Introduction to the design of FM radio reception for mobile phones based on built-in antenna

FM radio receiver modules are a standard feature in most modern mobile phones. Short-range FM transmission (Tx) has recently become a popular means of transferring audio from a portable MP3 player to a home or car radio, and this feature will soon be available for mobile phones.

Laird Technologies has developed an internal antenna for FM radio reception in mobile phones , the RadioAnt, that delivers performance similar to outdated wired headphone antennas through the integration of a radiating device and a co-designed preamplifier.

This approach has several advantages over traditional passive solutions. One is that the requirement for the antenna impedance to be 50 ohms is effectively eliminated. This is very important at FM frequencies where a radiating impedance of about 1 meter can be achieved. While the inherent radiating amplifier impedance interface in an active antenna is not around 50 ohms, the output can be adjusted to any impedance level, including 50 single-ended or 200 differential impedance for appropriate connection to the receiver input.

The gain of the preamplifier suppresses the noise of the FM receiver by about 6dB. This is equivalent to using a passive antenna with 6dB higher gain. The high gain of the active antenna provides a more suitable signal level for the FM receiver because it limits the dynamic range of the standard receiver automatic gain control. While the higher gain (because noise and signal are amplified equally) does not improve the signal-to-noise ratio (SNR) at RF frequencies, it greatly improves the SNR at down-converted audio frequencies. However, the need for an impedance load is eliminated, which severely reduces gain and increases antenna noise, and the amplifier does not need to be unconditionally stable.

This type of active antenna does have drawbacks, but they can be dealt with. Specifically, the design and features are more complex, and the preamplifier consumes power and PCB area. In addition, the active device must be protected from ESD without degrading sensitivity. Most importantly, stability and linearity must be achieved without resistive loading, even if the antenna presents a nearly open or short impedance at the amplifier input.

Characteristics of active antennas

The main metric that is useful for active antennas is the total gain (antenna + amplifier) ​​normalized by the total output noise temperature, G/T (G to T for short). Currently, if you increase the amplifier gain, the output noise will increase, with no improvement in G/T. For example, the G/T of a lossless, perfectly matched short dipole or loop antenna is -22.8 dB/K at room temperature (with a directivity of 1.8 dBi, 1.8 dBi - 10log(290K)). The concept of G/T degradation presented here in relation to a perfectly matched lossless short dipole antenna is similar to that of noise figure (NF) in that the SNR is compared at two different nodes, but a matched source is not required at the input (as defined by the NF metric) at 290K noise temperature. Typically, since most electrically small antennas have a directivity of 1.8 dBi, the gain, G, is considered to be the "average gain" over all angles, which is consistent with standard antenna efficiency (so, the maximum is 0 dB or 100%). Throughout this article, gain is synonymous with efficiency, and therefore, directivity is not included. For example, if G/T drops by 10dB, the system performance is equivalent to a passive antenna with -10dB efficiency (if all antennas were connected to noise-free receivers).

In addition to the antenna characteristics, the degradation value of G/T in practical applications is affected by two external factors: the ambient noise temperature Ta, which increases the output noise, and the receiver noise figure NFre, which increases the antenna output noise Tout (and thus reduces G/T). It has been shown that the value of Ta is much higher than room temperature T0 (e.g. 290 K or -174 dBm/Hz) at FM frequencies due to artificial RF noise. The increased noise level means that the impact of the active device and resistor noise contributions is reduced, unless an internal antenna is used, the gain of the radiating device is so low that the antenna physical temperature determines the noise temperature. In addition, the high background noise level means that the efficiency requirements of the radiating device can be reduced, for the ideal low-noise case without being reduced as much as G/T. This can be understood qualitatively as a high-efficiency antenna will receive a larger signal level than a low-efficiency antenna, but it will also receive more noise. Therefore, the SNR at the antenna output is not significantly improved. The second effect, NFrec, also contributes noise to the antenna output, but can be made negligible by choosing a very high gain amplifier (Gamp > NFrec), thus improving the NF performance of the system compared to using a passive antenna. It should be noted that these two effects of background noise and NFrec are usually inseparable, for example a high background noise temperature can achieve a receiver-independent noise figure and vice versa.

If the efficiency of the radiator and the gain Gamp of the amplifier are known (assuming that the antenna is at room temperature T0 and the surrounding noise temperature Ta is "known"), the degradation value of the active antenna G/T can be calculated by the following formula (where the temperature unit is K):

Calculate the G/T degradation value of active antenna

However, the efficiency of the radiator is usually not known separately from the gain of the amplifier (at least not by measurement, but simulation or analytical models can be used to obtain these data). Instead, the degradation value of G/T can be obtained by measuring the total output noise power of the antenna when placed in a specific environment (e.g. in a shielded room with Ta = T0) and measuring the gain by, for example, comparing it with an antenna of known gain. Careful care must be taken to eliminate the noise contribution from the measuring equipment by calibration and that no metal objects (coaxial measuring lines or voltage supply lines) are attached during the measurement, for reasons explained previously.

