Can 3dB amplifier compensate for 3dB loss in filter at 1420MHz?
If I use a filter at 1420 MHz that causes 3 dB loss and in amplifier beforehand that gives 3dB gain, will that cancel it out?
Not exactly — a 3 dB gain followed by a 3 dB loss does not fully cancel out in practice, especially when noise is considered. Here’s why:
1. Signal Power:
- A 3 dB amplifier boosts power by a factor of 2.
- A 3 dB attenuator (or filter loss) reduces power by a factor of 2.
- So, signal power-wise, yes, the net signal level is roughly the same as it started.
2. Noise Performance:
This is the key issue. If the amplifier comes before the filter:
- The amplifier boosts both the signal and the noise coming from the antenna.
- The filter then attenuates the boosted signal and noise by 3 dB.
- But the noise figure (NF) of the filter is added directly to the system noise figure, because you can’t undo the noise introduced after amplification.
So even though the signal level looks the same, the system noise figure is degraded compared to if the filter came before the amplifier or was lossless.
Summary:
| Scenario | Result on Signal | Result on Noise Figure |
|---|---|---|
| Amp → Filter (3 dB gain, 3 dB loss) | Signal power ≈ unchanged | NF worsens (filter adds noise) |
| Filter → Amp (3 dB loss, 3 dB gain) | Signal power ≈ unchanged | NF worsens even more (filter loss before amplification) |
| No filter, just amp | Signal boosted | Best possible NF |
Best Practice:
- For radio astronomy, noise figure is crucial.
- Use a low-noise amplifier (LNA) first, as close to the antenna as possible, before any lossy filter.
- Then apply the filter after the LNA, ideally a low-loss filter (or one integrated into the LNA module).