Measured RF Performance Summary

Transcription

1 Summary Application Note The AP603 is a high dynamic range power amplifier in a lead-free/rohs-compliant 5x6mm power DFN SMT package. It features an internal active-bias circuit that provides temperature compensation and dynamic bias adjustment. The input and output matching networks are realized with surface mount components allowing the part to be tuned over a wide range of frequencies. This application note examines the performance of the AP603 in a push-pull configuration tuned for MHz. In this frequency range, the amplifier has approx. 14.5dB gain, good gain flatness, +41 dbm P1dB, and 2.5% EVM ( OFDMA, 64QAM-1/2,1024-FFT, 20 symbols, 30 subchannels signal, %) at 7dB of back-off from P1dB. Measured RF Performance Summary Frequency (MHz) Units Small Signal Gain db Input Return Loss db Output Return Loss db dbm/tone dbc ( f=1mhz) dbm/tone dbm 72.5 na na 7dB back-off % Output P1dB dbm Quiescent Current, Icq ma 320 Vpd, Vbias V +5 Vcc V Measured S-Parameters (db) m3 m3 freq= 450.MHz db(s(2,1))=14.8 m7 m7 freq= 577.MHz db(s(2,1))=14.1 m5 m5 freq= 800.MHz db(s(2,1))= (db) Frequency (MHz) db(s(2,1)) db(s(1,1)) db(s(2,2)) WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #1 August 2008

2 Table of Contents Summary...1 Table of Contents...2 List of Figures...2 List of Tables...2 Background...3 Discussion of the Push-Pull Topology...3 AP603 Push-Pull Reference Design...3 Matching Network Design Methodology...3 Matching Network Measurement-Based Tuning Method...8 AP603 Push-Pull Reference Design Schematic...8 Bias Circuit Description...8 Board Layout...11 Bill Of Material...12 Measured Results...13 List of Figures Figure 1 - Push-Pull Amplifier Block Diagram...3 Figure 2 Load impedance as seen by AP603 with output matching network tuned from 400MHz to 800MHz with 50Ω and 25Ω ports. As shown in Figure 3, S11 is for the 50Ω case and S22 is for the 25Ω case...4 Figure 3 AP603 Output Matching Network with Zo = 50Ω and 25Ω...5 Figure 4 AP603 with Input and Output Matching for MHz...6 Figure 5 S-parameters with Input Matching Network Tuned for S11 < -10dB and Flat Gain...6 Figure 6 Push-Pull AP603 AWR Simulation Schematic...7 Figure 7 Simulated Small-Signal Performance in Push-Pull Configuration...7 Figure 8 - AP603 Push-Pull Simulated Stability...8 Figure 9 - Input Impedance Measurement using a 'Pig-tail'...8 Figure 10 AP603 Push-Pull Reference Design Schematic...10 Figure 11 AP603 Push-Pull Evaluation Board Showing Vpd and Vcc Jumper Wires...11 Figure 12 Picture of Assembled PCB...12 Figure 13 - Measured Performance vs Output Power...13 Figure 14 - S-parameters with Vcc = 18V, 28V, 32V...14 List of Tables Table 1 - EVM vs Output Power and Frequency ( O-FDMA, 64QAM-1/2, 1024-FFT, 20 symbols and 30 subchannels)...14 Table 2 - P1dB vs Vcc and Frequency...14 WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #2 August 2008

