关于AD8347自带AGC的一些问题

dushan1

我把ad8347的agc接成片内控制，射频输入前加了一级20dB增益的放大器，用信号源输入单载波信号-55dBm,此时测量agc的电压为0.67V,-45dBm入，电压为0.78V,输出功率基本保持不变。如果我再在前面加LNA模块，增益35dB,NF为1dB,内有带通滤波器带宽大概40MHz.用信号源输入-105dBm信号，此时agc的电压为0.67V,此时我认为是噪声功率发挥了主要作用，但是为什么我把信号加到-95dBm时，agc的电压才为0.68呢？输出也相应增加了10dB.好像agc没有起作用。请各位高手不吝赐教。谢谢!

dushan1

没有人知道吗？

RFsage

According to your description, the power input to AD8347 is  -35 ~ -25dBm after one amplifier and its output voltage is 0.67V ~0.78V. right?
When you inject power with -95dBm, and the power input to AD8347 will be -40dBm due to the total gain 55dB (LNA 35dB+ driver Amp 20dB). So the voltage was 0.67V around.
Please to check AD8347 operating range and filter frequency respone.

dushan1

thank you ,i have the answer.when the input power is -105dBm,after amplified is -50dBm,but the noise power within 40MHz bandwith also amplified,about -35dBm.so the voltage is 0.67V,when the input power increase to -95dBm,after amplified is -40dBm .the total pow still is about -35dBm (think about the noise power) .so the volatge dont change.
    but what shall i do ,if i want to keep the output power constant when the input power is -95dBm.thanks!

RFsage

If the linear range of AD8347 is from 0.67V to 0.78V, and it map to its input power from -35dBm to -25dBm. So the input power -95dBm must be added higher gain (more amplifier) to achieve enough power for AD8347.
For the AGC function, what is your design concept?
AS I know: If we hope the input power is variety and the output power is fixed.
Design concept:
The input path need one coupler device, and the coupling power transfer to voltage by AD83xx. The mapping voltage by AD83xx to control anttenuator of main path, and the output power can fixed constant.