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ZHCSMR4A november 2020 – november 2020 DS90UB662-Q1

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8.1.1 Power Over Coax

The DS90UB662-Q1 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 and bidirectional control and diagnostics data transmission. The 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.

Figure 8-1 Power-over-Coax (PoC) System Diagram

The PoC networks' impedance of ≥ 1 kΩ over a specific frequency band is typically sufficient to isolate the transmission line from the loading of the regulator circuits provided good layout practices are followed and the PCB return loss requirements given in Table 8-3 are met. The lower limit of the frequency band is defined as ½ of the bidirectional control channel's frequency, f_BC. The upper limit of the frequency band is the frequency of the forward high-speed channel, f_FC.

Figure 8-7 shows a PoC network recommended for a 4G FPD-Link III consisting of DS90UB63x CSI-2 Serializer and DS90UB662-Q1 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).

Figure 8-2 Typical PoC Network for a 4G FPD-Link III

Table 8-1 lists essential components for this particular PoC network.

Table 8-1 Suggested Components for a 4G FPD-Link PoC Network

COUNT	REF DES	DESCRIPTION	PART NUMBER	MFR
1	L1	Inductor, 10 µH, 0.288 Ω maximum, 530 mA minimum (Isat, Itemp) 30-MHz SRF min, 3 mm × 3 mm, General-Purpose	LQH3NPN100MJR	Murata
		Inductor, 10 µH, 0.288 Ω maximum, 530 mA minimum (Isat, Itemp) 30-MHz SRF min, 3 mm × 3 mm, AEC-Q200	LQH3NPZ100MJR	Murata
		Inductor, 10 µH, 0.360 Ω maximum, 450 mA minimum (Isat, Itemp) 30-MHz SRF min, 3.2 mm x 2.5 mm, AEC-Q200	NLCV32T-100K-EFD	TDK
		Inductor, 10 µH, 0.400 Ω typical, 550 mA minimum (Isat, Itemp) 39-MHz SRF typ, 3 mm × 3 mm, AEC-Q200	TYS3010100M-10	Laird
		Inductor, 10 µH, 0.325 Ω maximum, 725 mA minimum (Isat, Itemp) 41-MHz SRF typ, 3 mm × 3 mm, AEC-Q200	TYS3015100M-10	Laird
3	FB1-FB3	Ferrite Bead, 1500 kΩ at 1 GHz, 0.5 Ω maximum at DC 500 mA at 85°C, SM0603, General Purpose	BLM18HE152SN1	Murata
3	FB1-FB3	Ferrite Bead, 1500 kΩ at 1 GHz, 0.5 Ω maximum at DC 500 mA at 85°C, SM0603, AEC-Q200	BLM18HE152SZ1	Murata

Application report Sending Power over Coax in DS90UB913A Designs (SNLA224) discusses and defines the PoC networks in more detail.

Figure 8-3 Typical PoC Network for a 2G FPD-Link III

lists essential components for this particular PoC network.

Table 8-2 Suggested Components for a 2G FPD-Link III PoC Network

Count	Ref Des	Description	Part Number	MFR
1	L1	Inductor, 100 µH, 0.310 Ω max, 710 mA MIN (Isat, Itemp) 7.2 MHz SRF typ, 6.6 mm × 6.6 mm, AEC-Q200	MSS7341-104ML	Coilcraft
1	L1	Inductor, 100 µH, 0.606 Ω max, 750 mA MIN (Isat, Itemp) 7.2 MHz SRF typ, 6.0 mm × 6.0 mm, AEC-Q200	NRS6045T101MMGKV	Taiyo Yuden
1	L2	Inductor, 4.7 µH, 0.350 Ω max, 700 mA MIN (Isat, Itemp) 160 MHz SRF typ, 3.8 mm × 3.8 mm, AEC-Q200	1008PS-472KL	Coilcraft
		Inductor, 4.7 µH, 0.130 Ω max, 830 mA MIN (Isat, Itemp), 70 MHz SRF typ, 3.2 mm × 2.5 mm, General Purpose	CBC3225T4R7MRV	Taiyo Yuden
		Inductor, 10 µH, 0.288 Ω max, 530 mA MIN (Isat, Itemp) 30 MHz SRF min, 3 mm × 3 mm, AEC-Q200	LQH3NPZ100MJR	Murata
1	FB1	Ferrite Bead, 1500 kΩ at 1 GHz, 0.5 Ω max at DC 500 mA at 85°C, SM0603, General Purpose	BLM18HE152SN1	Murata
1	FB1	Ferrite Bead, 1500 kΩ at 1 GHz, 0.5 Ω max at DC 500 mA at 85°C, SM0603, AEC-Q200	BLM18HE152SZ1	Murata

Application report https://www.ti.com/lit/an/snla224/snla224.pdf?ts=1602885821293 (SNLA224) discusses and defines the PoC networks in more detail.

In addition to the PoC network components selection, their placement and layout play a critical role as well.

Place the smallest component, typically a ferrite bead or a chip inductor, as close to the connector as possible. Route the high-speed trace through one of its pads to avoid stubs.
Use the smallest component pads as allowed by manufacturer's design rules. Add anti-pads in the inner planes below the component pads to minimize impedance drop.
Consult with connector manufacturer for optimized connector footprint.
Use coupled 100-Ω differential signal traces from the device pins to the AC-coupling caps. Use 50-Ω single-ended traces from the AC-coupling capacitors to the connector.
Terminate the inverting signal traces close to the connectors with standard 49.9-Ω resistors.

The suggested characteristics for single-ended PCB traces (microstrips or striplines) for serializer or deserializer boards are detailed in Table 8-3. The effects of the PoC networks need to be accounted for when testing the traces for compliance to the suggested limits.

Table 8-3 Suggested Characteristics for Single-Ended PCB Traces With Attached PoC Networks

PARAMETER			MIN	TYP	MAX	UNIT
L_trace	Single-ended PCB trace length from the device pin to the connector pin				5	cm
Z_trace	Single-ended PCB trace characteristic impedance		45	50	55	Ω
Z_con	Connector (mounted) characteristic impedance		40	50	62.5	Ω
RL	Return Loss, S11	½ f_BC < f < 0.1 GHz			–20	dB
		0.1 GHz < f < 1 GHz (f in GHz)			–12 + 8 × log(f)	dB
		1 GHz < f < f_FC			–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 V_POC noise must be kept to 10 mVp-p or lower on the source / deserializer side of the system. The V_POC fluctuations on the serializer side, caused by the sensor's transient current draw and the DC resistance of cables and PoC components, must be kept at minimum as well. Increasing the V_POC voltage and adding extra decoupling capacitance (> 10 µF) help reduce the amplitude and slew rate of the V_POC fluctuations.

Figure 8-3 shows a PoC network recommended for a "2G" FPD-Link III consisting of a DS90UB633A-Q1 serializer and DS90UB662-Q1 with the bidirectional channel operating at the data rate of 2.5 Mbps (½ fBC = 1.25 MHz) and the forward channel operating at the data rate as high as 1.87 Gbps (fFC ≈ 1 GHz).