National Semiconductor Application Note 2028 Gerrit Segers and Jaimy de Koning July 21, 2010 General Description Detailed Operating Description The evaluation board is designed to help the evaluation of the LMH2191 Dual Channel 52 MHz Clock Tree Driver. The LMH2191 is ideal suitable for the distribution of a system clock to peripheral devices in mobile handsets. It provides a solution to clocking issues such as limited drive capability, protection of the master clock from varying loads and frequency pulling effects, isolation from noisy modules, and crosstalk isolation. It has very low phase noise which enables it to drive sensitive modules such as Wireless LAN and Bluetooth. The setup of the evaluation board consist of three distinguished segments. The first part is the TCXO oscillator that generates the clock signal that is fed to the clock buffer. The second part is the circuit around the LMH2191 itself. The DUT needs a stable supply to be able to operate adequately. Furthermore there is the possibility to switch each CLK output ON and OFF separately using a CLK request jumper setting or an external connector. Each clock output has several different capacitive load settings which are selectable by using jumper shorts. Each clock output is also fed to an SMA connector. The third part is the output buffer, that makes it possible to monitor both clock outputs or the TCXO voltage one by one by jumper selection. The output buffer is capable to drive a 50 ohm cable in order to connect the test board to measuring equipment. The LMH2191 evaluation board is designed to have maximum flexibility in evaluating the LMH2191 in various configurations. The schematic, Bill of material and board layout can be found at the end of this document. In the following sections a description will be given on how the circuit works and how to setup the measurement bench. For the factory default jumper setting, please refer to the Configuration section. FIGURE 1. LMH2191 Evaluation Board © 2010 National Semiconductor Corporation 301137 TCXO SUPPLY The supply for this on-board oscillator is done by the LMH2191 via the VOUT pin. The VOUT voltage is decoupled by C13 and C14 and is connected to jumper J8 pin 2. For normal operation pin 1 and pin 2 are shorten and the LDO voltage is connected to the supply pin of the TCXO. If, for any reason, an external supply is needed, this supply voltage can be connected to J8 at the pins 3 and 4. Pin 3 is the supply of the TCXO and pin 4 is the ground connection. (NOTE: The jumper short for the pins 1 and 2 must be removed then). CLOCK BUFFER LMH2191 The oscillator frequency of the TCXO or any external input frequency is fed to the SCLK_IN pin of the LMH2191 (DUT). The input of the DUT is AC coupled because of an internal coupling capacitor of around 30 pF. This means that any DC voltage added to the input frequency doesn't disturb the DUT from functioning. The DUT has two clock outputs which can be switched ON or OFF independently. When both outputs are switched OFF the part is in shutdown mode and that means that the LDO is switched OFF too. In case the TCXO is powered by the on board LDO of the DUT, the input frequency is switched OFF too. The current consumption drops down to zero. ON and OFF switching of the outputs is performed by both CLK_REQ inputs. Any of the clock outputs is OFF when the matching request input is low, thus connected to ground or a voltage below 400 mV. The testboard provides two jumpers J5 and J6 and the SMA edge launch connectors CON9 and CON10 connected to the CLK_REQ inputs, to switch the clock outputs ON or OFF. Each clock request input has an internal pull down resistor of 200kΩ which means that when no jumper is placed the clock output is OFF. To force a clock output to ON the voltage on the request pin must raise above 1.4V. The voltage used to toggle the outputs ON or OFF is VBAT or an external voltage applied to the connectors CON9 and CON10. If an external voltage is used, the jumpers for J5 and J6 must be removed. www.national.com AN-2028 30113705 POWER SUPPLY The board is powered by the use of banana connections, which is mostly a preferable connector type rather than a multi pole connector. The DUT (LMH2191) uses a single supply called VBAT while the output buffer, used for monitoring the CLK