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AD6630 데이터시트(PDF) 5 Page - Analog Devices |
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AD6630 데이터시트(HTML) 5 Page - Analog Devices |
5 / 8 page AD6630 –5– REV. 0 AD6630 DSP MAIN DIVERSITY AD6600 –28dBm –29dBm –14dBm LOCAL OSCILLATOR –23dBm –25dBm–10dBm–15dBm 9dBm 4dBm –1dB 15dB –9dB –2dB 15dB –5dB 24dB –5dB SAW SAW ANTENNA –104dBm –43dBm –28dBm –16dBm –15dBm AD6630 INPUT –91dBm –30dBm –15dBm –3dBm –2dBm AD6630 OUTPUT –67dBm –6dBm +9dBm +9dBm +9dBm AD6600 INPUT –71dBm –10dBm +4dBm +4dBm +4dBm AD6620 Figure 6. GSM Design Example THEORY OF OPERATION The AD6630 amplifier consists of two stages of gain. The first stage is differential. This differential amplifier provides good common-mode rejection to common-mode signals passed by the SAW filter. The second stage consists of matched current feedback amplifiers on each side of the differential pair. These amplifiers provide additional gain as well as output drive capa- bility. Gain set resistors for these stages are internal to the de- vice and cannot be changed, allowing fixed compensation for optimum performance. Clamping levels for the device are normally set by tying CLLO or CLHI pins to the negative supply. This internally sets bias points that generate symmetric clamping levels. Clamping is achieved primarily in the output amplifiers. Additional input stage clamping is provided for additional protection. Clamping levels may be adjusted to lower levels as discussed below. APPLICATIONS The AD6630 provides several useful features to meet the needs of radio designers. The gain and low noise figure of the device make it perfect for providing interstage gain between differential SAW filters and/or analog-to-digital converters (ADC). Addi- tionally, the on-board clamping circuitry provides protection for sensitive SAW filters or ADCs. The fast recovery of the clamp circuit permits demodulation of constant envelope modulated IF signals by preserving the phase response during clamping. The following topics provide recommendations for using the AD6630 in narrowband, single carrier applications. Adjusting Output Clamp Levels Normally, the output clamp level is set by tying either CLLO or CLHI to ground or VEE. It is possible to set the limit between 8.5 dBm and 12 dBm levels by selecting the appropriate exter- nal resistor. To set to a different level, CLLO and CLHI should be tied together and then through a resistor to ground. The value of the resistor can be selected using the following equation. R OUTPUT dBm CLAMP = 14 4 0 0014 . – . () This equation is derived from measured data at 170 MHz. Clamp levels vary with frequency, see Figure 5. Output clamp levels less than 8.5 dBm will result in damage to the clamp circuitry unless the absolute maximum input power is derated. Similarly, the output clamp level cannot be set higher than 12 dBm. CLAMP GENERATOR R VEE Figure 7. Clamp Level Resistor Matching SAW Filters The AD6630 is designed to easily match to SAW filters. SAW filters are largely capacitive in nature. Normally a conjugate match to the load is desired for maximum power transfer. Another way to treat the problem is to make the SAW filter look purely resistive. If the SAW filter load looks resistive there is no lead or lag in the current vs. voltage. This may not preserve maximum power transfer, but maximum voltage swing will exist. All that is required to make the SAW filter input or output look real is a single inductor shunted across the input. When the correct value is used, the impedance of the SAW filter becomes real. 400 3pF 47nH 9.7 15.2pF Figure 8. Saw Filter Model (170 MHz) EVALUATION BOARD Figures 9, 10 and 12 refer to the schematic and layout of the AD6630AR as used on Analog Devices’ GSM Diversity Re- ceiver Reference Design (only the IF section is shown). Figure 14 references the schematic of the stand-alone AD6630 evalua- tion board and uses a similar layout. The evaluation board uses center tapped transformers to convert the input to a differential signal and AD6630 outputs to a single connector to simplify evaluation. C8, C9 and L2 are optional reactive components to tune the load for a particular IF frequency if desired. |
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