The Research And Key Circuit Design Of 1.2GHz CATV Cable Trunk Amplifier

This paper introduces a new 1.2GHz CATV trunk amplifier which is used to compensate the signal loss caused by cable.The trunk amplifier owns wider frequency band,higher gain and higher output power capacity.With these characteristic,the trunk amplifier can upgrade the existing cable network and expand the available spectrum.The cable amplifier used in Europe and America CATV network only can support 1GHz bandwidth.The MSO try many tools to increase the bandwidth of the network.In order to feed wider bandwidth need of users,the MSO need to expand the unused spectrum of CATV network.On the other hand,the cable amplifier used in our country's CATV network,work with 870 MHz bandwidth.Under the background of tri-networks integration,many EOC techniques are introduced to transform the cable network to support two way communications.The common point of those EOC techniques is working above 870 MHz.So we need to design a new cable amplifier with wider bandwidth to applying the EOC technique.The bandwidth expanding of cable amplifier causes some problems needed to be solved.The cable loss is proportional to the square root of frequency.The 1.2GHz signal has more cable loss and so the trunk amplifier should provide higher gain and high output level.We design the 1.2GHz trunk cable amplifier with the cascade of MMIC power amplifier to solve this problem.The tilt of coaxial cable also increases at 1.2GHz.We expand the EQ working frequency and use input and output EQ module to offset the increased cable tilt.The worse gain flatness is caused by passive devices.We design a trim module to adjust flatness dynamically.The trunk amplifier works in the field and the output level vary with the temperature.The variation of 1.2GHz is larger.We expand compensation range of the gain adjustment circuit and controlled by AGC to compensate the output level variation caused by temperature.Finally,we debug the amplifier frequency response and port return loos based on the qualified module and test the S-parameter,distortion and noise figure over room temperature.The result shows that the gain is 48 dB,frequency response flatness is less than 0.8dB,return loss is less than-16 dBc,the channel output power 58 dBmV with CSO>62dB and CTB>60dB and the worst noise figure is 7.7dB.Over high and low temperature test,the channel output level variation is limited in 1dB and the distortion meets specification too.