I am designing a transmitter and receiver part of LoRa and GPS. It requires DC Blocking capacitors. I was looking for capacitor over digikey and I am amazed most of the manufacturer haven't specified
impedance vs frequency curve.
I am interested in RF capacitor which can be used as DC blocking capacitor and pass signal of 900 MHz (LoRa) and 1.5 GHz (GPS).
Even if very few manufacturer have specified the graph but it looks like they have SRF at around 200 MHz and I think they can't be used in my application. Should I ignore SRF and check impedance at my frequency of interest? I am clueless now how to select the capacitor for RF.
ESR is also specified at 100 kHz or some other low frequency but I want to interpolate it to my frequency of interest. I came across one application note which states that
If, for example, you are designing for a 900 MHz wireless application, and the ESR is specified at 150 MHz, the ESR at 900 MHz may be calculated by multiplying the specified ESR at 150 MHz by √ 900/150.
Is it correct? Can someone comment on what other parameters I should consider while selecting capacitor for RF application? I have mentioned few of the parameters below. Please someone validate my understanding.
- Tolerance- I am looking for capacitor having less than 5% tolerance.
- Voltage Rating: I am selecting caps having voltage double of my max RF signal voltage.
- Quality Factor: I am looking for high Q.
- ESR: I am interpolating ESR to my frequency of interest and trying to select the lowest ESR caps available on the digikey.