Figure 13-107.  The stator of an antiskid wheel sensor is mounted in the axle, and the rotor is coupled to the wheel hub spider that rotates  with the wheel.  The Boeing anti-skid control valve block diagram in  Figure 13-109  gives further detail on the functions of an  anti-skid control unit. Other aircraft may have different logic  to achieve similar end results. DC systems do not require an  input converter since DC is received from the wheel sensors,  and the control unit circuitry operates primarily with DC.  Only the functions on one circuit card for one-wheel brake  assembly are shown in  Figure 13-109 . Each wheel has its own  identical circuitry card to facilitate simultaneous operation.  All cards are housed in a single control unit that Boeing calls  a control shield. The converter shown changes the AC frequency received  from the wheel sensor into DC voltage that is proportional to  wheel speed. The output is used in a velocity reference loop  that contains deceleration and velocity reference circuits.  The converter also supplies input for the spoiler system  and the locked wheel system, which is discussed at the end  of this section. A velocity reference loop output voltage is  produced, which represents the instantaneous velocity of the  aircraft. This is compared to converter output in the velocity  comparator. This comparison of voltages is essentially the  comparison of the aircraft speed to wheel speed. The output  from the velocity comparator is a positive or negative error  voltage corresponding to whether the wheel speed is too  fast or too slow for optimum braking efficiency for a given  aircraft speed. The error output voltage from the comparator feeds the  pressure bias modulator circuit. This is a memory circuit  that establishes a threshold where the pressure to the brakes  provides optimum braking. The error voltage causes the  modulator to either increase or decrease the pressure to the  brakes in attempt to hold the modulator threshold. It produces  13-63 Figure 13-108.  A rack mounted antiskid control unit from an airliner. $^{RESE}$T VERIFY $^{PRES}$S/ TE ST BIT $^{NOR}$M BREAK TEST INBD A/B 1 2 3 4 OUTBD OFF$^{ENABLE/}$ VERIFY 1 L R 2 3 4 Hubcap fairing Spider Hubcap assembly Lockring Wheel Axle Transducer alignment rivet Electrical connector Transducer Transducer adapter Drive coupling