Mixer parameters (frequency converter)

As we already considered earlier, in order to transfer the input radio frequency to the intermediate frequency, it is necessary to multiply the input signal by the sinusoidal voltage of the local oscillator (local oscillator). Devices that multiply the input signal by a signal from a local oscillator (local oscillator) are called mixers. Real mixers are difficult to analyze, and therefore their performance characteristics are determined by many parameters.

1. Working frequency range

Mixers can be built in a wide range of frequencies, from very low frequencies (almost from zero frequency) to tens of GigaHerts. Commercially available mixers have a maximum operating frequency of 100 MHz to 2.5 GHz. The operating frequency range is the main parameter that determines the choice of the type of mixer.

2. Dynamic range

Dynamic range is one of the most important technical characteristics of the mixer. A significant increase in the number of working transmitters and the presence of other sources of interference means that modern radios, as a rule, work in a harsh interference environment. Even in the case when the useful signal has a very low level, the receiver is required to maintain its characteristics in the presence of strong interfering signals.

The lower limit of the dynamic range of the mixer is determined by its noise figure, while the upper limit is determined by the levels of compression of the transfer coefficient, intermodulation components and thermal destruction.

To estimate the intermodulation components at the mixer output, the parameter IP3 and the associated value of the local oscillator signal are usually used. To estimate this parameter, the following classification is usually used:

                         IP3 Pget
 Very low level + 7dBm 0dBm
 Low level + 13dBm + 7dBm
 Average level + 20dBm + 13dBm
 High level + 25dBm + 17dBm
 Very high level + 30dBm + 20dBm

3. Noise ratio

As a rule, mixers have a noise figure ranging from 6 to 20 dB. The noise figure of passive mixers is numerically equal to the conversion loss. The noise figure of the active mixers depends on the configuration of the circuit and the types of elements used in it.

The intrinsic noise of a mixer implemented on Schottky diodes usually does not exceed 0.5 dB, therefore they are usually not taken into account. The remaining noise value is provided by the noise value of the local oscillator at the received frequency and the suppression of the image channel (3 dB).

4. Transfer ratio

The availability of ready-made amplifiers, which overlap different parts of the frequency range, removes the requirement for the mixer to have any gain. In most cases, the presence of large insertion loss conversion of the mixer is also undesirable, especially when using passive mixers. Active mixers provide a transmission coefficient in the range from 1 to 4 ... 17 dB, while passive mixers have a typical conversion loss of 5.5 to 8.5 dB.

5. The signal level of the local oscillator

An ideal mixer should not be sensitive to the level of the local oscillator; in a real mixer, the parameters of the local oscillator should correspond to its parameters. Passive double balanced diode mixers require a local oscillator level of +7 to +23 dBm. Active mixers require a local oscillator level between -20 and +30 dBm. It follows that the development of the local oscillator is closely connected with the selected type of mixer.

6. Decoupling

Decoupling is a parameter characterizing the degree of suppression of parasitic signal propagation applied to one of the inputs to the remaining outputs of the mixer. The only signal that must be present at the output of the mixer is the intermediate frequency signal. The magnitude of the junction depends on whether the mixer is unbalanced, simple balanced, or double balanced (ring). Unbalanced mixers do not have any interchange between the conclusions. In this case, to ensure the required signal isolation, special buffer amplifiers must be used. Double balanced mixers provide the best possible isolation between all three terminals. However, to provide modern parameters of the frequency converter, additional buffer amplifiers are often necessary.

7. Matching Impedances

All inputs and output of the mixer must be carefully coordinated with signal sources and load. In active mixers, the gain is usually reduced as a result of misalignment. Passive mixers are particularly sensitive to the error at the output of the intermediate frequency, resulting in large conversion losses and a greater level of parasitic conversion products. Regardless of which mixer is used in the system, in order to realize its optimal parameters, careful matching of its inputs and output with the corresponding receiver nodes must be performed.

When coordinating the receiver (or transmitter) nodes, attention should be paid to the possibility of connecting measuring equipment to the control points of the radio path. Therefore, it is desirable to bring the input and output impedances to a standard value of 50 ohms. The same value of the input and output impedances is required for the ability to transmit a signal over standard 50-ohm strip or coaxial cables (transmission lines).

8. Simplicity

Quite complex systems are difficult to develop and produce. Using fewer parts reduces cost, increases reliability, eases maintenance and requires fewer spare parts. An overly complex scheme leads to a significant increase in the cost of equipment, so developers should strive to obtain maximum performance with a minimum of parts used.