In olden days, when driving a filter that needed a high source impedance, we used a pentode with its high plate resistance, absorbing the device capacitances into the filter.
Today, you can use the similarly high output impedance of a common-emitter BJT, or a common-source FET.
For testing, one just uses an appropriate high or medium-high resistance resistor after the 50 ohm generator.
Similarly, if you need a low source impedance, you should drive it from a "follower", but it may be difficult to obtain much less than 50 ohms at very high frequencies.
To test, one just uses a voltage divider whose total resistance is 50 ohms to load the generator.
L-C filter design must include the source and load impedances.
In general, a parallel L-C circuit works with relatively high impedances, and a series L-C circuit works with relatively low impedances.
(High and low means with respect to the reactances of the L's and C's at the frequency of interest.)