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Thermal analysis of RF Band-Stop Filter with an Open metal housing

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Introduction

Band-stop filters (BSFs) are one of the indispensable components for radio frequency and microwave circuits` applications. They are widely used in RF wireless communication systems for their effective suppression of spurious signals and unwanted wideband noises, to allow desired signals to pass through. Recently, BSFs have recently become an attractive research field for scientists since they can be used in both receivers and transmitters and has been integrated into nonlinear circuits such as mixer, oscillator, and amplifier to enhance their performances.

The RF device studied in this analysis is made of microstrip line (MSL) band-stop filter (BSF) by using a quarter-wavelength open circuited stub. Figure 1 shows the working prototype of the proposed implementation for the first order narrow-band BSF with an open metal housing.
In order to predict its maximum achieved temperature for a given input power under a well-defined environmental condition, an S-parameters study with thermal coupling of HFWorks tool, was used.

Figure 1 - Working prototype of the proposed BSF [1].

Problem description

The working prototype was realized by a standard printed board fabrication technique. The circuit is consisting of a uniform width strip-line of a 50 $begin mathsize 14px style capital omega end style$ impedance with a quarter-wave central open-circuit stub. It is implemented on a Megtron 6 substrate (from Panasonic) and an open metal housing made of Aluminum, like shown in the figure 2. The geometrical dimensions are detailed in table 1.

Figure 2 - a)-3D design and b)-Geometrical dimensions of the BSF

Table 1 - Geometrical properties

Geometrical parameter	Dimension (mm)
$W subscript b o x end subscript$	30
$L subscript b o x end subscript$	25
$w subscript s$	20
$h$	0.93
$w$	2
$l subscript s t u b end subscript$	4.3
$w subscript s t u b end subscript$	0.15
$T$	5.93

Simulation Setup

The S-parameters solver of HFWorks is used coupled to the thermal case for a working frequency range of [4GHz-16GHz]. The properties of the used materials are summarized in table 2.

Table 2 - Material properties

Material	Relative permittivity	Dielectric loss tangent	Electrical conductivity (S/m)	Thermal conductivity (W/m.K)
Megtron 6	3.6	0.006	0	0.4
Aluminum	1	0	3.5 E+7	237
Copper	1	0	5.96E+7	401

Electromagnetic boundary conditions

1- Wave port: The ports are applied to the lateral faces of the substrate and the corresponding faces of air box.
2- PEC: The bottom substrate face (Ground metal) is supposed to be a perfect electric conductor.
3- IEC: The microstrip lines (Copper conductor) is supposed to be an imperfect electric conductor.

Thermal boundary conditions

For an excitation power of

P subscript i n end subscript equals 2 W

applied to the input port, a thermal boundary convection is applied to the outer air box at an ambient temperature of 22°C and a convection coefficient set to 9

W divided by m squared. C

Mesh

To enhance the accuracy of the obtained results, a fine mesh control was applied to the both ports and open-circuit stub like shown in the next figure of the meshed model.

Figure 3 - the meshed model.

Results

A fast sweep S-parameters study for the frequency range of [4GHz-16GHz] revealed the next results for a resonant frequency of 10GHz:

Figure 4 - Electric field distribution at 10GHz

The next figures are showing the 2D plot of the S-parameters results: S11 and S21 (Return and insertion losses) for HFWorks tool and the experimental measurement, versus frequency. The deepest rejection is about 25dB at the mid-band frequency of 10GHz. The obtained 10dB Stopband bandwidth $B W$ =5.1%: [9.8 GHz-10.41GHz].

(a) Return loss results

(b) Insertion loss results

Figure 5 - 2D plot of Return and insertion losses versus frequency.

After solving the main electromagnetic study, HFWorks feeds the thermal loads (conductor and dielectric losses) to the thermal solver. Under a power excitation of 2W applied to the input port, the simulation revealed the next temperature distribution results across the BSF by taking into consideration the applied thermal boundary conditions. The temperature achieves a maximum value of 47°C which is in a good coincident with the experimental measurement according to the Reference [1].

Figure 6 - Temperature distribution for a frequency of 10 GHz

Conclusion

An electro-thermal analysis of an open stub BandStop Filter was investigated using HFWorks. The analysis allowed to predict how hot the RF device can get under a fixed low excitation power, accounting for external boundary conditions and with the use of an outer metallic housing. The maximum power handling capability of the Microstrip circuit was analyzed through its heating process (generated by its ohmic and dielectric losses).
The good agreement between the simulation and measurement results shows how much scientists could rely on the calculate heat estimation by HFWorks for RF applications.

References

[1]. Sánchez-Soriano, Miguel Á., et al. "Average power handling capability of microstrip passive circuits considering metal housing and environment conditions." IEEE Transactions on Components, Packaging and Manufacturing Technology 4.10 (2014): 1624-1633.