3 ms·
A first order filter will attenuate at a rate of 20 dB per decade away from its cutoff frequency. For example, at a frequency 1000 times higher than its cutoff
by jacquesgt 15y ago
A first order filter will attenuate at a rate of 20 dB per decade away from its cutoff frequency. For example, at a frequency 1000 times higher than its cutoff frequency, a first order filter will attenuate by 60 dB.
An n-th order filter will attenuate at 20*n dB per decade.
Let's assume based on your numbers the GPS band is 20 MHz away from LightSquared's 1.5 GHz-ish band. That's log(1.52/1.5) = 0.00575 of a decade.
We need 180 - 70 = 110 dB of attenuation to push the LightSquared signal amplitude well below the GPS signal amplitude. Solving for n above, 1/0.000575 x 110/20 = 956.
An order 956 filter is not something you can design for a buck or two. It's certainly not something you would design if you expected to filter out other space-based -160 dBm signals. Plus, the numbers above are very aggressive. You're going to need to do even better if you don't want to distort the signal you care about.