phyCAM-L Cable Length

Overview

This article is intended to provide an overview when considering the permissible cable lengths of the phyCAM-L interface.

According to TI, the following regulations apply to the FPD link. The entire system consisting of cable, connector, and PCB must be considered here:

• The attenuation of the forward channel of the FPD-Link III may be between 1.2 bB and 20 dB (2GHz).
• The return loss must be -25dB up to 25MHz. Between 25MHz and 1GHz S11=-12dB+8*log(f(GHz)). Above 1GHz, -12 dB is typically allowed. According to TI, -10 dB is permissible in the worst case. However, this is strongly dependent on the subsequent operating conditions.
• The Back Channel may be attenuated by a maximum of 4 dB.
• For further details on the limits of the FPD-Link III, please refer to the TI datasheets and application notes of the DS90UB95x.

Since the attenuation of the forward channel is usually the more critical limit value, only this is considered here. However, the back channel must also be checked during verification.

The maximum attenuation recommended by TI is not specified in more detail with regard to EMC immunity. If industrial interference immunity is to be maintained, the maximum cable length should not be exhausted and the possibility of an early interference diversion should be implemented, e.g. by means of pigtails placed on the shield at the camera and baseboard.

The operating frequency of the FPD-Link III can be between 1840 MHz and 2080 MHz. The typical phyCAM-L interface operates at a frequency of 1920 MHz. The attenuation values must be considered at the operating frequency.

The following values are assumed as a flat rate for the considerations:

• For a UMCC-type connector, a flat rate of 0.24 dB is used.^[1]

• Approx. 0.07 dB can be estimated for one centimeter of the conductor path.^[2]

• A flat rate of 3 cm is assumed for the conductor lengths on the PCB. This corresponds to an attenuation of approx. 0.21 dB.

This results in a constant attenuation of 0.69 dB for connectors and traces. The resulting cable length represents the theoretical maximum. It is recommended to always consider a buffer of at least 1 dB.

When dimensioning, it should also be noted that the insertion loss is also temperature-dependent. If no curve is available for the attenuation over the temperature, it should be assumed that the specified attenuation applies to room temperature. Measurements have shown that operation at 40°C can already strongly influence the attenuation values. It is recommended to add at least 10% to 20% to the typical damping value to ensure that the design has a safe and good functional reliability.

Verification by means of a climatic chamber and an assessment of the signal quality are recommended (see next chapter).

Calculation of the permissible insertion loss (worst case insertion loss of the cable known):

(mathjax-inline(a=a_{MaxDämpfung}-2*a_{UMCC-Dämpfung}-a_{Leiterbahn}-a_{Puffer}=20dB-2*0,24dB-0,21db-1db=18,31dB)mathjax-inline)

Calculation of the permissible insertion loss (worst case insertion loss of the cable not known):

(mathjax-inline(a=(a_{MaxDämpfung}-2*a_{UMCC-Dämpfung}-a_{Leiterbahn}-a_{Puffer})*Temperaturabzug=(20dB-2*0,24dB-0,21db-1db)*0,9=16,4dB)mathjax-inline)

(mathjax-inline(a=(a_{MaxDämpfung}-2*a_{UMCC-Dämpfung}-a_{Leiterbahn}-a_{Puffer})*Temperaturabzug=(20dB-2*0,24dB-0,21db-1db)*0,8=14,6dB)mathjax-inline)

This leaves 18.31 dB for the cable. If longer traces or vias are used, this value must be reconsidered.

If the device is to be operated in a temperature range of up to 85°C and only the typical insertion loss is known, the loss should be limited to a maximum of 14.6 dB to 16.4 dB.

Definition of Line Lengths through Signal Integrity Verification

The maximum line length can be approximated using signal integrity.

Due to the equalizer (EQ) and its configuration, a correct evaluation of the eye diagram is not possible, TI recommends using the MAP test^[3]to verify the quality of the signal integrity. The program is included in TI LaunchPAD^[4]and is based on TI's Application Note SNLA301. PHYTEC provides Python3-based software for performing this test (See PHYTEC Application Note LAN-107). When performing the test, make sure that the image sensor is working and that any additional load on the expansion connector of the camera module is also in operation. Ideally, the test should also be performed under different expected temperatures.

It is recommended to always perform the MAP test even if the attenuation and ripple values of the signal path have been considered!

Furthermore, analyzing CRC errors (back channel) and parity errors (forward channel) provides information about the quality of the link. No errors should occur during verification. During this test and also in serial use, the ideal strobe and EQ settings used by the MAP analysis can be configured in the DS90UB954 so that invalid settings are directly prevented. If single parity errors occur, the DS90UB954 automatically corrects the EQ setting. The EQ settings or parity errors should therefore always be monitored since any parity error can theoretically represent a data error. CRC errors cannot be optimized by changing EQ settings. Monitoring the CRC errors and checking whether write and read operations were performed correctly helps here. An initial reset of the strobe and EQ setting is recommended before the first start.

