#ACTIVE RF ISOLATOR INSTALL#

During signal detection time T/R switch directs signals from resonator to LNA. During the high RF power application, a T/R switch directs RF power to the resonator and isolates the LNA from the excitation power. They also suffer from a lower filling factor leading to lower B 1/√W RF power-field conversion efficiency than that of a unimodal resonator ( 6, 7).Īnother approach has been the use of a single mode reflection resonator which employs a T/R switch to isolate the LNA from the excitation power ( 2). However, bimodal resonators typically suffer from geometric complexity that limits sample accessibility.

This isolates the RF excitation power from appearing at the detector LNA by as much as 60 dB, obviating the need for a dedicated transmit/receive (T/R) switch. Most versions use resonators whose excitation RF field B 1 is perpendicular to the readout RF field B 1 ( 5). One approach to isolation of the power from the LNA has been the use of bimodal resonators ( 1, 3, 4). Several strategies have been adopted to protect that LNA from the high input power. Under these circumstances, high spectral bandwidth requires shorter pulses and, consequently, higher power. This is exacerbated in the context of EPR imaging of large objects with size on the order of several cm that exhibit important spectral features separated by tens of Gauss. The output of the LNA is saturated and otherwise distorted by high power pulses and recovers slowly from these pulses contributing to the dead time of an imager. The amplifier can be destroyed by RF pulses of sufficient power.

The first stage in the detection of the response of the magnetization to the high power RF pulses is a sensitive, low noise amplifier (LNA). This requires the application of tens of nanosecond long RF pulses that have broad bandwidth and kilowatts of incident power. Time domain EPR imaging techniques utilize simultaneous excitation of the whole EPR line broadened by the applied magnetic field gradient ( 1, 2). In time domain EPR, a short duration radio frequency (RF) high power (hard) pulses are used to rapidly change the macroscopic spin system magnetization. In conventional continuous wave (CW) Electron Paramagnetic Resonance (EPR) the use of a balanced bridge and narrow band low instantaneous power prevents saturation of the first stage of amplification in the detection chain. This level of isolation is sufficient to fully protect the LNA of pulse EPR bridge. For high input power levels the isolation may exceed 60 dB.

We find that the present design passively limits the output power to 14 dBm independent of the input power.

This brings the device into conduction mode. Only when the power diminishes below the diode conduction threshold will the resistance of the PIN diode rise to a value much higher than the system impedance. The mismatch reflects the high power to the remaining port of the second circulator, dumping it into a system impedance matched load. The low resistance of the forward biased PIN diode passively generates an impedance mismatch at the second circulator output port during the high power excitation pulse and resonator ring down. We discuss a combination of techniques to accomplish this involving the power-routing circulator in the bridge, a second circulator acting as an isolator with passive shunt PIN diodes immediately following the second circulator.

#ACTIVE RF ISOLATOR INSTALL#

In order to protect the low noise amplifier (LNA) in the receive arm of a pulsed 250 MHz EPR bridge, it is necessary to install as much isolation as possible between the power exciting the spin system and the LNA when high power is present in the receive arm of the bridge, while allowing the voltage induced by the magnetization in the spin sample to be passed undistorted and undiminished to the LNA once power is reduced below the level that can cause a LNA damage.

Active rf isolator