Design007 Magazine


Issue link:

Contents of this Issue


Page 75 of 107

76 DESIGN007 MAGAZINE I FEBRUARY 2020 insertion loss. Insertion loss is made up of four other losses and is a summation of dielectric loss, conductor loss, radiation loss, and leak- age loss. A circuit using a very low loss material with a Df of 0.002 and very smooth copper will have relatively low insertion loss. The same circuit using the same low-loss material, but using a high-profile electrodeposited (ED) copper in- stead of smooth copper will lead to an increase in insertion loss. Copper surface roughness will have an im- pact on the conductor loss of a circuit. To be clear, the surface roughness that is a concern for losses is the copper surface roughness at the copper-substrate interface as the laminate is made. Additionally, if the circuit substrate is thin, the copper planes will be closer together, and the copper surface roughness will have a larger impact on insertion loss compared to a circuit using a thicker laminate. For high-power RF applications where ther- mal management is typically an issue, choos- ing a laminate with low Df and smooth copper can be advantageous. Additionally, choosing a laminate with high thermal conductivity is generally a smart thing to do as well. The high thermal conductivity will assist in moving the heat more effectively out of the circuit and into the heat sink. The frequency-heat relationship basically causes more heat to be generated when there is an increase in frequency, with the assump- tion of the same RF power being applied at both frequencies. As an example taken from some thermal management experiments done at Rogers, we found that a microstrip transmis- sion line with an applied RF power of 80 watts at 3.6 GHz had a heat rise of ≈50°C. When that same circuit was tested with 80 watts applied at 6.1 GHz, the heat rise was ≈80°C. There are several reasons for having an in- crease in temperature with an increase in fre- quency. One reason is the Df of a material will certainly increase with an increased frequen- cy, which will cause more dielectric losses and will ultimately cause an increase in insertion loss and heat. Another issue is the fact that conductor losses naturally increase with an increase in frequency. Some of the conductor loss increase is due to a thinning of skin depth as frequency increases. Additionally, with in- creased frequency, the fields will condense, and there will be more power density in a giv- en area of the circuit, which will also increase the heating effects. Lastly, TCDk, which has been mentioned several times in this column previously be- cause it is a material property that is often overlooked, is basically how much the Dk will change with a change in temperature. In the case of power amplifier circuits, they of- ten have ¼ wavelength matching networks, and these networks are sensitive to Dk fluc- tuations. When the Dk changes greatly, the ¼ wavelength matching will shift, and the pow- er amplifier can vary in efficiency, which is very undesirable. In summary, when selecting high-frequency materials for high-power RF applications, the material should have low Df, relatively smooth copper, high thermal conductivity, and a low TCDk. There are many tradeoffs when consid- ering these material properties, along with the final end-use application requirements. There- fore, it is always wise for the designer to con- tact their material supplier when choosing ma- terials for high-power RF applications. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coon- rod, click here. Insertion loss is made up of four other losses and is a summation of dielectric loss, conductor loss, radiation loss, and leakage loss.

Articles in this issue

Links on this page

Archives of this issue

view archives of Design007 Magazine - Design007-Feb2020