Is the QT-FHP-3G-18G a 3–18 GHz band-pass filter?
No, it is a high-pass filter. The datasheet specifies a 3.0 GHz cutoff, a 3.0–18 GHz operating passband, and at least 60 dB rejection from DC to 2.6 GHz. 18 GHz is the upper limit of the stated passband; it does not mean rejection is guaranteed above 18 GHz. Treating this part as a 3–18 GHz band-pass filter would leave the upper stopband unprotected.
Key points
- High-pass topology: passes above 3.0 GHz, rejects below
- Rejection of 60 dB or better from DC to 2.6 GHz
- 18 GHz is the passband ceiling, not a stopband specification
- For upper-stopband suppression, add a separate low-pass element
- Insertion loss is 2.0 dB or better from 3.0–3.2 GHz and 1.0 dB or better above 3.2 GHz
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