Help selecting the RF power detector/controller

Hi,

I am designing a broad band 40 dBm power amplifier (PA) in the frequency range of 0.5-2.5 GHz. I would like to implement Automatic Gain Control (AGC) to precisely control the output power of the PA. After much research, I decided to use HMC1120, mainly because from the RMSOUT vs. Pin diagram (as shown below), it appeared to me that HMC1120 offers consistent RMSOUT for my frequency range of 0.5-2.5 GHz and also good sensitivity. (I understand that I need to convert the RMSOUT with something like AD8062 to provide a negative loop for AGC). 

When I started looking for variable gain amplifiers (VGA), the ADL5330 appeared to be an attractive choice. However, going through the datasheet, I noticed that the example shown in Figure 37 of the ADL5330 datasheet (shown below) has used AD8318 for detecting the power in AGC. 

I started exploring the AD8318 datasheet and noticed that Figures 42 and 43 of the datasheet shows a similar diagram.

AD8318 also appeared to offer consistent Vout for my frequency range but obviously the slope is negative and this will eliminate the need for converting the slope:

More research led into the Figure 44 of the ADL5513 datasheet, which also offers consistent Vout for my frequency range (as shown on page 8 of the data sheet) 

So, now I am confused as to which components to use. Here are my questions:

1) Can you please explain which component I should use in my design (HMC1120, AD8318, or ADL5513) and why? 

2) As to VGA, can I use ADL5330 to cover the frequency range of 0.5-2.5 GHz? Is there a better choice that I should look into?

3) I am planning to use a directional coupler at the output of the PA, and then connect the coupler (with correct attenuation) to the HMC1120 to monitor output power. In addition, I need to use a gain stage between my PA and ADL5330 to provide sufficient input for my PA.  That is, I will have three "stages" or "modules" in my control loop. Would the same control loop --such as the one shown in Figure 42 of the AD8318 datasheet-- work stably in my configuration? (In all examples --such as Figure 42 of the AD8318 datasheet-- the AGC is a single stage.). 

Thanks again for your help. 

Parents Reply
  • For most precise power leveling up at the high end of the power scale, a linear-in-Volts/Volt rms detector is very precise at the max power, while trading off some accuracy down at lower power levels. If your RF signal is always a constant envelope signal, then a log power detector can be calibrated to give excellent accuracy over a very wide power range and across temperature. From the RF power detector homepage, you can choose rms or log, then there will be a separate online product selector guide for each.  

    Thanks Bruce for your help. I am designing this amplifier for RF power applications and the input will be constant amplitude sine waveform. Based on what you mentioned, I think the best shot would be to use linear-in-volts detector (something like ADL5511?) because I need precise measurement at high power (lets say 0.5-10W) and I do not care about the low end. The RF power detector search does not appear to filter based on linear-in-volt or dB. Is that right or am I missing something? Are there any component similar to ADL5511 that comes with the controller?

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