QUESTION:

What is the maximum frequency at which I can use the various Rogers materials?

ANSWER:
The question of maximum frequency of operation is quite complicated because of the many variables encountered. Issues pertaining to circuit performance tend to dictate which materials can or cannot be used. If the circuit only needs to act as a transmission line then the maximum frequency will be much higher for a given material than if the circuit needs to perform complex signal functions (filters, VCO’s and low noise amplifiers for example). Also, the type of construction used, microstrip or stripline, will have an effect on the maximum frequency. Material properties like loss tangent, dielectric constant, thickness and metal cladding play a very important part in determining the maximum frequency of operation. This article will state the typical frequency range of operation for Rogers materials, while keeping in mind that the maximum value is circuit specific.

Two important factors for determining the use of a material for a given frequency are dielectric constant,(K') , and loss tangent, (tand). These are needed to calculate the insertion loss of a transmission line as well as the frequency for transverse modes. Another factor that is also critical in selecting a material is line width size. At high frequencies, lines can become too small, particularly for higher K' materials, to use standard PCB fabrication processes and the design cannot be produced. This can be resolved by using thicker substrate, which will give wider lines and lower loss but at the expense of encountering transverse modes at lower frequencies.

Insertion loss is made up of conductor loss and dielectric loss, and in the case of non-homogeneous media radiation loss. To better understand the effect, we will analyze a 50W microstrip line on 0.025" TMM® material with 1/2 oz electrodeposited copper (1.9 micro-inch copper surface roughness, treated side). A low and a high dielectric constant material were selected, as well as a low and high frequency to point out the effect of K' and tand on material frequency use. Table I displays the calculated losses for both dielectric and conductor.

At lower frequencies (for low loss tangent materials), conductor loss plays a major role by making up to 80% of the total circuit loss. The overall numbers are low so selection of a material will not be dominated by the loss tangent alone. For higher frequencies, dielectric loss becomes an important contributor to the total loss, increasing by a factor of 10x. Conductor losses at this point are also quite high and need to be addressed as well when selecting a material. From Table I, we observe that low dielectric constant/loss tangent materials tend to be better suited for higher frequency use because of the lower insertion loss.

Now that we understand why lower dielectric constant materials work better at higher frequencies, it will be clearer why Figure I has been laid out in the format that it has. Rather than presenting which material can be used, based on tand, it makes more sense to do so with the K'. Along with the typical maximum frequency use, the figure also provides frequencies at which various existing applications operate or will operate. As the frequency increases, thickness of the material decreases in order to avoid generating transverse modes on the transmission lines. For example, if using RT/duroid® 5880 0.031”, transverse modes for a 50W microstrip line are generated at 32 GHz, but if the thickness selected becomes 0.005”, for the same impedance, then the modes are pushed out to 235 GHz because of the narrower line width. Of course, insertion loss more than triples for the lower thickness.

FIGURE I. Typical Maximum Frequency of Operation for Rogers Microwave Materials

Selection of a material depends on many factors which make each application unique. What is not suitable for one application might be quite acceptable for another. For low loss materials, dielectric constant plays and important role in determining what is the maximum frequency of operation for a given material. A recommended maximum frequency chart has been presented, but should be used to provide the designer assistance when selecting Rogers microwave materials rather than using it to make final decisions.

The information and guidelines contained in this document are intended to assist you in designing with RO4000® materials. They are not intended to and do not create any warranties express or implied including any warranty of merchantability or fitness for a particular application. Results may vary as conditions and equipment may vary. The user should determine the suitability of Rogers materials for each application.

TMM, RT/duroid, ULTRALAM, RO4000 and RO3000 are licensed trademarks of Rogers Corporation.