of the receiver. As a result, it provides a constant output voltage level over a wide range of RF input signal levels. AFC circuit generated AFC signal which is used to adjust and stabilize the frequency of the local oscillator.

Advantages of the superheterodyne receiver

• IF stage permits use at very high frequencies.
• Because many components operate at the fixed IF, they can be optimized.
• Less expensive.
• Better selectivity
• Improved circuit stability.
• Uniform gain over a wide range of frequencies

Receiver Types
There are many different types of receivers, each designed to receive and process specific types of signals. Here are some common types of receivers:

1. Radio Receiver: This type of receiver is designed to receive and process radio signals. Radio receivers are used in a variety of applications, including in radios, televisions, and cell phones.

2. Television Receiver: A television receiver is designed to receive and process television signals. This type of receiver is used in televisions and set-top boxes.

3. GPS Receiver: A GPS receiver is used to receive signals from GPS satellites and determine the user's location.

4. Satellite Receiver: A satellite receiver is designed to receive signals from a satellite, which may include television signals, weather data, or other types of information.

5. Bluetooth Receiver: A Bluetooth receiver is used to receive and process Bluetooth signals, which are commonly used to connect wireless devices such as headphones, speakers, and keyboards.

6. Infrared Receiver: An infrared receiver is used to receive signals from an infrared transmitter, which is commonly used in remote controls for televisions and other electronic devices. 

7. Radar Receiver: A radar receiver is used to receive and process radar signals, which are commonly used in aviation, military, and weather applications.

8. Optical Receiver: An optical receiver is used to receive and process optical signals, which can include light waves used in fiber optic communication or signals from optical sensors.

9. Wi-Fi Receiver: A Wi-Fi receiver is used to receive and process Wi-Fi signals, which are commonly used to provide wireless internet access.

Overall, the specific type of receiver used will depend on the application and the type of signals that need to be received and processed.