アプリケーションノート 6166

MAX20328 Adds FM Radio Antenna Path to Smartphones with USB Type-C Interface


要約:

Modern smartphones with USB Type-C interfaces gain greater market acceptance with the addition of a broadcast frequency modulation (FM) radio receiver feature. The MAX20328 manages the USB interface which can use an attached audio headset as an antenna, routing the radio frequency (RF) signal to the FM tuner module. This application note describes four different implementations and the performance of each solution. The solution with the lowest insertion loss over the FM broadcast band in the United States employs two inductors, while the loss varies 8dB to 6.4dB from the antenna to the tuner input.


Introduction

This application note describes how to use the MAX20328 in a smartphone design to add an FM band broadcast receiver function. The focus on the loss performance is over the U.S. FM band of 88MHz to 108MHz. Because there is no antenna model or FM tuner input model, the results are captured in a laboratory environment at room temperature with a signal generator and spectrum analyzer presenting a 50Ω input and output impedance.

Motivation

Many users of modern smartphones expect their mobile, handheld device to provide constant connectivity and entertainment. Some users like to use their device to listen to broadcast FM radio stations for music and talk, which does not consume the monthly data allocation of their cell phone service plan. With some extra design work to meet this user need, it is feasible to add a broadcast radio receiver feature to a smartphone during the telephone's development. Because phones have great computing power, the ability to process digital bit streams of audio content, and the inter-integrated circuit (IIC) bus for control functions, it is straightforward to "graft on" this radio receiver.

A Solution and the Antenna Challenge

Figure 1 presents a proposed signal path solution for the added FM receiver capability. The MAX20328 manages the USB Type-C interface for the smartphone and serves to route the RF signal through the coupling network to the tuner module. It also supplies protection features on the SBU1, SBU2, and the D+ and D- USB data signals. The MAX20328 contains many analog switch functions to route the USB Type-C signals under processor control. Because the USB connection can serve as an analog audio path, we borrow the SBU1 or SBU2 signals to serve as the antenna input to our FM receiver when a set of "earbuds" are attached with an audio adapter. The RF signal is obtained from the earphone (or headset) cable, which acts as the antenna. Since the SBU1 and SBU2 switches inside the MAX20328 have sufficient bandwidth, the RF antenna signal is routed through the analog ground (AGND) signal of the IC to a coupling network. The coupling network should efficiently transfer the RF energy to the tuner for downconversion, demodulation, and digitization. Because USB Type-C connectors are inserted with 0°or 180° orientations, the "antenna" can be connected to either the SBU1 or SBU2 pin 1 of the connector. The control system must select which analog switch is closed to route the desired RF signal to the AGND location.

Signal path details from antenna to FM tuner using the MAX20328Figure 1. Signal path details from antenna to FM tuner using the MAX20328.

The antenna of any radio receiver is crucial to setting the sensitivity since it captures the weak signals from the air and presents the RF energy to the various filters, amplifiers, and mixers. An efficient whip antenna should measure roughly ¼ of the signal's wavelength to do a good job capturing the signal from the distant transmitter. Using Equation 1, the geometric center frequency (fCENTER) of the USA FM band is calculated to be approximately 97.5MHz, assuming the band ranges from 88MHz (fLOWER) to 108MHz (fUPPER).

(Equation 1)

Next, use Equation 2 to calculate how long a quarter wavelength (λ) is if fCENTER = 97.5MHz with relation to the speed of light (c).

f × λ = c(Equation 2)

Using c as 2.998 × 108m/s, the wavelength is computed to be 3.075m for a 97.5MHz signal.

The length of the whip antenna is λ/4, which is 0.769m, or roughly 30in. If the earphone/headset cable is approximately this long, it acts as an antenna that provides efficient reception characteristics.

Keep in mind that this "earphone" antenna suffers from many issues in use. A user might roll up any excess cable or use an extender audio cable. Both of these conditions impact the antenna performance. However, the good news is that the desired electrical length is reasonable for a typical use scenario. Had the quarter wavelength antenna been estimated for, say, 9.75MHz operation, the length would have been approximately 300in, and this approach would suffer from serious mechanical and efficiency challenges. In that case, a different antenna technology would be advantageous.

Hardware and Lab Methodology

A small printed circuit board, configured to interface with RF test equipment and support experimentation with coupling network topology and values, was created, as shown in Figure 2.

Circuit board with SMA connectors for easy RF test equipment interfaceFigure 2. Circuit board with SMA connectors for easy RF test equipment interface.