3 Background Discussion of the Push-Pull Topology The push-pull topology consists of two amplifiers operating in parallel with signals 180 out of phase. One implementation (Figure 1) uses a transmission-line balun to convert a single-ended input signal to a differential signal that is applied between the inputs of the two parallel amplifiers. Matching networks () are used at the input and output to present appropriate impedances to the device terminals. At the output of the amplifiers a second balun is used to convert the differential signal back to a single-ended signal. Figure 1 - Push-Pull Amplifier Block Diagram 25Ω 25Ω 50Ω 50Ω 25Ω 25Ω At the cost of using two amplifiers (and thus twice the power consumption) and two baluns the following benefits are achieved: 1. 3dB higher P1dB since the output power of the amplifiers is combined each amplifier operates 3dB lower than the total output power of the push-pull amplifier, 2. Reduction of 2 nd order IMDs (higher OIP2) the 2 nd order distortion products of each amplifier are in-phase and therefore cancel because the output signal is taken differentially. Because the 2 nd order rejection is dependent on how well the two signal paths are matched, push-pull amplifiers are sometimes implemented with adjacent transistors on a single die to minimize process variation. The AH22S is an example of this. 3. Lower Q impedance transformations resulting in wider bandwidth the baluns typically have 50Ω unbalanced input impedance and 25Ω balanced input impedance so the input and output matching networks have lower impedance transformations compared to a single-ended amplifier and hence lower Q and wider bandwidth. Because of the rejection of 2 nd order distortion products (IMD2), push-pull amplifiers are often used in broadband applications in which these products fall in-band such as in cable TV and public radio applications. AP603 Push-Pull Reference Design Matching Network Design Methodology Prior to this work an AP MHz to 800MHz reference design had been created 1 using a standard AP60x Rogers Ultralam eval board. The load impedance of this design (as seen by the AP603) was simulated with AWR (Figure 2) and the output matching network was retuned to provide a similar load impedance with Z o = 25Ω rather than 50Ω (Figure 3). The input match was then tuned (Figure 4) to provide a broadband input match from 400MHz to 800MHz (Figure 5). Note, a large-signal ADS model was not available at this time and s- parameters (available from the website) were used. The small-signal performance was then simulated (Figure 6) in a push-pull configuration. 1 TQS Application Note AP MHz Reference Design WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #3 August 2008

4 Figure 2 Load impedance as seen by AP603 with output matching network tuned from 400MHz to 800MHz with 50Ω and 25Ω ports. As shown in Figure 3, S11 is for the 50Ω case and S22 is for the 25Ω case. Z load MHz r Ohm x Ohm 800 MHz r Ohm x Ohm MHz r Ohm x Ohm Swp Max 800MHz MHz r Ohm x Ohm S(1,1) load S(2,2) load Swp Min 400MHz Zo = 25Ω Zo = 50Ω WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #4 August 2008

5 Figure 3 AP603 Output Matching Network with Zo = 50Ω and 25Ω CAP ID=Cblk1 C=10000 pf ID=TL2 W=32 mil L=1700 mil ID=L_via L=1 nh PORT P=1 Z=50 Ohm ID=TL3 W=43 mil L=150 mil ID=L2 L=6.8 nh ID=TL1 W=43 mil L=350 mil CAP ID=C1 C=22 pf CAPQ ID=C2 C=3.3 pf Q=100 FQ=1000 MHz ALPH=1 RES ID=R1 R=50 Ohm CAP ID=Cblk2 C=10000 pf ID=L_via L=1 nh ID=TL5 W=32 mil L=1700 mil ID=L_via L=1 nh PORT P=2 Z=50 Ohm ID=TL6 W=43 mil L=150 mil ID=L5 L=4.2 nh ID=TL4 W=43 mil L=350 mil CAP ID=C3 C=22 pf MSUB Er=2.45 H=14.7 mil T=1.4 mil Rho=1 Tand=0.002 ErNom=2.45 Name=SUB1 RES ID=R2 R=25 Ohm WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #5 August 2008

6 Figure 4 AP603 with Input and Output Matching for MHz CAP ID=C_bypass C=1e4 pf PORT P=1 Z=25 Ohm CAP ID=Cblk1 C=27 pf ID=TL7 W=42 mil L=41 mil RES ID=R1 R=5.1 Ohm ID=TL4 W=42 mil L=126 mil ID=TL1 W=42 mil L=160 mil SUBCKT ID=S2 NET="AP603" 1 2 ID=TL3 W=32 mil L=1700 mil ID=TL6 W=43 mil L=150 mil ID=L_via L=1 nh ID=L6 L=4.7 nh ID=TL2 W=43 mil L=350 mil CAP ID=C1 C=22 pf PORT P=2 Z=25 Ohm ID=L8 L=4.3 nh ID=L_via L=1 nh CAPQ ID=C4 C=14 pf Q=100 FQ=1000 MHz ALPH=1 ID=L_via L=1 nh CAPQ ID=C3 C=16.8 pf Q=100 FQ=1000 MHz ALPH=1 ID=L_via L=1 nh 3 MSUB Er=2.45 H=14.7 mil T=1.4 mil Rho=1 Tand=0.002 ErNom=2.45 Name=SUB1 Figure 5 S-parameters with Input Matching Network Tuned for S11 < -10dB and Flat Gain 20 AP603 AMP 400 to 800 MHz 25 ohms MHz 16.7 db 600 MHz 15.6 db 800 MHz 16.7 db 0 (db) DB( S(2,1) ) AP603 wideband tuning 25ohms DB( S(1,1) ) AP603 wideband tuning 25ohms DB( S(2,2) ) AP603 wideband tuning 25ohms Frequency (MHz) WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #6 August 2008

