Page 35 of 39 RTE6701-OEM Integration Manual VC27U V1.03 DRAFT.doc
Appendix A – EMC Compliance Techniques
Note that the latest versions of the VC27U PCB (59-2004-06 and -07) have additional EMC filtering
components.
The VC27U may need a number of precautions when integrating within a kiosk or
terminal in order to ensure EMC compliance. The PCB has 3 terminal capacitor noise
suppression on the main +5V input connector J5.
Try to keep the VC27U PCB as close to the read head as possible.
There are two main frequencies one from the CPU clock giving 25MHz harmonics and the other at around
2.1MHz which is from the CCD head cable.
The processor noise can be suppressed by ferrites on the PSU and USB cables - ensuring
that the ferrites are placed close to the PCB. You may also need a second ferrite at the
host end. The cables should be shielded and experimentation is required to see whether
the screens should be connected at the PCB end, the PSU/host ends or both ends. Care
should be taken to avoid earth loops.
Note that the PCB only has a 0V digital ground and does not have a separate shield
connection. Mounting holes are not connected.
The head cables consist of a three wire ribbon cable driving the illumination and a 10 way carrying the CCD
data. The illumination cable does not normally cause any problems (there are no clocks) and nobody has ever
placed suppression components on it.
The 10 way ribbon cable is the main cause of radiated EMC especially if it has been
lengthened.
3M generally supplies an unshielded ribbon cable which is used in many of 3M's products but can also supply a
shielded ribbon cable which is earthed at the PCB end. If radiation is suspected on this cable a shield can be
fabricated from EMC foil and experiments should be made with earthing at one or both ends.
Another suppression technique is to use a ferrite on the cable but this can reduce the
signal quality and cause bad reads.
Experiment with placing a small ferrite in the centre of the cable or preferably both ends
(but do check the read quality as well – 3M can supply tools to do this). A shorter cable is
also a good technique and 3M recommends that the cable is no longer than 15cm.
The ten way ribbon cable should not be bundled into a round cable (especially if using twisted pairs) as the
clocks interfere with the data causing bad read quality.
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