Superhet Radio Circuit Blocks:
Block diagram / overall receiver RF amplifier & tuning RF mixer IF amplifier & filter Automatic gain control, AGC
Superhet Radio Includes:
Superhet radio Superhet theory Image response Block diagram / overall receiver Design evolution Double & multi-conversion superhet Specifications
The performance of the 1st mixer within any superhet receiver can govern many of the characteristics of the overall receiver and therefore its performance can be of great importance to the overall receiver.
The ideal mixer should accept the incoming signal and the local oscillator and produce an output of the sum and difference frequencies and transfer the modulation precisely to the IF output.
Real-life mixers produce a variety of other undesired outputs, including noise and they may also suffer overload when very strong signals are present.
Although very basic non-linear devices can actually perform a basic RF mixing or multiplication process, the performance will be far from the ideal, and where good receiver performance is required, the specification of the RF mixer must be match this expectation.
Basic RF mixers
The basic process of RF mixing or multiplication where the incoming RF signal and a local oscillator are mixed or multiplied together to produce signals at the sum and difference frequencies is key to the whole operation of the superhet radio.
Note on RF Mixing / Multiplication:
RF mixing or multiplication is a key RF technique. Using a local oscillator, it enables signals to be translated in frequency, thereby enabling signals to be converted up and down in frequency.
Read more about RF mixing / multiplication
There are many different forms of mixer that can be used, and the choice of the type depends very much upon the receiver and the anticipated performance.
For some very basic superhet receivers, the mixer may be incorporated into the same circuit as the oscillator where cost is a major issue and low performance can be accepted, whereas others are very high performance for a high performance receiver.
Superhet mixer considerations
There are a number of considerations when looking at the receiver design and topology with respect to the RF mixer.
On frequencies below about 30MHz or so, the noise picked up by the antenna - both atmospheric and man made noise - has a higher level than a modern high performance low noise figure receiver.
By keeping the pre-mixer gain low, the mixer will be able to tolerate higher level input signals without entering a non-linear region and specifications like the intercept point and other overload specifications will be improved.
For HF receivers, a noise figure of 10dB is quite adequate and many modern designs have a noise figure of between 10 and 14 dB.
Above about 30 MHz the receiver noise performance becomes more of an issue and noise figure levels of between 4 and 8 dB are common. For preamplifiers operating over a single band it is possible to achieve noise figure values of less than 2 dB.
To achieve these levels of noise figure it is generally necessary to use amplifiers before the mixer, but in these cases care must be taken to ensure that the mixer can accommodate the strong amplified signals that it will encounter without running into compression and overload..
Combined mixer / local oscillator circuits
Where cost and component count is of prime importance, a single transistor can be made to act as RF stage, oscillator and mixer. These very low component count circuits, offered low performance, but were nevertheless very inventive in combining several functions within one transistor stage.
The circuit shown above has a ferrite rod antenna as the input, and the signal from this is fed into the transistor which acts as an oscillator and amplifying mixer. The inductor between the collector and emitter circuits provides the feedback and this is tuned by the variable capacitor which is ‘ganged’ with the one on the ferrite rod antenna so that the two circuits are tuned together. The output is taken from the IF transformer in the collector of the circuit.
In view of the fact that the circuit is undertaking multiple functions, it cannot be optimised for any particular aspect. Accordingly the performance is poor, but adequate for many cheap broadcast transistor portable radios.
Valve / vacuum tube oscillator mixer
In the days of thermionic valves or vacuum tubes, there were very many different designs of oscillator mixer circuit. Some used a single valve section - a heptode tube or valve could act as a single oscillator frequency changer circuit. However some valves / vacuum tube types had two separate sections in them like the 6U8 / ECF82 type and others. This triode pentode could act as an oscillator mixer using a single valve, but preserving the separate identify of the oscillator and mixer.
The circuit below shows a typical oscillator / RF mixer arrangement using thermionic valve / vacuum tune technology. It would typically have been used on frequencies below 30 MHz.
Even though this was far more satisfactory than a single section of a valve being used for the combined oscillator / mixer function, often a separate oscillator was needed to ensure high stability and overall optimum performance. The oscillator could be run under its own conditions with a buffer to provide optimum stability, and this would require a separate valve for the mixer.
Many performance superhet radio receivers opt for the use of a passive mixer. These passive mixers often use diodes as the main element and can take a variety of forms.
Although a single diode can be used as an RF mixer, it is not very satisfactory. This is because the wanted signals of the sum and difference frequencies are supplemented by the presence of the RF and LO signals as well as harmonics and other odd and even mixing products. In addition to this the circuit does not provide any isolation between the local oscillator and the input. This means that the LO signal can appear at the input along with its harmonics. Accordingly this circuit would need to rely on the isolation provided by any RF amplifier to isolate the LO from the antenna.
To obtain much better performance double balanced mixers would be used for this application as they provide good isolation.
In addition to the improved isolation provided, a double balanced mixer also produces very low levels of spurious signal, and therefore the overall receiver will have low levels of spurious responses. This is a key element of any design, ensuring that spurious signals and strong signal performance are optimised
When considering the design of any superhet radio, the mixer conversion loss must be accommodated. Figures for passive diode double balanced mixers are around 8 dB and this needs to be accommodated within the design. In view of this, often receivers have some pre-mixer gain.
There are many different forms of active mixers ranging from simple single deice mixers like dual gate MOSFET mixers through to a variety of forms of balanced active mixers using either bipolar or FET devices.
When well designed FET mixers can provide an excellent level of performance whilst also providing some gain as well.
The choice of RF mixer for any receiver is a key decision in the design process. It needs to be able to have low noise, especially at VHF and above, but also be able to provide a wide dynamic range and have good strong signal handling.
More Essential Radio Topics:
Radio Signals Modulation types & techniques Amplitude modulation Frequency modulation RF mixing Phase locked loops Frequency synthesizers Passive intermodulation RF attenuators RF filters Radio receiver types Superhet radio Radio receiver selectivity Radio receiver sensitivity Receiver strong signal handling
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