7 Figure 6 Push-Pull AP603 AWR Simulation Schematic AB=80 PORT P=1 Z=50 Ohm 3 BALUN1 ID=BU1 Zo=50 Ohm L=500 mil Er=1 A=AB F=1000 MHz N=10 AL=100 nh 1 SUBCKT ID=S1 NET="AP603 wideband tuning 25ohms" BALUN1 ID=BU2 Zo=50 Ohm L=500 mil Er=1 A=AB F=1000 MHz N=10 AL=100 nh 3 2 SUBCKT ID=S2 NET="AP603 wideband tuning 25ohms" 2 PORT P=2 Z=50 Ohm 1 2 Figure 7 Simulated Small-Signal Performance in Push-Pull Configuration 20 AP603 Push Pull Simulated Performance MHz 15 db 670 MHz 14.3 db 860 MHz 13.2 db (db) 0 DB( S(2,1) ) AP603 Push Pull DB( S(1,1) ) AP603 Push Pull DB( S(2,2) ) AP603 Push Pull Frequency (MHz) WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #7 August 2008

8 Figure 8 - AP603 Push-Pull Simulated Stability 10 AP603 Push Pull Stablity 8 6 K() push pull Frequency (MHz) Matching Network Measurement-Based Tuning Method Subsequently, an evaluation board was fabricated and tuned based on measurements. The method for tuning the input matching network involved biasing both amplifiers and using a pig-tail co-axial line to measure the input impedance of one of the input matching networks with the unbalanced terminal of the balun terminated to ground with a surface mount 25Ω resistor (Figure 9). Note, the network analyzer has Zo = 50Ω and this introduces an error to the measurement, but this technique is still useful for matching the amplifier. When tuning, of course, component values should be changed in both amplifier matching networks at the same time. A similar technique can also be used to measure the load-line, or output return loss. Figure 9 - Input Impedance Measurement using a 'Pig-tail' 50Ω termination 50Ω termination 25 Ω 0603 SMT resistor Coaxial Pigtail Network Analyser AP603 Push-Pull Reference Design Schematic The AP603 push-pull reference design (Figure 10) uses Anaren 3A325 baluns (with a power rating of 275W) and 0603 surface mount components for the matching networks. Many inexpensive surface-mount baluns such as a M/A-Com ETC are available, but are only rated to 1W. For an amplifier with a P1dB of 42 dbm and a saturated Pout in the approximate range of dbm, a balun with a power rating of at least 50W is required. The design of the matching networks has already been discussed. Bias Circuit Description The AP603 features an internal active bias circuit. The Vpd (power down) pin (pin 14) is used to supply a reference current to the bias circuit. The pin 1 provides bias to an internal current buffer for the bias circuit. This bias is provided using Vcc (28V) dropping through a 2K resistor (R15 and R18). The Vcc pins (pins 9, 10 and 11) supply bias to the collector of the RF device. HBT power amplifiers can use emitter resistive ballasting to mitigate thermal runaway. For this circuit, Vpd must be shut off before Vcc because with Vpd biased Vcc will draw a small amount of current (approx 30mA) when Vcc is less than 15V WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #8 August 2008

9 The AP603 push-pull reference design draws bias voltage through Vcc pin (28V) to ensure bias is not applied to the pin1 before the Vcc pin. The trade-off is efficiency versus simplicity. Ibias is approx 8mA for the AP603. The bias sequence circuit allows Ibias to be sourced from 28V. Q1-B acts as a PNP switch to removes Vpd to the device when Vcc is not present. Q2 controls the reference current to Q1-B. In case of no supply on Vcc, Q1-B switch shuts off and therefore, no current on Vpd. AP603 can be toggled ON-OFF by using the ENABLE_N pin. This pin can be controlled by a 3.3V (2.5 to 5V compliant) logic device such as a microcontroller. ENABLE_N is used to control the reference current to Q1-B through Q2. AP603 can also be switched on-off using Vpd (power down) pin. WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #9 August 2008