This circuit board implements in simplified form some of the circuitry shown in Figure 1, with the exception that the USB Type-C connector is absent, and a single SMA coaxial connector is attached to the SBU1 signal. This is where the RF signal generator is connected. The tuner module is also absent and is replaced with another SMA connector such that a spectrum analyzer is connected to capture the output signal. This simple circuit board also has header pins for the IIC bus (labeled I2C in the silk screen image) connection along with power and ground connection locations. This I2C bus interface is used to set the internal configuration of the MAX20328 such that a connection is formed between the SBU1 signal and AGND.

The coupling network (Figure 3) found near the AGND signal of the MAX20328 consists of five generic PCB "0805 footprints" in an array supporting different network topologies. These five locations are labeled with R1 through R5 reference designators but can be used to mount appropriately sized R, L, C components for ease of experimentation. In this way, various topologies and values can be used and characterized without the need for a unique PCB for each idea considered. For this application note, the locations R1 and R2 have a short piece of bus-bar wire installed.

oupling network PCB layout on the AGND signal of the MAX23028 to the SMA connectorFigure 3. Coupling network PCB layout on the AGND signal of the MAX23028 to the SMA connector.

The ideal method to conduct the design of the RF coupling network for use with an antenna and FM tuner is to employ a network analyzer. The reality is that often in life we must use the test gear available in the laboratory. In this case, no network analyzer is on hand. There is, however, an HP8656B RF signal generator and an Agilent® E4411B spectrum analyzer, which cover the needed frequency range. By setting the spectrum analyzer to a peak detect mode and forcing the signal generator to produce -10dBm and to "sweep" from 1MHz to 300MHz, it is possible to simulate a scalar network analyzer.

Using the laboratory setup shown in Figure 4, the performance of four different coupling solutions are tested:

  1. A bias resistor at location R3.
  2. A single inductor at location R5.
  3. A single inductor at the VCC location.
  4. Two inductors, one at R5 and another at the VCC location.

The test setup used to measure RF performanceFigure 4. The test setup used to measure RF performance.

Inductor Selection

The inductors used in RF matching networks impact the performance obtained, so consider the following for inductor selection:

  • The Q should be high enough to pass as much of the RF energy as possible through to the tuner. Q is an inverse measure of inductor losses, so a high Q device indicates a low loss. The Q should also be examined over the frequency band of interest. An inductive device with little loss at 10MHz might produce significant loss at 100MHz.
  • The self-resonant frequency (SRF) of the inductor should be well above the frequency band of interest. This ensures that the inductor acts as an inductor, producing inductive reactance at the proper frequencies. An inductor beyond the SRF appears to act as a capacitor.

To meet the needs of high Q and high SRF, a wire-wound surface-mount device is a good solution. Murata and others manufacture surface-mount inductors that are very appropriate for the 50MHz to 300MHz frequency range, and provide good Q, small size, and sufficiently high SRF. For examples of these inductive devices, see Table 1 for typical specifications of 0603 and 0402-sized inductors made by Murata.

Table 1. Typical Wirewound RF Inductor Specifications

Manufacturer Part No. Inductance (nH) Size DC Resistance (ꭥ max) SRF (GHz) Q
Murata LQW18AN15NG00 15 0603 13 6.0 min ~40 min
Murata LQW18ANR10G00 100 0603 68 1.8 min ~34 min
Murata LQW15AN15NG00 15 0402 16 5.0 min ~30 min
Murata LQW15ANR10J00 100 0402 2.52 1.5 min ~20 min

Case 1 - No Inductors and One Resistor

The simplest solution involves rapid design speed, the fewest components, and a non-optimal frequency response. A non-optimal response solution might be fully acceptable to the end users that simply want to listen to their favorite, strong-signal local FM broadcast station. This first proposal for coupling the antenna RF signal through to the FM tuner uses no reactive components and only needs a single resistor to provide a bias path (Figure 5).

The simplest coupling network with one resistorFigure 5. The simplest coupling network with one resistor.

The MAX20328 analog switches connect directly to the USB Type-C receptacle. When FM reception is desired, the appropriate switch closes under software control using the I2C bus. The RF signal is then routed through to the AGND, which is then connected through a PCB trace to the FM tuner's input. Some FM tuners require a DC-blocking capacitor and present a non-50Ω input impedance. While this non-reactive coupling solution is not optimal, it is a baseline for insight on the intrinsic loss, and other solutions can be weighed for cost versus performance comparisons.

The spectrum analyzer measures the insertion loss frequency response in the one-resistor coupling network. Figure 6 shows the expected single-pole RC response below 100MHz. A single-pole RC response rolls off with a -6dB/octave slope.

Spectrum analyzer plot of insertion loss frequency response in the one-resistor coupling networkFigure 6. Spectrum analyzer plot of insertion loss frequency response in the one-resistor coupling network.

The x-axis on this spectrum analyzer plot is linear in the frequency domain, so the straight lines normally expected with Bode plots are distorted if extrapolated more than approximately 1 octave. A short red guideline indicates the -6dB per octave slope in the vicinity of 30MHz to 60MHz.

