L'ultimo numero della newsletter Radio TechCheck del NAB affronta un problema apparentemente marginale ma a tutti gli effetti critico nel contesto del moltiplicarsi di dispositivi telefonici e mp3 che incorporano uno stadio di ricezione FM. E' il problema dell'antenna, che a queste frequenze non è per niente banale, considerando le lunghezze d'onda in gioco. Come forse sapete, quando il ricevitore FM è così minuscolo, il filo d'antenna coincide con il cavetto dell'auricolare. Una soluzione poco ottimale perché basta un leggero movimento per indurre evanescenza e rumore. TechCheck fa notare come l'ultima generazione di radio "single on chip" basate su componentistica Silicon Labs (un nome che i miei lettori ormai conoscono bene), risolva invece il problema con antenne molto corte ma incorporate e soprattutto attive, attraverso un circuito, un vero e proprio antenna tuner, che modifica elettricamente la frequenza di risonanza, il Q, dell'ingresso. TechCheck rivela anche l'uscita di un breve white paper in cui Silicon Labs discute il funzionamento dell'antenna attiva e sintonizzabile integrata a bordo dei chip della famiglia Si4704/05. Il documento è gratuito ma bisogna compilare un formulario presentandosi come responsablie di un progetto. Se non avete la mia stessa faccia tosta fatevi vivo ché vi mando il pdf in forma privata. Nel 4704/05, basato su near zero IF e DSP di demodulazione e filtraggio, un commutatore permette di passare dall'ingresso con auricolare come antenna a una eventuale antenna integrata che viene sintonizzata da un varactor integrato sul chip e un algoritmo che ne calcola il valore ottimale alla variazione della sintonia.
An increasing number of portable consumer electronics devices are including radio broadcast receivers, with the most exciting recent examples being the 5th-generation iPod nano (which includes an FM radio) and the Zune HD (with an FM-band HD Radio IBOC receiver™). Invariably one of the biggest challenges faced by manufacturers of these products is how to incorporate the receiver antenna into the design.
For both of the new products just mentioned, the headphone cord is also the FM receiver antenna, but for a variety of reasons it would be more desirable to incorporate the antenna into the device itself. One of the leading manufacturers of FM receiver integrated circuits (ICs), Silicon Labs (Austin, Texas, www.silabs.com) has published an interesting white paper which discusses some of the technical issues surrounding how to embed FM receiver antennas in portable devices. A good example of a device with an integrated FM antenna is the Motorola ROKR™ EM35 cell phone (see the March 23, 2009 issue of Radio TechCheck for more information on cell phones with FM radios and integrated FM antennas).
Entitled “Solving FM Antenna Design Challenges in Portable Devices,” the SiLabs white paper focuses on the theory behind and implementation of so-called “tuned antennas,” referring to an antenna which incorporates tunable, resonant elements allowing for optimization of antenna performance at a particular frequency. One of SiLabs’ FM receiver IC products, the Si4704/05, supports the use of a short, embedded antenna. The Si4704/05 implements a self-tuned resonant network with a patented advanced signal processing algorithm. The antenna algorithm tunes the capacitance value of an on-chip, tunable varactor to ensure the antenna stays at its highest efficiency for each frequency tuned, resulting in optimum performance.
The graph below illustrates this operation. If an FM receiver using the SiLabs IC is tuned to station #1 in the graph (around 100 MHz), the antenna algorithm tunes the antenna resulting in the peaked response shown (with a high quality factor, Q, of 20). For receiving station #2 (around 84 MHz in the graph), the algorithm would re-tune the antenna such that the peaked response is aligned with the new frequency.
Also shown in the graph is the response of a low-Q antenna (with a Q=3) representing the response of an equivalent short antenna designed to receive the entire FM band without tuning. For this example, implementation of the tuned antenna increases the antenna gain by approximately 18 dB compared to an equivalent untuned short antenna. An added benefit of using a tuned antenna is improved receiver selectivity. In addition to increasing the gain, resonating the antenna at the desired frequency also attenuates interference at other frequencies, significantly increasing the selectivity of the receiver.
A copy of the full Silicon Labs white paper is available for download from the Silicon Labs Web site. Additional information on the iPod nano with FM receiver is available on the Apple Web page at www.apple.com/ipodnano; see www.zune.net/en-us/products/zunehd/default.htm for additional information on the Zune HD. The Motorola ROKR EM35 cell phone with built-in FM radio and integrated FM antenna is described on the Motorola Web page at www.motorola.com/motoinfo/product/details.jsp?globalObjectId=275#.
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