UMX9989AP™

The UMX9989AP is the first MRI switching diode module, designed to optimize performance and reduce assembly labor, cost, and polarity errors.

There are two principle applications for which the UMX9989AP modules are intended: 1) MRI receiver protection from high RF energy fields, including long RF pulses and RF spike pulses present in most MRI machines. The UMX9989AP acts as a passive protector (limiter) for the MRI receiver's LNA. The diode assembly exhibits extremely low insertion loss, both in the "on" state (high power present) and the "off" state (receiver power present) so the Receiver's Noise Figure is not increased by the protector circuit. 2) Passive switching of surface coil detuning and blocking circuits. In this case, the flow of loop current during transmitter pulse turns on the diodes, without a switch driver.

If the UMX9989AP is combined with a PIN diode (UM7201SM) the combination can be used to implement a semi-active detune or block circuit design. The UMX9989AP's turn on the PIN diode (used for higher power switching) during the sinc(x) sidelobes, before the main pulse of the transmitter waveform, sinc(x) = [sin (x)]/x, occurs. The mechanical drawing shows the structure of the diode pair. Manufacture of dual anti-parallel pairs of UMX9989's ensures that the matched pair of diodes can be inserted in a coil with the correct diode polarities and with the minimum parasitic inductance and capacitance, thermal impedance and labor for the coil manufacturer.