Introduction to SAR test method for wristwatch-type mobile phone FCC certification

    Watches are very common accessories in daily life. With the advancement of technology, various watches with wireless functions have entered the market, and their certification methods have become more complicated. Recently, MORLAB successfully completed the FCC certification of a wristwatch mobile phone. Engineers share the most tedious SAR test method and experience in this certification with readers.

    For products such as wristwatch phones, the FCC's TCB cannot complete the final review, and the test results must be reviewed by the Federal Communications Commission (FCC) itself. Before the test begins, the laboratory or TCB needs to send a corresponding Inquiry to the FCC and wait for the FCC to give corresponding suggestions.

    At present, the FCC pays special attention to the measurement and evaluation of the SAR value of such products. Therefore, in this article, we focus on introducing the FCC's evaluation requirements for the SAR test of wristwatch-type mobile phones.
                                             
    First of all, before the certification test, Moore's engineers will conduct a technical evaluation of customer products based on the FCC's SAR test methods and requirements for wristwatch mobile phone products. Among them, the following 6 factors need to be evaluated and considered:

1) identify the FCC ID and all wireless modes (WWAN, Bluetooth etc.)

    a) maximum output power for each operating mode and technology

    b)identify power reduction and muli-slot classes for GPRS/EDGE

    You need to know the product's FCC ID and all the wireless standards it supports, as well as parameters such as power and time slot. For example, a product includes GSM850MHz, the corresponding maximum transmit power is 33dBm, GSM1900MHz, the corresponding maximum transmit power is 30dBm, Bluetooth 2.4GHz, the maximum transmit power is 0.5 dBm; in addition, the product supports GPRS class 10 Bluetooth 2.0, etc.

2) identify the material used in the body and bands of the wrist watch; especially metallic and conductive parts

    a) indicate if any components are flexible; leather, soft plastic etc.

    Understand the physical materials of each part of the watch, especially which ones are metal and conductive media. In addition, it is also necessary to specify whether other parts such as the strap are made of leather, soft plastic or other materials, etc. As shown in Figure 1 below: The strap of this product is made of all plastic and is bent and shaped.

Figure 1

3) identify the type, location and dimension of all antennas; indicate if the antenna(s) is for transmit and/or receive a) include antenna distances from the side edge, top and bottom of the wrist watch

    Specify the type, location and size of the various antennas in the product, indicate the distance between each antenna (side, top and bottom), and distinguish between transmitting antennas and receiving antennas. As shown in Figure 2 below: The distance between the GSM antenna and the Bluetooth antenna of this product is 4.5cm, and the distance between the top and bottom antennas is 1.5cm.

Figure II

4) identify the normally expected operating configurations and exposure conditions with respect to available features a) speaker mode, PTT mode, Bluetooth etc.

    Identify the various working modes of the product, the normal working configuration mode, and other typical configuration working modes based on the product application functions, such as voice call mode, speaker mode, PTT mode, Bluetooth mode, etc. For example: the product manual describes that the product has a speaker function, can use the amplification mode to make calls, supports Bluetooth headsets, and can also wear wired headsets to make calls, etc. Then it is necessary to conduct corresponding evaluation and testing for each mode.

5) identify if any part of the bands may contain electronic parts and components; eg keypad, antenna etc.

    Specify some accessories that the product may include, such as keyboards, antennas, etc. For example: the product comes with wired headphones, Bluetooth headphones, Bluetooth keyboards, etc.

6) With the bands open and the back side of the wrist watch placed on a flat surface, identify the separation distance between the back side of the wrist watch and the flat surface.

    Open the watch strap, place the watch with the back facing down on a flat surface, and mark the distance between the back of the watch and the flat surface. As shown in Figure 3: The distance between the front and back of the watch is 1.5cm, and the distance between the back and the flat surface is 0cm.

Figure 3

    Next, we need to develop a SAR test plan that complies with the product according to the SAR test standards and submit the test plan to the FCC for evaluation. FCC engineers usually provide corresponding review opinions or their recommended test plans within one week.

    The following is an example of a test solution for SAR of a wristwatch phone, and its product specifications are as follows:

    A wristwatch phone with dual band GSM850Mhz and GSM1900Mhz and supports Bluetooth 2.0. Its accessory is a wired headset with details as in the example mentioned in element 1.

    1. The front of the product is 1cm away from the body phantom, using tissue fluid from the head, in the amplification mode, test the three channels of high, medium and low. The FCC limit is 1.6w/kg (1g average).

    2. The back of the product is placed against the body phantom test, using body tissue fluid to test three channels: high, medium and low. And add an evaluation accessory mode (such as the state of making a call with a wired headset). The FCC limit is 4w/kg (10g average).

    3. The product needs to evaluate voice mode and data mode, and evaluate the SAR value of multi-antenna multi-transmitter according to KDB648474.

    In addition, assuming that the bottom of the watch-type phone cannot fit closely to the phantom plane, it is necessary to consider using a hand model to achieve a curved surface that fits closely to the bottom of the watch-type phone.

    The test layout is shown in Figures 4 and 5:

          Figure 4: EUT front 1cm away from body Figure 5: EUT + earphone back close to body
                         
    From our evaluation method for SAR of wristwatch-type mobile phones mentioned above, we can see that the FCC requirements for such products are very strict. Its FCC engineers will personally review the relevant reports and materials, so this will make the entire certification cycle relatively long.