Cascaded Triplet
Understanding Cascaded Triplet
Asymmetric Rejection in Filter Design
In many wireless communication scenarios, the interference environment is asymmetric. For example, a receiver may have a strong, co-located transmitter operating just below its passband, while the spectrum above the passband is relatively clear. Designing a symmetric filter to reject this single interferer requires a high order, which increases cost and insertion loss. A cascaded triplet (CT) solves this problem by using cross-coupling to place a single transmission zero close to the band edge where the rejection is needed.
A cascaded triplet consists of three resonators arranged in a closed loop. The signal propagates sequentially from resonator 1 to 2, and 2 to 3. Additionally, a direct cross-coupling is introduced between resonator 1 and resonator 3. The interference of these two paths generates a single transmission zero in the stopband.
Tuning and Coupling Matrix Synthesis
The position of the transmission zero in a cascaded triplet is governed by the sign of the cross-coupling coefficient. If the cross-coupling has the same sign as the sequential couplings (positive coupling), the transmission zero falls on the low side of the passband. If the sign is opposite (negative coupling), the transmission zero falls on the high side of the passband. This versatility allows engineers to design customized co-existence filters. The tuning of triplets is highly interactive, requiring precise adjustment of coupling screws or aperture sizes to align the zero location.
Key Mathematical Relations
Technical Specifications Comparison
| Filter Parameter | Standard Triplet | Super-Triplet | Quadruplet Comparison |
|---|---|---|---|
| Resonator Count | 3 | 3 (with source-load coupling) | 4 |
| Transmission Zeros | 1 (Asymmetric) | 2 (Asymmetric) | 2 (Symmetric or Asymmetric) |
| Passband VSWR | Good | Excellent | Very Good |
| Insertion Loss | Low (fewer resonators) | Low | Higher (more stages) |
Frequently Asked Questions
What is the benefit of a cascaded triplet filter configuration?
The main benefit is the ability to place a single transmission zero on one side of the filter passband. This provides very high selectivity and steep roll-off close to the band edge to reject a specific, nearby interfering signal, without the size and loss penalty of a larger filter.
How do you control which side of the passband the transmission zero is placed on?
The location of the zero is determined by the mathematical sign of the cross-coupling between the first and third resonators. A positive cross-coupling places the transmission zero below the passband, while a negative cross-coupling places it above the passband.
What physical technologies are used to build cascaded triplets?
They are commonly built using metallic coaxial cavities, ceramic monoblocks, dielectric pucks, microstrip hairpins, or high-temperature superconducting (HTS) planar microstrip circuits where low loss and high selectivity are required.