FAQ

This Product Brief applies to both the Census and CypherTag products, but make sure you are referring to the relevant product.

What causes the noise?

The kind of electrical noise that will affect an Identec system is a magnetic field. These are generally caused either by something that deliberately causes a magnetic field, or because currents are flowing through nearly cables. Of these, magnetic fields caused by nearby cables are the most likely to be an issue.

The magnetic field of the noise must be in the relevant frequency range for it to affect performance.

Although we can give rough figures for noise levels, it must be stressed that all sorts of noise are not equal. Readers have a lot of filtering that tries to minimise the effect of noise, but this works better for some types of noise.

Cryptag Census - 98kHz

Cryptag Census uses a frequency of 98.4kHz to transmit from tag to reader. (115.2kHz for FCC approved systems.) Noise will reduce the reading range. To meet a range of 3 metres the noise level should ideally be below the field created by a tag at this distance, which is about 10μA/m. Very often the noise can be filtered out by the reader, but this is the noise level that should be aimed for. If a lower range is required, the noise level can be higher, so for 2 metres range the level can be nearer 40μA/m. (Inverse cube law)

A figure of 10μA/m applies to noise in the range of approximately 93 to 103kHz. Outside this range noise will have an effect, but the further away from 98kHz it is the higher the noise level that can be tolerated.

Very often the noise is caused by spikes, for instance from motor drives. Spikes produce what is known as broadband noise, which affects a wide range of frequencies. Spikes higher than 100μA/m will have a similar effect to narrowband noise of 10μA/m.

10μA/m is 20dBμA/m. 100μA/m is 40dBμA/m.

Cryptag Census 131kHz

Cryptag Census readers transmit to tag at a frequency of 131kHz (153.6kHz for FCC approved readers). The magnetic field produced by the reader is much greater than what a tag can produce, so it is much rarer for 131kHz noise to activate tags. If it does happen it is most commonly because there is another tagging system around.

Tags are activated by a magnetic field of about 10mA/m, approximately 1000 times greater than the tag field. Any field greater than this will make a tag wake up, even if it is just a short spike. This should be avoided because every time the tag is woken up it consumes power, and the battery will be flattened. (Typically, it takes about a month in a magnetic field to flatten the battery if the tag is left in a continuous field that will activate the tag.)

As well as flattening the battery, 131kHz fields will prevent the tags from being read. For reliable reading the maximum field should ideally be less than 2ma/m.

10mA/m is 80dBμA/m. 2mA/m is 66dBμA/m.

CypherTag 4MHz

CypherTag tags transmit at a frequency of 4MHz. The magnetic field produced by the tag is around 3μA/m at a distance of 3 metres – less than a Census tag, but at higher frequencies noise levels are also generally lower. To achieve a range of 3 metres the noise level should be kept below 3μA/m.
CypherTag has better filtering so higher levels of noise can more often be tolerated. As with Census the inverse cube law applies, so for 2 metres range the noise can be about 4 times higher at 12μA/m.

The frequency range is 3.95 to 4.05MHz. Outside this range noise will have a lower impact, and higher levels can be tolerated.

CypherTag readers will be affected by spike noise (e.g. from motor drives), but if this is the main source of noise the reader can be configured for spike suppression. This works best when the pulse repetition rate of the spikes is not too high.

3μA/m is 10dBμA/m.

CypherTag 125kHz

CypherTag readers transmit to tag at a frequency of 125kHz. As with Census, peak noise of around 10mA/m will activate the tag and flatten its battery, and lower levels down to 2mA/m may affect reading.

Background

CypherTag readers and tags communicate through radio frequency fields. This Product Brief summarises the emission levels and also how they should be interpreted when answering specific questions.

CypherTag readers transmit at 125kHz, and the tags reply at 4MHz.

A little science

Radio waves are electromagnetic waves. An electric current causes a magnetic field, and a changing magnetic field causes an electric field. Even in a vacuum a changing electric field gives rise to an electric current. The electromagnetic wave propagates as energy is transferred backwards and forwards between the electric and magnetic fields.

At the relatively low frequencies used by CypherTag the aerials are much smaller than the wavelength of the electromagnetic wave. CypherTag aerials create a magnetic field, but the distances involved are too small for the corresponding electric field component to build up. This is normally referred to as the “near field” region, which normally extends up to around one sixth of a wavelength. In the near field region it is usually best to think in terms of loosely coupled transformers. It should also be noted that ideas such as “radiated power” which apply in the far field region are practically meaningless in the near field region.

Measured Emission levels

These are the magnetic field levels measured during approval testing of CypherTag products. The tests were carried out to EN300330.

Tags were measured at being below -24.5dBÇA/m at a distance of 10 metres. The distance of 10 metres is used for European approvals.

Mullion Reader (REVx) was measured at being -26.5dBÇA/m at a distance of 10 metres.

The RVx Loop Controller was tested with 4 transmit aerials, each 4 metres square, and gave a net emission level of 49dBÇA/m at a distance of 10 metres.

Magnetic Field with other aerial sizes and at other distances (RV1)
RV1 Loop Controllers can be used with aerials with a wire length up to 16 metres, corresponding to the largest size tested. Most installations will use smaller aerials, which create a smaller magnetic field. The magnetic field is proportional to the area of the aerial loop and to the current in the loop. The current is inversely proportional to the length of the wire, so a smaller loop has a higher current.

A loop 2m by 2m has one quarter the area of a 4m by 4m loop, but half the wire length, and therefore twice the current. Overall the magnetic field is half that of a 4m by 4m loop.

A loop 6m by 2m has the same wire length but only É of the area. The magnetic field is therefore also only É that of the 4m by 4m loop.

The magnetic field varies considerably with distance. At large distances the magnetic field has an inverse cube law, so that at half the distance the magnetic field is 8 times greater. This is valid until the distances involved are similar to the size of the aerial, when the variation starts to level off. The highest fields are seen very close to the wire of the loop.

For a loop 2 metres by 2 metres the field at a distance of 10cm from the wire is about 0.8A/m. This magnetic field is inversely proportional to both the wire length of the aerial and the distance from the wire.

Human Body Exposure

CypherTag meets requirements on human body exposure. Although it is possible to create large magnetic fields these cannot induce enough current in a human body to exceed limits.

Power Levels

We are sometimes asked about the maximum power level from an aerial. This might for instance be to ensure that it is not possible for this energy to cause an explosion. Unfortunately it is difficult to give a hard and fast answer that fits all possible situations.

The maximum power that can be extracted from the output circuit of the RVx reader is 0.9W with a duty cycle of just over 30%, giving an average figure of 300mW. This could only be extracted by a perfectly matched coil of the same dimensions as the transmit coil, and with near perfect coupling. This is not practical. The highest power we have seen in a test coil was 210mW peak/70mW average. This coil has the same size and shape as the transmit coil and was placed close to it (Interestingly maximum power transfer is not when the impedances match or when the coupling is too good because the transmit aerial is detuned).

If an application requires that it is not possible to induce an average power of less than 70mW then it would be necessary to consider what the likely coupling will be. In almost all cases there should not be a problem but if it is a safety issue then Identec recommends a test.