10 Figure 10 AP603 Push-Pull Reference Design Schematic Board Substrate: FR4; Substrate thickness: 21 mil; εr = 4.2 Bias Feed Line: Width = 30 mil, Spacing = 35 mil; 50 Ω trace: Width = 39 mil, Spacing = 50 mil WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #10 August 2008

11 Board Layout Application Note The evaluation board uses 21mil thick FR4 (εr = 4.2). The 50Ω traces are 39mil wide with 50mil spacing to the top-layer ground. The bias feed is 30mil wide with 35mil spacing to the top-layer ground. Vpd, Vbias and Vcc are provided to the lower amplifier using backside jumper wires. Figure 11 AP603 Push-Pull Evaluation Board Showing Vpd and Vcc Jumper Wires WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #11 August 2008

12 Figure 12 Picture of Assembled PCB Assembled Board Picture is for the Rev0 board. Substrate: FR4; Substrate thickness: 21 mil; εr = 4.2 Bill Of Material Item No. Ref Des Value Part Style Size 1 U1 U2 AP603-F DFN14 5x6mm 2 B1 B2 na Surface mount balun - Anaren 3A mil x 890mil 3 Q1 na Transistors DDA123JH SOT C22, C16, C11, C9, C7, C10, C pF Chip cap C5, C18 15pF Chip cap C3, C19 18pF Chip cap C1, C2 27pF Chip cap C17, C24 6.8pF chip cap C6, C21 4.7pF Chip cap C13, C uF Chip cap C12, C8 10uF Tantalum J2 J6 na RF edge connector na 13 D1 5V zener diode na 14 D2 33V zener diode na 15 D3 6.8V zener diode SOT L4, L5 3.6nH Wire wound chip inductor L1, L6 No load Wire wound chip inductor L2, L3 5.6nH Wire wound chip inductor R7 1.0k Chip resistor R6, R4 10k Chip resistor R15, R16 2.0k Chip resistor R9, R2 698 Chip resistor R3, R10 4.3ohm Chip resistor C20, C4 22pF Chip capacitor FB1, FB2 22 ohm, 4A Ferrite Bead J1 na 4-pin DC connector na 27 Q2 na Transistors - UMX1N na WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #12 August 2008

13 Measured Results Figure 13 - Measured Performance vs Output Power Push-Pull AP603 - Gain vs Output Power (CW, Icq = 320mA, Vcc = 28V) Gain (db) MHz 600 MHz 800 MHz Output Power (dbm) Push-Pull AP603 - Collector Efficiency vs Output Power (CW, Icq = 320mA, Vcc = 28V) Push-Pull AP603 - Icc vs Output Power (CW, Icq = 320mA, Vcc = 28V) Efficiency (%) MHz 600 MHz 800 MHz Icc (A) MHz 600 MHz 800 MHz Output Power (dbm) Output Power (dbm) -10 IMD3 vs Output Power (CW, Icq = 320mA, Vcc = 28V, 1 MHz Tone Spacing) OIP2 vs Output Power (CW, Icq = 320mA, Vcc =28V, T1 = 400MHz, T2 = 401MHz) IMD3 (dbc) Output Power per Tone (dbm) OIP2 (dbm) IMD3 low side 400MHz IMD3 low side 600MHz IMD3 low side 800MHz IMD3 high side 400MHz IMD3 high side 600MHz IMD3 high side 800MHz Output Power per Tone (dbm) WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #13 August 2008

14 Table 1 - EVM vs Output Power and Frequency ( O-FDMA, 64QAM-1/2, 1024-FFT, 20 symbols and 30 subchannels) Frequency Pout Icc EVM (MHz) (dbm) (ma) (%) Table 2 - P1dB vs Vcc and Frequency Vcc P1dB (dbm) (V) 400 MHz 600 MHz 800 MHz Figure 14 - S-parameters with Vcc = 18V, 28V, 32V 20 Push-Pull AP603 - S21 vs Vcc vs Freq 0 Push-Pull AP603 - S11 vs Vcc vs Freq (db) freq, MHz 18V 28V 32V -20 (db) freq, MHz (db) Push-Pull AP603 - S22 vs Vcc vs Freq Push-Pull AP603 - Stability Factor vs Vcc vs Freq (db) freq, MHz freq, GHz WJ Communications, Inc Phone WJ FAX: sales@wj.com Web site: Page #14 August 2008