ZHCSIC4C June 2018 – November 2018 LMX2615-SP
PRODUCTION DATA.
Another strategy is to choose an inductor pull-up (L). This allows a higher impedance without any concern of creating any DC drop across the component. Ideally, the inductor should be chosen large enough so that the impedance is high relative to the load impedance and also be operating away from its self-resonant frequency. For instance, consider a 3.3 nH pull-up inductor with a self-resonant frequency of 7 GHz driving a 25-Ω spectrum analyzer input. This inductor theoretically has j50-Ω input impedance around 2.4 GHz. At this frequency, this in parallel with load is about j35-Ω, which is a 3 dB power reduction. At 1.4 GHz, this inductor has impedance of about 29-Ω. This in parallel with the 50-Ω load has a magnitude of 25-Ω, which is the same as you would get with the 50-Ω pull-up. The main issue with the inductor pull-up is the impedance does not look nicely matched to the load.
As the output impedance is not so nicely matched, but there is higher output power, it makes sense to use a resistive pad to get the best impedance control. A 6 dB pad (R1 =18-Ω, R2=68-Ω) is likely more attenuation than necessary; 3 dB or even 1 dB might suffice. Two AC coupling capacitor is required before the pad. In the configuration below, one of them is placed by the resistor to ground to minimize the number of components in the high frequency path for lower loss.
For the resistive pad, here are some common values:
Attenuation | R1 | R2 |
---|---|---|
1 dB | 2.7 Ω | 420 Ω |
2 dB | 5.6 Ω | 220 Ω |
3 dB | 6.8 Ω | 150 Ω |
4 dB | 12 Ω | 100 Ω |
5 dB | 15 Ω | 82 Ω |
6 dB | 18 Ω | 68 Ω |