Simulation Report based on filter

he filter in the schematic is as follows:

According to ADS, the key parameters are offered:

In my opinion, this circuit could be divided into two blocks and their function and role could be explained separately.

Part A: Main part of the filter

                           (2) the main part of the filter

This key block is composed of the main part of this filter and it offers two reject points with parallel resonant and series resonant cavities, described below the schematics (2).

1  parallel resonant circuit

As you know, we can calculate the resonant frequency of this circuit is 81.11Mhz but mismatching effect would occur in other frequency (e.g 64MHz ) and the forward transmission also be aggravated 3.294dB obviously .

2  series resonant circuit

Similarly, we can calculate the resonant frequency is 51.18Mhz but mismatching would occur in other frequency (e. g 64 MHz ) and the forward transmission also be aggravated 2.855dB ..

The importance rejection points produced by the resonant circuits connected can pass the signal with valuable information between the frequency band 51.19MHz and 81.11MHz and reject the other frequency signal.

But we can find that the forward transmission in the band between 51.19MHz and 81.11MHz is aggravated seriously because of the mismatch in input or output. So the capacitance C1 and C3 could be used to regulate the input reflection coefficient and to improve the forward transmission (3).

                    (3) the main part of the filter with matching capacitance

The simulation results with using ADS about this block above are demonstrated:

The designer uses two capacitances to optimize the performance easily and improve the forward transmission and input reflection coefficient perfectly. I think the key point of all process of design is tuning the two capacitances and reducing the parasitic effect.

Part B  Improving the suppression out of the band

We notice that the suppression in the frequency band above 81.11MHz and below 51.19MHz is not very strong and the tow resonant circuits with wide band(ease to degeneration of the forward transmission if added to the circuit) and central frequency 64MHz approximately can be adapt to strengthen the ability of rejection.

In short, we should balance the suppression of adjacent band and the forward transmission in band using the resonant circuit in the front stage and back. So we can develop the filter finally and observe the strong rejection out of the band marked 7 and 8.