For the purposes of this discussion it will be sufficient to treat noise figure as a relative figure of merit. The conventional wisdom concerning noise figures can then be summarized as follows:
Noise figure is expressed in dB. Smaller numbers correspond to lower noise and a greater ability to detect weak signals. It is not possible to have a noise figure less than 0 dB. In the context of amplifier noise figures, 8 dB is extremely poor, 5 dB is poor, 3 dB is reasonable, 2 dB is good, 1.5 dB is very good and anything below 1 dB is truly excellent. Mixer stages are generally noisier than amplifier stages. Each stage of a receiver contributes to the overall noise figure of a receiver, but the relative contribution of each stage is reduced by the cumulative gain of the preceding stages.
Two consequences of this conventional wisdom are as follows. If the first stage of amplification has a reasonably high gain, it will effectively determine the overall noise figure of a receiver; and the use of a stage of low noise amplification before a noisy mixer will greatly reduce the impact of mixer noise on the overall noise figure. Both considerations support the use of a stage of low noise amplification with reasonably high gain prior to the first mixer. How much low noise gain is needed? Most engineers would say somewhere between 10dB and 20dB.
With this background, consider the FCD Pro. The LNA of the E4000 has a nominal noise figure of <4dB. This is acceptable for consumer TV applications. Depending on the overall configuration of the receiving station (including the antenna, and perhaps a preamplifier) it may also be acceptable in the context of the Fun Cube satellite project. However, when the FCD Pro marketed as having general broad-band capabilities there are applications, including ham radio applications, for which a noise figure of ~4dB is unacceptably high.
The designers of the FCD Pro seem to have responded to this noise figure problem by using a low cost Si MMIC to provide lower noise amplification in front of the E4000. The BGA2717 MMIC has a nominal noise figure of 2.3 dB at a frequency of 1 GHz. This is acceptable. It is significantly better than ~4dB, and people who need a noise figure below 2 dB would probably expect to use a special-purpose preamplifier that uses a more advanced (and more expensive) technology, such as pHEMT technology.
The noise figure trap is that lowering the noise figure from ~4dB to ~2.3 dB increases the gain in front of the mixer by up to 24 dB. This results in the default gain of the FCD Pro from antenna to mixer output reaching close to 60dB (with up to +24dB from the BGA2717 amplifier, + 20 dB from the E4000 LNA and + 15 dB from the mixer.) The gain can be brought back down towards 20 dB if the gain of the E4000′s LNA and mixers are set at their minimum values – but lowering the gain seems counter-intuitive to casual users. Faced with a seemingly deaf receiver, their instinct will be to increase the gain of the E4000′s LNA to 30 dB, not to reduce it to -5dB!
There can be situations in which the uncontrolled gain of the BGA2717 alone is high enough to cause problems. In this case it may be helpful to put an attenuator in front of the FCD Pro. Other things being equal, overload problems get worse when a preamplifier is used in front of the FCD Pro. With another 20 dB of gain from a preamplifier, the total gain between the antenna and the mixer can be around 80 db. This corresponds to a voltage gain of around 10,000, which is much too high for a wideband system with puny little mixers.