If I use HMC472A at a low frwquency(DC~100MHz).
How much capacity AGC cap ?
In datasheet 380pF for 700 to 2700 MHz.
Or about 1nF it would be reasonable at a low freq ?
The ACG and DC blocking capacitors should behave like a short at operating frequency. The HMC472ALP4E utilizes internal ACG capacitors that are large enough for operating at frequencies greater than 700 MHz. However large capacitors should be externally connected to ACG pins for operating frequencies less than 700 MHz.
The HMC472ALP4E evaluation board uses 330pF at ACG pins and 1nF at RF pins. With these capacitor values, attenuation accuracy of HMC472ALP4E degrades below 100 MHz especially at high attenuation states (see left plot below). When ACG and DC blocking capacitors are changed to 1uF, attenuation accuracy can be improved down to 100 KHz (see right plot below). If you operate the HMC472ALP4E at frequencies lower than 100 kHz, you will need to use ACG and DC blocking capacitors larger than 1uF.
Even if large enough ACG capacitors are used, HMC472ALP4E's power handling degrades at frequencies lower than 200 MHz of which is a normal characteristic of digital attenuators in GaAs process. The plot below shows the HMC472ALP4's input P1dB at low frequency region. Maximum input power should not exceed P1dB. At frequencies where input P1dB is higher than 28 dBm, maximum input power should be set to 28 dBm.
I hope this helps.
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