outputs and the TCXO voltage, is powered by a dual supply (called V+ & V-). The supply for the DUT ranges from 2.5V to 5.5V while the supply for the output buffer (LMH6559) has a fixed voltage of +5V and –5V. The output voltage of the LDO is 1.8V and can be monitored on an extra banana connector called VOUT. Evaluation Board for the LMH2191 Evaluation Board for the LMH2191 AN-2028 In default configuration VBAT is used via the short on J5 and J6 to create a HIGH or LOW level for the CLK_REQ inputs. In case there is a need to use an external voltage to create the HIGH or LOW level on the CLK_REQ inputs this external voltage can be applied to J4 at the pins 3 and 4. Pin 3 is the ground connection and pin 4 is the alternate voltage for VBAT.(NOTE: The jumper short for the pins 1 and 2 must be removed then). Both outputs are connected to an SMA edge launch connector (CON6 and CON7) and to the header blocks J1 and J3. These headers are used to load the output with a specific capacitive load. There are four capacitors that can be chosen to load the output. The lowest capacitor value is 10 pF and the highest value is 47 pF. Using these capacitors each of the outputs can have a capacitive load varying from 10 pF to 112 pF. When no jumper is placed on J1 or J3 the capacitive load is reduced to the capacity that arises through the connected pcb tracks. When a cable connection is made at CON6 and CON7 be aware that the capacitive load increases, and that due to insufficient termination and 50 ohm drive capability the output signals can deteriorate. is included on the pcb. The output of this buffer is connected to an SMA edge launch connector (CON8). This buffer is capable to drive a 50 ohm cable and load with a high magnitude. The input of the buffer consists of a 6pin header block (J2) and has a pull down resistor of 100 kΩ to assure the output is zero in case no jumper is placed on J2. Both CLK1 and CLK2 and the TCXO output signal are connected to J2. By placing a jumper between pin 1 and 2 the CLK1 output signal is buffered and fed to CON8. Placing the jumper between pin 3 and 4 connects the TCXO signal to CON8 and with the jumper placed over the pins 5 and 6 the CLK2 signal is connected to CON8. The resistor R1, placed as a series resistor in the output line, creates the possibility to use series termination in combination with the connected cable and load. The default value for this resistor is zero ohm. Configuration The LMH2191 evaluation board can be configured via jumper settings. An overview of the various jumper positions on the board is given in Figure 2. The settings of these jumpers and their functions are listed in Table 1. BUFFERING THE CLOCK SIGNALS To be able to connect the output signals and the TCXO signal to measuring equipment a high bandwidth buffer (LMH6559) 30113706 FIGURE 2. Jumper Positions www.national.com 2 AN-2028 TABLE 1. Jumper and Header overview Bold face jumper settings refer to the factory default configuration. Jumper J1 J2 J3 Function CLK1 Capacitive Load selection Input signal selection for buffer LMH6559 CLK2 Capacitive Load selection Jumper Position Desription 1-2 Connects 10 pF from CLK1 to GND 3-4 Connects 22 pF from CLK1 to GND 5-6 Connects 33 pF from CLK1 to GND 7-8 Connects 47 pF from CLK1 to GND 1-2 CLK1 selected 3-4 TCXO selected 5-6 CLK2 selected 1-2 Connects 47 pF from CLK1 to GND 3-4 Connects 33 pF from CLK1 to GND 5-6 Connects 22 pF from CLK1 to GND 7-8 Connects 10 pF from CLK1 to GND J4 CLK Request Supply Voltage 1-2 VBAT 3-4 External Supply. 3=ground; 4=VEXT J5 Manual CLK_REQ1 setting Open J6 Manual CLK_REQ2 setting CLK_REQ1 can be controlled externally via CON9 1-2 CLK_REQ1 = GND 2-3 CLK_REQ1 = High. Open CLK_REQ2 can be controlled externally via CON11 1-2 CLK_REQ2 = GND 2-3 CLK_REQ2 = High. J7 Frequency input selection for SCLK_IN 1-2 External input frequency 2-3 On board TCXO J8 TCXO Supply 1-2 TCXO is supplied by VOUT 3-4 External Supply. 