Additional power extraction at the expansion connector can strongly influence the properties of the PoC filter so that extensive verification of the system is always necessary. 

The CMLOUT output of the DS90UB954 can be used for jitter measurement. However, since this already evaluates the processed signal, this measurement does not provide any information about the actual eye level/width and any buffer. Limits for the jitter can be taken from the datasheet.

Line Length (without PoC) through Theoretical Consideration of Attenuation

As it is not possible to make a general allowance for interference caused by Power-over-Coax (PoC), the maximum cable lengths without PoC are considered first. The table below shows typical values for corresponding cable types. Since even the same cable types can have manufacturer-specific deviations, the datasheet of the specific cable must always be used or the S-parameters must be measured by means of a network analyzer in order to be able to determine correct values.

It should be noted that the UMCC connector cannot accommodate a stronger cable than RG1.37. Stronger cables are adapted with a pigtail. This must be included in the calculation.

A pigtail should be considered as follows:

• A connector, which attenuates with approx. 0.25 dB.
• For example, 5 cm RG1.37 cable, which attenuates with another 0.12 dB. 

(mathjax-inline(a_{Pigtail}=a_{Steckverbinder}+a_{Kabel}=0,30dB-0,12db=0,42dB)mathjax-inline)

If two pigtails are used, this adds approx. 0.84 dB.

One major advantage of using pigtails is that the cable shield can be attached to the housing by means of these adapters, enabling a more robust overall system against EMC influences.

Line Length (with PoC) through Theoretical Consideration of Attenuation and DCR

To ensure that the interference caused by PoC remains as unproblematic as possible, the following limits have been set:

• The maximum voltage loss between the complete transmission path must not exceed 1 V.
  • The measurement can be performed on the expansion connector of the camera module and the Power Domain in front of the PoC-Filter of the Baseboard. This means that the already filtered voltage on the camera module is evaluated.
• The edge steepness (falling and rising) at V_POC must not fall below 200 µs (10% - 90%).
  • The measurement is performed at the expansion connector of the camera module. The already filtered voltage on the camera module is evaluated.
• A maximum of 6 watts may be drawn.
  • Compatibility of the phyCAM interface
• The maximum current flow must not exceed 750 mA.
  • Compatibility of the phyCAM interface

In order to approximate the maximum voltage variation, the system DCR values are needed as well as the power consumption of the camera. There are several variables, such as the current flow, the line length, the line type, and the voltage for PoC.

• The voltage VCAM and V_POC must be sufficiently high to ensure that a current flow is as low as possible. A voltage of 12 volts is recommended.
• The line type defines the attenuation and DCR to a great extent.
  • Note that the DCR is usually specified at a certain temperature. For example, if the DCR is specified for 25°C and the cable is operated at 85°C, the resistance value of copper will increase by about 25%. Reserve should be planned for.
  • When dimensioning, it should also be noted that the insertion loss is also temperature-dependent. If no curve is available for the attenuation over the temperature, it should be assumed that the specified attenuation applies to room temperature. Measurements have shown that operation at 40°C can already strongly influence the attenuation values. It is recommended to add at least 10% to 20% to the typical damping value to ensure that the design has a safe and good functional reliability.
• The cable length defines most of the total attenuation and DCRs.
• The voltage V_CAM at the camera must not fall below 4.5 volts.
  • The measurement can be done at the expansion connector of the camera module. The already filtered voltage on the camera module is then evaluated. The DCR of the corresponding camera module can be taken into account.
• The power requirement of the camera must be taken into account.
• The DCR of the baseboard and, if necessary, the DCR of the camera must also be included.

It should be noted that the UMCC connector cannot accommodate a stronger cable than RG1.37. Stronger cables are adapted with a pigtail. This must be included in the calculation.

• The connector is assumed to have a flat attenuation of approx. 0.25 dB.
• Furthermore, a 5 cm RG1.37 cable is used, for example, which attenuates by a further 0.12 dB. 
• The DCR is assumed with approx. 25 mΩ per pigtail.

A great advantage of using pigtails is that the cable shield can be connected to the housing by means of these adapters, enabling a more robust overall system against EMC influences.

The DCR of the hardware used must also be taken into account. If DCR values for other PHYTEC boards are required, please contact our support team. The DCR of the camera module can be found in the following table.

Power consumption and DCR of camera modules and baseboards:

It is recommended to provide approx. 100 µF to 200 µF (X7R) on the expansion modules Expansionconnector when using an additional load. By verification with MAP analysis and monitoring of the parity and CRC errors, the influence of the expansion module must be evaluated individually.

Here are some example considerations: