数据搜索系统,热门电子元器件搜索 |
|
AD6122 数据表(PDF) 11 Page - Analog Devices |
|
AD6122 数据表(HTML) 11 Page - Analog Devices |
11 / 20 page REV. B AD6122 –11– THEORY OF OPERATION The CDMA Transmitter IF Subsystem (Figure 21) consists of an I and Q modulator with a divide-by-two quadrature genera- tor, high dynamic range IF amplifiers with voltage-controlled gain, a low dropout regulator and power-down control inputs. I and Q Modulator The I and Q modulator accepts differential quadrature baseband inputs from CDMA baseband converters. The LO is injected at twice the IF frequency. A divide-by-two quadrature generator followed by dual polyphase filters ensures ±1° quadrature accu- racy (Figure 22). For 500 mV p-p differential I and Q input signals, the output power of the modulator will be –21 dBm referred to 1 k Ω when the output of the modulator is loaded with a 1 k Ω differential load. With the maximum input conditions stated above, the modulator outputs are a 225 µA p-p differential current; conse- quently, the output load will greatly affect the output power of the modulator. 2 180 2 POLYPHASE FILTERS I Q I Q 2 IF LO INPUT QUADRATURE OUTPUT TO MODULATOR Figure 22. Simplified Quadrature Generator Circuit The I and Q modulator also provides a common mode reference signal at the MODCMREF pin. This voltage is a dc voltage set to 1.408 V when a 2.7 V supply is used. It is used to dc bias the output of the DAC that provides I and Q inputs to the modulator. IF Amplifiers and Gain Control The IF amplifiers provide an 86 dB linear in dB gain control range. The input stage uses a differential, continuously variable attenuator based on Analog Devices’ patented X-AMP™ topol- ogy. This low noise attenuator consists of a differential R-2R ladder network, linear interpolator and a fixed gain amplifier. The IF amplifier’s input impedance is 1 k Ω differential. Similar to the I and Q modulator’s output, the IF amplifier’s output is a differential current, which will vary depending upon the gain control voltage. In order to achieve the specified gain, the out- put of the IF amplifiers should be loaded with a 1 k Ω differen- tial load. The gain control circuits contain both temperature compensa- tion circuitry and a choice of internal or external reference for adjusting the gain scale factor. The gain control input accepts an external gain control voltage input from a DAC. It provides 97 dB of gain control range with a nominal 75 dB/V scale factor. The external gain control input signal should be a clean signal. It is recommended to filter this signal in order to eliminate the noise that results from the DAC. If a noisy signal is used for the gain control voltage, VGAIN inband and adjacent channel noise peaking can occur at the output of the AD6122. A simple RC filter can be employed, but care should be taken with its design. If too big a resistor is used, a large voltage drop may occur across the resistor, resulting in lower gain than expected (as a result of a lower voltage reaching the AD6122). An RC filter with a 20 kHz bandwidth, employing a 1 k Ω resistor is appropri- ate. This results in an 8.2 nF capacitor. The resulting circuit is shown in Figure 23. Note that the input resistance at the VGAIN pin is approximately 100 k Ω. FROM BASEBAND CONVERTER AD6122 VGAIN 1k 8.2nF 109k Figure 23. Gain Voltage Filtering I INPUT Q INPUT LOCAL OSCILLATOR INPUT ATTENUATOR COMMON-MODE REFERENCE OUTPUT VREG POWER- DOWN 1 1.23 V REFERENCE OUTPUT VPOS TEMPERATURE COMPENSATION GAIN CONTROL VOLTAGE INPUT GAIN CONTROL REFERENCE VOLTAGE INPUT TRANSMIT OUTPUT IF AMPLIFIERS QUADRATURE MODULATOR VCC POWER- DOWN 2 IF AMPLIFIER INPUT QUADRATURE MODULATOR OUTPUT 2 AD6122 GAIN CONTROL SCALE FACTOR LOW DROPOUT REGULATOR Figure 21. Block Diagram X-AMP is a trademark of Analog Devices, Inc. |
类似零件编号 - AD6122 |
|
类似说明 - AD6122 |
|
|
链接网址 |
隐私政策 |
ALLDATASHEETCN.COM |
ALLDATASHEET是否为您带来帮助? [ DONATE ] |
关于 Alldatasheet | 广告服务 | 联系我们 | 隐私政策 | 链接交换 | 制造商名单 All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |