Amplifier Tutorials

7 RFPA SYSTEM INTEGRATION

With the performance of the RFPA verified, its time to integrate the RFPA into the system. A few topics are worth mentioning when installing an RFPA:

  • Rack-mounting: Most RFPAs come in a standard 19 inch rack mountable package. A rack without front/rear doors is preferable (but not mandatory) from an air flow perspective. Mount the RFPA on the lowest spot available for a low center of gravity and a stable rack. If you are to make one and only connection properly, be certain to adequately ground the RFPA chassis to the rack ground (using 6 gauge green/yellow band wire) and the ground the rack to the system ground.
  • Location: The amplifier should be as close to the coil as possible to minimize cable losses (especially at high fields such as 7T). Most RFPAs can work in fringe fields of 50 gauss, so a balance would be as close to the coil without materially exceeding this level of fringe field.
  • RF Connectors:A few quick words on connectors. First, there is no magic number for maximum peak/average power handling capability; several factors weigh in; operating frequency range, load mismatch, ambient temperature, altitude and manufacturer. Whats listed here is fairly conservative. You only need to be familiar with the following connectors:BNC: good for RF input (0 dbm power levels) cables and gating signals, thats it.The following connector power ratings apply up to 2GHz:SMA: Can be used up to 1kW peak, 100W average power, Type N: 10kW peak, 500 W average power. Type SC: 50kW peak, 2kW average power. Type 7/16th: 13 kW peak, 3kW average.
  • RF Coaxial Cables: There are literally dozens of RF coaxial cable types. The primary parameters to bear in mind when selecting an RF Cable are: Insertion Loss (in db per foot), Peak/Average power handling capability, flexibility (if you intend to move it frequently) and cost. And speaking of cost, especially at high fields (4, 7, 9.4T etc), the RF cable is not an item to cut corners on. An RFPA for 9.4T may cost $50-$100,000.00. A coax cable run with poor loss characteristics of -3db can cost you $25-$50,000.00 in RF power! A few recommendations are Times Microwave LMR-400, Andrew heliax and HF Electronics Ecoflex.
  • Protection: Coil research can be a trial and error process in which the RFPA can be subjected to extreme load VSWRs that can be detrimental. Poor load VSWRs can send large amounts of reflected power back at the output of the RFPA and may damage its transistors. Electronic protection (i.e. VSWR fault circuitry) can shut the amplifier down in the event excessive reflected power is detected, however VSWR fault circuits engage on the order of microseconds and some RFPA damage can occur in this time duration. The most effective RFPA protection involves the installation of circulators in line with the output. Any reflected RF power will be instantaneously diverted to a high power RF isolation load. The main drawbacks with circulators are they are narrowband, lossy and only available at frequencies down to about 120MHz.