SNLS696A April 2021 – May 2021 TSER953
The TSER953 is designed to support the Power-over-Coax (PoC) method of powering remote sensor systems. With this method, the power is delivered over the same medium (a coaxial cable) used for high-speed digital video data, bidirectional control, and diagnostics data transmission. This method uses passive networks or filters that isolate the transmission line from the loading of the DC-DC regulator circuits and their connecting power traces on both sides of the link as shown in Figure 8-1.
The PoC network impedance of ≥ 2 kΩ over a specific frequency band is typically sufficient to isolate the transmission line from the loading of the regulator circuits. The lower limit of the frequency band is defined as ½ of the frequency of the bidirectional control channel' (fBCC). The upper limit of the frequency band is the frequency of the forward high-speed channel (fFC).
Figure 8-2 shows an example PoC network suitable for a "4G" V3Link consisting of TSER953 and TDES954 or TDES960 pair with the bidirectional channel operating at 50 Mbps (½ fBCC = 25 MHz) and the forward channel operating at 4.16 Gbps (fFC ≈ 2.1 GHz). Other PoC networks are possible and may be different on the serializer and the deserializer boards as long as the printed-circuit board return loss requirements listed in Table 8-2 are met.
Table 8-1 lists essential components for this particular PoC network. Note that the impedance characteristic of the ferrite beads deviates with the bias current. Therefore, keeping the current going through the network below 150 mA is recommended.
|COUNT||REF DES||DESCRIPTION||PART NUMBER||MFR|
|1||L1||Inductor, 10 µH, 0.288 Ω maximum, 530 mA minimum (Isat, Itemp)|
30 MHz SRF minimum, 3 mm × 3 mm, General-Purpose
|Inductor, 10 µH, 0.288 Ω maximum, 530 mA minimum (Isat, Itemp)|
30 MHz SRF minimum, 3 mm × 3 mm, AEC-Q200
|Inductor, 10 µH, 0.360 Ω maximum, 450 mA minimum (Isat, Itemp)|
30 MHz SRF minimum, 3.2 mm × 2.5 mm, AEC-Q200
|Inductor, 10 µH, 0.400 Ω typical, 550 mA minimum (Isat, Itemp)|
39 MHz SRF typical, 3 mm × 3 mm, AEC-Q200
|Inductor, 10 µH, 0.325 Ω maximum, 725 mA minimum (Isat, Itemp)|
41 MHz SRF typical, 3 mm × 3 mm, AEC-Q200
|3||FB1-FB3||Ferrite Bead, 1.5 kΩ at 1 GHz, 0.5 Ω maximum at DC|
500 mA at 85°C, 0603 SMD , General-Purpose
|Ferrite Bead, 1.5 kΩ at 1 GHz, 0.5 Ω maximum at DC|
500 mA at 85°C, 0603 SMD , AEC-Q200
In addition to the selection of PoC network components, their placement and layout play a critical role as well.
The suggested characteristics for single-ended PCB traces (microstrips or striplines) for serializer or deserializer boards are listed in Table 8-2. The effects of the PoC networks must be accounted for when testing the traces for compliance to the suggested limits.
|Ltrace||Single-ended PCB trace length from the device pin to the connector pin||5||cm|
|Ztrace||Single-ended PCB trace characteristic impedance||45||50||55||Ω|
|Zcon||Connector (mounted) characteristic impedance||40||50||60||Ω|
|RL||Return Loss, S11||½ fBCC < f < 0.1 GHz||-20||dB|
|0.1 GHz < f < 1 GHz (f in GHz)||–12 + 8 × log(f)||dB|
|1 GHz < f < fFC||–12||dB|
|IL||Insertion Loss, S12||f < 0.5 GHz||–0.35||dB|
|f =1 GHz||–0.6||dB|
|f =2.1 GHz||–1.2||dB|
The VPOC fluctuations on the serializer side, caused by the transient current draw of the sensor, the DC resistance of cables, and PoC components, must be kept to a minimum as well. Increasing the VPOC voltage and adding extra decoupling capacitance (> 10 µF) help reduce the amplitude and slew rate of the VPOC fluctuations.