This simple no-inductor solution produces almost 18dB loss at 100MHz. If this receiver sensitivity solution is acceptable, this is all the RF engineering work required. Most users can compare field studies of the simplest, lowest cost solution to an improved, lower loss implementation.

Case 2 - One Inductor at AGND Location

Case 2 considers a more complex solution (Figure 7) using an inductive device to compensate for the capacitance present in the RF path.

Spectrum analyzer plot of insertion loss frequency response in the one-resistor coupling networkFigure 7. A single inductor coupling network at AGND does not require a resistor.

This approach adds a single peaking inductor at the AGND output from the AO11 device and produces a defined peak in the response at 60MHz. One minor benefit of this inductor to ground on the AGND signal is that the small DC bias currents have a path to ground, so the 332Ω resistor is not required. The 332Ω resistor to ground is also not needed in situations where the tuner input presents 50Ω impedance at DC.

The response with a single inductor at AGND produces a peak in the response, but the bandwidth is too narrow for full coverage of the USA FM band (Figure 8).

Insertion loss with a single inductor at AGND shows too narrow a bandpass responseFigure 8. Insertion loss with a single inductor at AGND shows too narrow a bandpass response.

Case 3 - A Single Inductor at VCC

If one treats the MAX20328 as a black box, the usual place to compensate for capacitance on the transmission line between the antenna and the tuner, is either on the SBU1, SBU2, or AGND signals. Due to internal design implementation, it can be efficient to compensate for switch and IC layout capacitance by using an inductor on the VCC connection of the die. Adding one inductor in the VCC path to the MAX20328 device compensates for much of the loss produced by the 50Ω RF environment working "against" the internal capacitance of the switch on the AGND signal (Figure 9).

A single inductor network at VCC to better compensate for internal capacitanceFigure 9. A single inductor network at VCC to better compensate for internal capacitance.

Over the FM band from 60MHz to 120MHz, the signal delivered to the tuner increases by 6dB to 9dB compared to the no inductor case. Beyond 180MHz, there is no appreciable difference in response. If the intended frequency band of use is restricted to these higher frequencies, the added inductor gives no significant benefit.

Compensating the internal capacitance with a VCC inductor produces a notch in the frequency response. As shown in Figure 10, the notch occurs near 25MHz. From this measured resonant notch frequency, one can estimate the effective capacitance on the AGND path of the MAX20328.

The resonant frequency of an LC circuit is calculated as follows:

(Equation 3)

Solving for the effective capacitance of the resonant circuit (CE) gives the following:

(Equation 4)

With the resonant frequency of 25MHz, and the inductance of 100nH, the effective capacitance, CE, is estimated to be 405pF.

Insertion loss with one inductor at VCC shows improved response over a wider range of frequenciesFigure 10. Insertion loss with one inductor at VCC shows improved response over a wider range of frequencies.

Case 4 - Two-Inductor Solution

Two inductors are used to form a network for efficiently conveying the RF energy of the antenna through to the tuner input, as shown in Figure 11.

The two-inductor coupling network configurationFigure 11. The two-inductor coupling network configuration.

Figure 12 shows lower loss and a wide enough peak to cover the USA FM broadcast band when compared to a single-inductor configuration..

The insertion loss response with two inductors compared with the single inductorFigure 12. The insertion loss response with two inductors compared with the single inductor.

This two-inductor solution produces a low loss response for the USA FM broadcast band between 88MHz and 108MHz. In this case, the VCC inductor is 100nH and the AGND inductor is 15nH, and the loss ranges from 7.5dB to 6.5dB, approximately.

Results Summary

Table 2 and Figure 13 illustrate the performance improvements over a range of frequencies that includes the USA FM broadcast band.

Table 2. Results Summary for Tested Configurations

Frequency (MHz)

Configuration
No Inductors
(dB Loss)
One Inductor at AGND
(dB Loss)
One Inductor at VCC
(dB Loss)
One Inductor at VCC and One Inductor at
AGND
(dB Loss)
60 11.4 7.7 4.8 12.5
90 17 15 9 8
100 17.9 16.2 10 6.7
120 18.7 18.2 10.3 6.4

Over the USA FM band, the insertion loss of the two-inductor network offers the best performanceFigure 13. Over the USA FM band, the insertion loss of the two-inductor network offers the best performance.

At 100MHz, the roughly 10dB improvement obtained with the two-inductor method results in a more sensitive FM receiver, able to tune in more distant stations with better signal quality. This extra 10dB of signal also helps to offset the expected issues of antenna orientation and length due to user behaviors.

References

Vizmuller, P., RF Design Guide: systems, circuits, and equations. Norwood, MA: Artech House, 1995.