Development of a low cost 94GHz imaging receiver using multi-layer liquid crystal polymer technology

The 94GHz imaging band is the most commercially focused of the mm-wave imaging "windows". However, the commercial uptake of imaging systems has been limited due to the production costs involved, of which a significant proportion is due to the front-end receivers. Conventionally, the receiver is machined from metal and made-up of either several modules or a single more integrated module containing the RF and DC circuitry. Even with the more integrated approach the cost is prohibitive, due to the cost of the MMICs, the machining of the metal and integration of different materials during assembly. The front end receiver cost is a potential limiting factor in the deployment of imaging systems. LCP multi-layer substrates remove the requirement for expensive metal machining and because the RF and DC circuitry is integrated in the same substrate the assembly cost of the module is also reduced. Cost is not the only consideration, LCP has excellent properties which are especially attractive for high-performance microwave applications. These properties include low permittivity, low loss tangent, low water-absorption coefficient and most importantly low cost. By means of heat treatments, their coefficients of thermal expansion can be tailored to make them more amenable to integration into packages that include other materials. The LCP is manufactured in large sheet/panel form allowing batch manufacture of circuits which ensures circuit to circuit repeatability and a high yield. LCP has a dielectric constant of 3.16±0.05 and a dielectric loss tangent of 0.0049 to 100GHz. These properties have resulted in measured line loss of 0.2dB/mm at 110GHz. This level of loss makes this material system a viable approach for low noise integrated imaging receivers and will allow sensitivities of <0.8°K NETD to be achieved. This paper describes the design, measurement and characterisation of the first 94GHz receiver manufactured using LCP reported in the literature.