Designing Matched Bandstop Filter for Very High Frequency (VHF) Band

A high-performance filter is always desired for wireless communication systems and applications. However, its performance is limited by the technology used to realize the filter due to the inevitable losses inherited from each technology. For instance, microstrip filters experience higher loss compared to coaxial or waveguide types which have higher inherent quality factor (Q-factor) up to tens of thousands whereas the microstrip is only in a few hundreds range.

With the use of the quadrature coupler, there exist a class of second order matched and highly selective bandstop filter. This filter has merits for high stopband attenuation and a perfect match at all frequencies even though high loss technology is used. As an example, the matched bandstop filter for very high frequency (VHF) band based on lossy network together with the coupler is worth to demonstrate its capability in achieving high stopband attenuation with lossy networks, filter compactness and a perfect matching at all frequencies.

Figure 1 shows the matched bandstop filter circuit implementation utilizing the 90 ohybrid branchline coupler together with the lossy resonant circuits comprising of inductance L, capacitance C, and conductance G (which is the inverse of resistance R).

Figure 1: Matched bandstop filter circuit

In this design, the Branchline coupler is a signal divider that can separate an incoming signal into two equal power splits with 90-degree phase shift. In order to achieve the broader bandwidth, the double branchline coupler must be used. For the higher frequencies, we must realize this coupler using transmission lines such as microstrip transmission line or coaxial line. On the other hand, at lower frequencies, lumped element is more suitable to realize this branch line coupler. Lumped elements can be used to approximate a quarter wave transmission lines in a branch line coupler realized with a pair of shunt capacitors of equal value, separated by a series inductor (a “pi” network). By optimizing the inductor and capacitor values, a lumped-element quadrature coupler can be realized with ideal lumped element values to operate at 90 MHz. Figure 2 below shows the ideal lumped element quadrature coupler.

Figure 2: Double section lumped branchline coupler circuit

Finally, the inverter K can be realized as a Pi network, where the negative shunt capacitors can be absorbed by the adjacent capacitors as shown in Figure 3.

Figure 3: Realization of Inverter

The final simulation result are shown in Figure 4.

Figure 4: Simulation result for lumped-matched bandstop filter

We can observe from Figure 4 for the response of the filter and we can conclude few things as follows.

1. The s21 value at the center frequency of 90 MHz has the value of — 40.97dB which equals to the suppression of incoming signal at that frequency at least 10,000 times. This conforms to the requirement for the filter to be bandstop at 90 MHz.
2. The s11 has return loss that is better than 10 dB for almost the entire range. This means, the reflection for this range frequency is at most 1%; hence the name matched bandstop filter.
3. It could be observed that the frequency beyond 110, the reflection goes higher than 1%, this could be lowered by optimizing the values of the circuit elements further but around the designed frequency at 90 MHz.

Check out this video on how to design a Matched Bandstop filter step by step using FILPAL EDS HF:

Originally published at http://filpal.wordpress.com on October 31, 2021.