Filter before amplification or amplify before filtering (and does it work for FPV?)

watertreader said:

Hi,

Would like to know which one is the common practice? which is better and is there a condition for one to be superior over the other?

I have a PMT(photomultiplier tube) which is currently connected to an amplifier before the amplified signal is then digitised for processing with the computer. I would like to include in a filter to clear up some interfering noise from both the electrical line and ambient source. Where should I place it? what other consideration do we need?

Thanks!

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halls said:

Always amplify BEFORE filtering.

When filtering you attenuate the signal, and when you amplify, you amplify both the signal and the noise.

If you put the filter before the amplifier you first filter your signal attenuating it. This means that the filtered signal level is very close to the noise level. When then amplifying it, you get both signal and noise amplified, thus having almost the same signal to noise ratio (SNR, which will be low) but with amplified levels.

In the other hand if you first amplify you will get both noise and signal amplified, BUT, then you filter the signal, thus eliminating most of the amplified noise, thus getting a much better SNR.

Hope it's clear :smile:

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laktronics said:

Hi,
It is desirable to put the filter before the amplifier for the following reasons:

- Since the level of the input signal is low, the filter components need not handle large signals and therefore need be less power rated.

- The amplifier need not handle noise, which might otherwise have introduced distortions due to clipping of total input (signal + noise), thus improves the usable signal range of the amplifier.

- Assuming amplifier itself is not the source of noise ( like Chopper amplifiers), the SNR of the overall circuit will remain same irrespective of where the filter is introduced.

Regards,
Laktronics.

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rcconcepts said:

I am thankful to watertreader for posting this question as I am wondering the same question.

Both Halls and Laktronics had provided very good points on the benefits of having the amplification first and having the filtering first respectively.

According to this video below, the band pass filter after the LNA combination really made A LOT of difference to an analog 1.2ghz video's reception :razz:

https://www.youtube.com/watch?v=-KIT70vCoiI

I planned to use an LNA to enhance 5.8ghz reception of my drone's video transmission and had ordered an expensive 50usd "gpiolabs" 6ghz LNA (0.8db NF) and a cheap 10usd "Aliexpress" 6ghz LNA (4.5db NF). Both appear to have a gain of 15db at 6ghz.
The Aliexpress one appears to be using the Mouser SBB-4082S IC (or equilvalent IC) while the gpiolabs one is using a better IC which is concealed by the aluminium shielding case. I am not even sure if a BPF is necessary for 5.8ghz analog video or for 5.8ghz digital HD video. As proven in the youtube video above, LNA + BPF appeared to have an improvement when a low gain omni was used on the receiver and it had little effect when a high-gain directional yagi was used on the receiver, which is kinda interesting.

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