Analysis Page-Surface/Multicoil Mode

Biasing -

Surface & Multi coil Mode:

In this mode we need to transmit on body and receive on surface or a particular multi-coil configuration. Because we need the body coil to transmit an RF pulse, the Body coil must "dynamically" changes from an "On" (Transmit) to an "Off" state.

In Surface coil mode the head receive circuitry is used to receive the signal.

In Multi-coil mode the circuitry becomes more complex due to the different configurations and multiple receivers necessary to acquire high quality signal data. (See Multi-coil Troubleshooting Flowcharts)

Dynamic Disable diode switches are biased "On" to enable body coil to Transmit RF, and "OFF", allowing the Receive signal to be handled by Surface or Multi-coil circuitry.
T/R diode switches are forward biased "On" with +4vdc. Reverse biased "Off" with -12.25vdc
Direct Drive diode switches are biased "On" to enable body coil to Transmit RF, and "OFF", allowing the Receive signal to be handled by Surface or Multi-coil circuitry.

Surface Mode	Body coil is Enabled	During Transmit	During Receive
Dynamic Disable	DD Switch action: On & Off	+1000vdc	-15vdc
T/R Bias to Hybrid Splitter	T/R Switch action: On & Off	+4vdc	-12.25vdc
Direct Drive	T/R Switch action: On & Off	+1000vdc	-15vdc

Rear of SSM [Photo]

Theory:

T/R switches are made up of Pin diodes. This type of special diode is extremely fast at switching states. They have an extremely stable junctions. They have a very small tolerance and are not replaceable with standard diodes.

Why 3 different biasing circuits?

Dynamic Disable- 3 purposes:

To allow the body coil to transmit in body mode.
To allow the body coil to be used to transmit RF power during Surface & Multi-coil mode.
To disable the body coil completely, in Head mode.

Direct Drive- 2 purposes:

To allow the body coil to transmit in body mode.
To allow the body coil to be used to transmit RF power during Surface & Multi-coil mode.

T/R Bias-1 purpose:

To allow the body coil to quickly disconnect ( Interrupt the transmission) from body coil during the receive time period in body mode and allow some of the circuitry to be used to combine the Receive RF signal into one high quality signal that will be passed on to the preamp.