To support these requirements, Laird Technologies has developed a fiber-based cable replacement system that, along with a battery-driven preamplifier, facilitates the proper characterization of electrically small antennas (Figure 1). The gain of monopole antennas of varying lengths protruding from a shielded box was measured using both coaxial and fiber-optic systems. The fiber-optic system measured an error of approximately 20 dB for lengths under approximately 10 m, which is a realistic value for internal antennas .

Finally, it should be noted that the presence of a human body increases the gain of small antennas at FM frequencies, especially if the user is touching the antenna or the shielding box. This is because the human body is a fairly efficient antenna around 100MHz, with a half wavelength of about 1.5 meters, and human tissue is conductive at such low frequencies. A cellular antenna may lose more than 10dB of gain at the talk position compared to a cellular antenna. The positive human body effect is shown in Figure 2, which shows the output spectrum of the receive antenna when the user is touching and not touching the antenna. The gain is much higher in the case of hand contact, and the figure also shows that the G/T degradation is increased by 10dB to 15dB in this case.

Output spectrum of the receiving antenna when the user touches and does not touch the antenna; Design and measured performance of the RadioAnt active antenna

The design of the RadioAnt active antenna is shown in Figure 3. The radiating device is a single-turn half-ring, where the radiator on the short side of the shielding box is surrounded by the ground wire and connected to the amplifier on the other side. The antenna is short-circuited at the maximum of the GSM E field by the short-circuited antenna on the short side and the AC short-circuited antenna with a parallel capacitor on the other short side (to obtain resonance), thus ensuring low crosstalk. There is a parallel capacitor between the gate-source node at the amplifier input, which, in addition to improving the gain, also provides better noise matching (improved stability) by increasing the real part of the antenna seen by the amplifier. The amplifier uses a microwave FET transistor configuration in a common source topology to minimize noise contributions. The entire amplifier consumes 3mA at 3V, which provides sufficient gain and linearity for the application. The bias point is stabilized by dc feedback, and the noise contribution from the bias network is reduced to near zero by design. Since microwave transistors have positive gain at frequencies of 10 GHz and above, careful consideration must be given to ensure stability at the source impedance provided by the antenna. The input impedance of the radiator is only sensitive to magnetic materials (because it is a short loop), which is rare, so the antenna is not de-tuned by nearby objects. The sensitivity to GSM crosstalk was measured by placing a reference dipole antenna (824MHz to 960MHz, and 1710MHz to 2170MHz) close to the phone and connecting it to a high-power CW transmitter. The onset of signal degradation was detected at 824MHz (the worst-case frequency) at about +36dBm, which is well above the peak output power of GSM.

The measured G/T degradation and the RadioAnt gain are shown in Figure 4 and the application to a Nokia 6125 phone is shown in Figure 5. The phone can be operated in two modes, open and closed, with different performances. In general, the open position outperforms the closed position by a few dB due to the longer shielding box length, but it is expected that the closed position will be used mainly during listening. Although the gain has an in-band variation of about 20 dB, the highly resonant G/T degradation, which is an important indicator for reception and audio quality, is almost flat, with an in-band variation of about 5 dB. Therefore, tunability is not required. However, the RadioAnt module supports frequency tunability (if a control signal can be obtained from the FM receiver), which will improve the received SNR level by several dB at the band edges (especially if the entire 76 MHz to 108 MHz band must be covered) and will also improve the tolerance to strong in-band blocking signals. This is an optional feature and is not necessary to obtain good performance.

Measured G/T drop and RadioAnt gain

Measured gain of an unmatched half loop antenna with the Rx mode preamplifier still connected and from a 50 ohm source input Small antenna FM radio reception

Field tests have shown that RadioAnt achieves the same performance as wired headset-based FM radio reception , despite the difference in antenna size. Although the active antenna is almost an optimal design that provides a small footprint, the radiation impedance of about 1m combined with parasitic loss resistance of at least 1 ohm (e.g. from finite conductivity in the radiating metal and interconnects) inevitably leads to a gain of -30 dBi to -50 dBi. For most RF engineers, it is difficult to believe that such a low gain is sufficient for any application involving long-distance communication, but high ambient noise temperature greatly relaxes the requirements for FM reception gain.

Most wireless systems operate above 1 GHz, where the ambient noise is close to room temperature and a gain of -10 dBi translates into a 10 dB reduction in SNR. However, due to man-made noise, the noise level is about 20 dB in most urban areas at FM frequencies (and even higher for AM). Therefore, a very poorly efficient antenna will collect as little noise as a small signal, for example compared to a perfect dipole antenna. As shown in Figure 7, the G/T degradation is compared at different gains (0 dB, -20 dB and -40 dB) for three antennas at different noise temperatures when all antennas are connected to a 6 dB NF receiver. For the real case of a -40 dB gain antenna (46 dB reduction in G/T at room temperature), the same configuration improves the performance by 19 dB at a typical temperature of 23.000 K. For the active antenna, a further 6 dB can be gained from the receiver noise figure suppression. Therefore, the SNR performance of a -40 dB gain passive antenna is only 27 dB for a perfect dipole.

By co-optimizing the radiating device and the low-noise preamplifier, RadioAnt achieves performance similar to that of a wired headset antenna. This enables mobile phones to support wireless headsets, transmit and receive FM radio signals, and thus support more user features.