3=VEXT ; 4=ground CON1 +5V Positive Supply for the buffer LMH6559. CON2 –5V Banana Receptacle Negative Supply for the buffer LMH6559. CON3 GND Banana Receptacle Ground Connection. CON4 VOUT Banana Receptacle LDO output voltage CON5 VBAT Banana Receptacle Supply Voltage of the DUT CON6 CLK1 Banana Receptacle Clock 1 output voltage CON7 CLK2 SMA Edge Launch receptacle Clock 2 output voltage CON8 VBUF SMA Edge Launch receptacle Buffered output voltage for CLK1, CLK2 and TCXO CON9 CLK_REQ1 SMA Edge Launch receptacle Connection for external signal for switching CLK1 ON or OFF CON10 CLK_REQ2 SMA Edge Launch receptacle Connection for external signal for switching CLK2 ON or OFF CON11 Ext CLK SMA Edge Launch receptacle Connection for external clock signal. TP1 +5V SMA Edge Launch receptacle Test Point Buffer Supply TP2 –5V Test Point Buffer Supply 3 www.national.com AN-2028 Jumper Function Jumper Position Desription TP3 GND Test Point ground TP4 GND Test Point ground TP5 GND Test Point ground TP6 GND Test Point ground TP7 GND Test Point ground TP8 VBAT Measured at the connector side TP9 VBAT Measured at the DUT side TP10 VOUT Measured at the DUT side TP11 VOUT Measured at the connector side TP12 CLK1 Clock 1 output Signal TP13 CLK2 Clock 2 output Signal TP14 TCXO Supply Oscillator Supply TP15 GND Test Point ground TP16 GND Test Point ground inputs are HIGH, which means that both CLK outputs are toggling at the TCXO frequency. The clock outputs are loaded with 33 pF and can be checked with a high impedance probe on TP12 (CLK1) and TP13 (CLK2). The clock signals are also connected to SMA edge launch connectors CON6 (CLK1) and CON7 (CLK2). All three signals (CLK1, CLK2, and TCXO) can be checked via another edge launch connector (CON8) and this signal is buffered via the LMH6559 and is capable to drive a 50 ohm cable and load. Jumper J2 selects which of the three signals is chosen to appear on this connector. The best way to check for pulse shape and bandwidth is to make use of the connector CON8. Measurement Setup The performance of the LMH2191 can be measured with the setup shown in Figure 3. To verify all the functions of the part on the evaluation board it is recommended to have the following equipment available. * Multimeter * Power Supply for dual supply of 5V and single supply of 3.5V * Oscilloscope * Signal Generator Using the default settings, the TCXO is active and its signal is connected to the input of the LMP2191. Both CLK_REQ 30113703 FIGURE 3. Test Setup www.national.com 4 AN-2028 Schematic 30113704 FIGURE 4. Evaluation Schematic 5 www.national.com AN-2028 Bill of Material Board Layout The Bill of Material (BOM) of the evaluation board is given in Table 2. As with any other device, careful attention must be paid to the board layout. If the board isn't properly designed, the performance of the device can be less than might be expected. Especially the input clock trace (SCLK_IN) and output traces (CLK1/2) should be as short as possible to reduce the capacitive load observed by the clock outputs. Also proper decoupling close to the device is necessary. Beside a capacitor in the µF range, a capacitor of 100 nF on VBAT and VOUT is recommended close to the device. The equivalent series resistance (ESR) of the capacitors should be sufficiently low. A standard capacitor is usually adequate. The component locations of the evaluation board are depicted in Figure 5 and Figure 6. The copper layers of are depicted in Figure 7 and Figure 8. TABLE 2. Bill of Material Designator Description Comment C1, C3, C12 Case A Capacitor 4.7 µF C2, C4, C5, C10, C11, 0603 Capacitor C14 0.1 µF C6, C16 0603 Capacitor 10 pF C7, C17 0603 Capacitor 22 pF C8, C18 0603 Capacitor 33 pF C9, C19 0603 Capacitor 47 pF C13 Case A Capacitor 2.2 nF C15 1206 Capacitor 0.1 µF C20 0603 Capacitor 0.01 µF C21 0603 Capacitor 470 pF CON1, CON2, CON3, CON4, CON5 Connector Banana CON6, CON7, CON8, CON9, CON10 Connector SMA J1, J3 Header 4x2 J2 Header 3x2 J4, J4 Header 2x2 J5, J6, J7 Header 3x1 J9 Test Socket 8 pin J10 Test Socket 6 pin R1, R4, R5, R6, R7, R8, R10 0603 Resistor 0Ω R2, R11 0603 Resistor 10 kΩ R3 0603 Resistor 100 kΩ R9 0603 Resistor TP1, TP2, TP3, TP4, TP5, TP6, TP7, TP8, TP9, TP10, TP11, TP12, TP13, TP14, TP15, TP16 Test Point 51Ω NC U1 IC LMH6559MA U2 IC LMH2191TM X1 TCXO - 26.000 MHz Kyocera Corporation Part#: KT2520F2600 0ACW18TAL www.national.com 6 AN-2028 30113707 FIGURE 5. Component Locations Top Side 30113710 FIGURE 6. Component Locations Bottom Side (View from Bottom side) 7 www.national.com AN-2028 30113711 FIGURE 7. Top Layer of Evaluation Board 30113712 FIGURE 8. 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