Frequency multiplier

Variable Crystal Filter Schematic Circuit Diagram

Adjustable Bandwidth in Crystal Filters for Enhanced Receiver Selectivity

Crystal filter serve as indispensable components in receivers, particularly in the role of intermediate frequency (IF) filters. In this context, the filter’s bandwidth plays a pivotal role in defining the selectivity of the receiver. The distinct characteristic of the filter under discussion lies in its ability to offer adjustability in bandwidth, a feature that significantly enhances the receiver’s selectivity.

Variable Crystal Filter Schematic Circuit Diagram

Optimizing Ladder Filter Configuration with Identical Crystals

The specified configuration adopts a ladder filter design, incorporating three crystals operating at the same frequency. To ensure optimal performance, it is imperative that these crystals are identical. We highly recommend purchasing all three crystals from a single production batch, a standard practice when placing an order, or acquiring them simultaneously. This ensures uniformity and coherence in the filter’s operation, enhancing its overall efficiency.

Adjustable Bandwidth Using Varicap Diodes

Varicap diodes typically come with a specified voltage rating of Ur= 0.5 V. Despite this specification, the measurement results for 0 V are still provided for reference. By employing a voltage range spanning Ur= 0 to 12 V, the bandwidth of the filter becomes adjustable, ranging from 2 to 6 kHz. This bandwidth range is particularly suited for various communication modes, ranging from Continuous Wave (CW) and Single Sideband (SSB) to standard Amplitude Modulation (AM). Such flexibility ensures adaptability to diverse communication needs and underscores the versatility of the filter design.

The ripple of the filter is determined by the input and output impedances Zin and Zout.

With smaller values of Zin and Zout, the ripple will increase, but the roll-off will be steeper. A compromise is Zin = Zout = 330 Ω resulting in a ripple of <3 dB. It is expected that the characteristics at other IFs such as 10.7 or 9 MHz will be much the same.

Understanding Crystal Filters: The Band Pass Filter

Crystal filters, commonly referred to as Band Pass filters, possess the unique ability to permit signals within a specific frequency range, known as the pass band frequency range, while simultaneously rejecting or attenuating signals falling within a designated frequency band called the stop band frequency range.

Front-End Filters in Two-Way Radio Communications

Crystal filters tailored for two-way radio communications often incorporate front-end filters. These filters are intricately designed using discrete crystal filters with multiple poles. Front-end filters find their niche in radio frequency settings characterized by robust signals, particularly in the presence of high-power transmitters.

Versatility of Monolithic Crystal Filters in Wireless Applications

Monolithic crystal filters also find utility as high-frequency filters in wireless applications. These filters, mounted directly onto integrated circuits, serve as vital components in wireless technology. Additionally, crystal ladder filters, comprising multiple crystal filters operating at the same frequency, are commonly employed in amateur radio applications to construct single sideband transmitters. Their adaptability underscores their significance in the realm of wireless communication.


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