Mechanical filter

Since its advent in 1947, mechanical filters have been widely used in electronic equipment due to their advantages such as high selectivity, high reliability, high stability, small size, light weight, low cost and suitability for automated production.

Mechanical filters are also called electromechanical filters or electromechanical filters. Mechanical filters can usually only be designed as bandpass filters . Only under certain conditions can they be designed as bandstop filters, so mechanical filters are also called bandpass mechanical filters.

The operating frequency range of mechanical filters is 100HZ~600HZ, of which 50KHZ and below are low-frequency mechanical filters, 50~200KHZ are medium-frequency mechanical filters, and 200~800KHZ are high-frequency mechanical filters. The relative bandwidth is 0.5%~15%, and some can reach 0.1%~30%.

1. Working principle of mechanical filter structure

(1) Basic structural forms At present, there are many types of mechanical filters in practical use. The basic structures are as follows, as shown in Figure 5.1-18.

1) The first structural form (see Figure 5.1-18A) consists of three parts: a mechanical resonator, a mechanical coupler, and an electromechanical transducer. The mechanical vibration unit includes a vibrator and a coupler, as shown in the dotted line part of the figure. Most mechanical filters, especially medium and high frequency mechanical filters, belong to this structure.

2) The second structural form (see Figure 5.1-18B) has no coupler, only the oscillator and transducer are combined into one, called a "combination". Generally, a combination constitutes a filter. Some low-pass filters use this structure.

3) The third structural form (see Figure 5.1-18C), the vibrator and the transducer are combined into one combination, and then two or more combinations are connected with wires to form a filter. Some medium and low frequency mechanical filters adopt this structure.

4) The fourth structural form (see Figure 5.1-19D) is different from the third one in that two or more combinations are connected by a coupler. Some low and medium frequency mechanical filters use this structure.

The above structure is the core of the mechanical filter. Most mechanical filters do not have a "matching network" composed of inductors and capacitors at the input and output ends, which are also the main components.

(2) Working principle of mechanical filter The working process of mechanical filter includes two parts: electromechanical transduction and mechanical vibration. The electrical signal is input through the matching network and added to the transducer, which converts the electrical signal into mechanical vibration of the same frequency and a certain vibration mode. This mechanical vibration has a selective characteristic for frequency, and transmits the selected mechanical vibration signal to the output transducer. Through energy conversion, the mechanical vibration is converted into an electrical signal of the same frequency, and then output through the matching network, thereby achieving the function of the filter transmitting the frequency band and suppressing the stop band.

The mechanical vibration system composed of the vibrator and the coupler is the main body of the mechanical filter. They are all made of constant elastic alloys - iron alloys containing nickel, chromium and titanium. The most commonly used is N142CRT1 alloy. The Young's modulus of constant elastic alloys changes very little within a certain temperature range. In mechanical filters, the temperature frequency coefficient B1 is also used to measure its temperature stability. The B1 of nickel, chromium and titanium in the range of -20 to +60 degrees is about ±5*10 to the negative sixth power. Therefore, the temperature stability of mechanical filters is better than that of ceramic filters.

(3) Classification of practical mechanical filters (see Table 5.1-14)

General characteristics of mechanical filters

In order to ensure that the transmission characteristics change minimally under the specified environment, the design and manufacturing must be as stable and reliable as possible, and parasitic responses and clear noise effects must be eliminated as much as possible.

The range of characteristics that can be achieved by mechanical filters (see Table 5.1-15)

2. Stability

As time goes by, changes in temperature and level will cause the transmission characteristics of the mechanical filter to deteriorate, so stability should include three aspects: temperature stability, time stability and level stability.

The stability of mechanical filters is far better than that of LC filters and ceramic filters, and is comparable to that of crystal filters. Moreover, when achieving the same order of magnitude of stability, the process of mechanical filters is much simpler than that of crystal filters.

1) Temperature stability Temperature stability refers to the degree to which the transmission characteristics of the filter change with temperature. Generally speaking, the center frequency offset is mainly caused by the oscillator frequency offset. Of course, changes in the capacitance of the static capacitor and changes in the component values ​​in the matching circuit will increase the passband fluctuation.

The transmission characteristics of mechanical filters remain almost unchanged in the temperature range of 0 to 40 degrees. In the temperature range of -45 to 85 degrees, mechanical filters can still work normally as long as special design and process treatment are carried out (no additional devices are required).

2) Time stability Time stability mainly refers to the degree to which the transmission characteristics of the filter change over time. It is mainly determined by the time-dependent rate of change of the constant elastic alloy of the special vibrator and transducer. Usually, this change is small and can reach 0.2~3.0*10-4/10A.

3) Level stability Level stability refers to the degree to which the transmission characteristics of the filter change with the input signal level. When the power of the filter input signal exceeds the energy it can store, the output will be nonlinearly distorted, causing the filter center frequency to drop, the bandwidth to increase, and the passband insertion loss to increase. The level characteristics of mechanical filters are far superior to LC filters.

(3) Parasitic responses and microphonic effects Any filter that uses a resonator will have other vibration modes in addition to the vibration mode they use, which will cause parasitic responses and microphonic effects that affect the transmission characteristics of the filter.

1) Parasitic response The parasitic response of mechanical filters is mainly caused by higher-order vibrations in other modes other than the main vibration mode and vibrations generated by other parts. These vibrations are collectively called stray vibrations. Parasitic response has a great influence on the transmission characteristics of the filter, so it is treated as an important issue in design, process processing and manufacturing.

2) Microphonic effect In low-frequency mechanical filters, the vibration frequency of the mechanical oscillator is low, and it is easily disturbed by external vibrations and produces noise output. This characteristic is called the "microphonic effect" of the mechanical filter. External vibrations come from many aspects, such as the engine in electrical equipment, the vibration of vehicles when driving, and various sounds. The size of the microphonic effect is related to factors such as the strength of the interference source, the frequency, and the anti-interference ability of the mechanical filter.

(4) Vibration and shock Mechanical filters have good vibration, collision, shock and centrifugal force resistance. When the vibration acceleration does not exceed 5G (G is the acceleration of gravity), the collision shock and centrifugal force acceleration are not greater than 50G, the mechanical filter does not need to take anti-vibration or vibration reduction measures. After special design, the vibration acceleration resistance can reach 15G, and the impact resistance, collision resistance and centrifugal force acceleration resistance can reach 1500G.