Superhet Receivers

Introduction:

The superhet radio or to give it its full name the superheterodyne receiver is one of the most popular forms of receiver in use today in a variety of applications from broadcast receivers to two way radio communications links as well as many mobile radio communications systems.

Although other forms of radio receiver are used, the superheterodyne receiver is one of the most widely used forms. Although initially developed in the early days of radio, or wireless technology, the superhet or superheterodyne receiver offers significant advantages in many applications. Naturally the basic concept has been developed since its early days, and more complicated and sophisticated versions are used, but the basic concept still remains the same.

Working Principle:

In the superhet radio, the received signal enters one inputs of the mixer. A locally generated signal (local oscillator signal) is fed into the other. The result is that new signals are generated. These are applied to a fixed frequency intermediate frequency (IF) amplifier and filter. Any signals that are converted down and then fall within the pass-band of the IF amplifier will be amplified and passed on to the next stages. Those that fall outside the pass-band of the IF are rejected. Tuning is accomplished very simply by varying the frequency of the local oscillator. The advantage of this process is that very selective fixed frequency filters can be used and these far out perform any variable frequency ones. They are also normally at a lower frequency than the incoming signal and again this enables their performance to be better and less costly.

To see how this operates in reality take the example of two signals, one at 6 MHz and another at 6.1 MHz. Also take the example of an IF situated at 1 MHz. If the local oscillator is set to 5 MHz, then the two signals generated by the mixer as a result of the 6 MHz signal fall at 1 MHz and 11 MHz. Naturally the 11 MHz signal is rejected, but the one at 1 MHz passes through the IF stages. The signal at 6.1 MHz produces a signal at 1.1 MHz (and 11.1 MHz) and this falls outside bandwidth of the IF so the only signal to pass through the IF is that from the signal on 6 MHz.

If the local oscillator frequency is moved up by 0.1 MHz to 5.1 MHz then the signal at 6.1 MHz will give rise to a signal at 1 MHz and this will pass through the IF. The signal at 6 MHz will give rise to a signal of 0.9 MHz at the IF and will be rejected. In this way the receiver acts as a variable frequency filter, and tuning is accomplished.

Basic Block Diagram

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.

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:
  • 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.
  • 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:
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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

Double superheterodyne Receiver

When choosing the intermediate frequency for a superheterodyne radio receiver there is a trade-off to be made between the advantages of using a low frequency IF or a high frequency one:

• High frequency IF:   The use of a high frequency IF means that the difference between the wanted frequency and the unwanted image is much greater and it is easier to achieve high levels of performance because the front end filtering is able to provide high levels of rejection.
• Low frequency IF:   The advantage of choosing a lower frequency IF is that the filters that provide the adjacent channel rejection are lower in frequency. The use of a low frequency IF enables the performance to be high, while keeping the cost low.

Accordingly there are two conflicting requirements which cannot be easily satisfied using a single intermediate frequency. The solution is to use a double conversion superheterodyne topology to provide a means of satisfying both requirements

Working Principle:

The basic concept behind the double superheterodyne radio receiver is the use of a high intermediate frequency to achieve the high levels of image rejection that are required, and a further low intermediate frequency to provide the levels of performance required for the adjacent channel selectivity.

Typically the receiver will convert the incoming signal down to a relatively high first intermediate frequency (IF) stage. This enables the high levels of image rejection to be achieved. As the image frequency lies at a frequency twice that of the IF away from the main or wanted signal, the higher the IF, the further away the image is and the easier it is to reject at the front end.

Once the signal has passed through the first IF at the higher frequency, it is then passed through a second mixer to convert it down to a lower intermediate frequency where the narrow band filtering is accomplished so that the adjacent channel signals can be removed. As the lower frequency, filters are cheaper and the performance is often higher. 

How Superhet works as Radar Receiver:

The input at RF is down converted to an intermediate frequency (IF). 

•Advantages: Excellent sensitivity, much lower conversion loss in detection. 

•IF amplifier is more effective and stable than RF amplifier 

•IF signal simplified filtering (narrow filter) improve selectivity 

•LO OSC can be changed to track the TX frequency IF and filtering 

•Duplexer: switches the common antenna between TX and RX (TR switch). 

•Input of RX to output of processor can vary from 100 to 200dB 

•STC (sensitivity time control): gain as a function of time (range)

 •AGC (automatic gain control)

Noise Considerations:

• Noise Characteristics 
• Noise Figure 
• Radar Receiver Noise Figure 
• Dynamic Range - Bandwidth
•  IF selection and Filtering                                                                                                                     
  

Considerations on Noise:

Usually the first characteristics specified for a radar receiver 

• The understanding of the receiver noise as the ultimate limitation on radar range performance is important. 
• The ability to detect received radar echoes is ultimately limited by thermal noise, even if receiver adds no additional noise 
• The lowest-noise receiver may need great a sacrifice in system performance and cost 
• It is seldom a dominant factor because the noise contribution has been reduced sufficiently. 

Advantages of this Receiver:

• 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

Reference:

http://www.microwaves101.com/encyclopedias/superheterodyne-receivers