Attenuators before or after the pre-amp?
Hi all,
I have been on holiday for a couple of weeks and in that time my deliveries have arrived.
I purchased a couple of pre rf amps to test and an AM and FM attenuator.
Which does it really matter in which order the go in?
[Antenna]<->[AMP] <->[AM filter]<->[FM Filter]<->[Kiwi] is how I was going to do it.
TIA.
Sean
Comments
Based on some tests = attenuators before the LNA is better - otherwise I end up with lots of AM broadcast up in the 80m band!!
Correct. The LNA is also affected by strong local signals so filtering must take place as close to the antenna as possible.
Ideally, you should place attenuators between the LNA and receiver, this ensures the best overall Noise Figure.
However, there is a trade in LNA design between strong signal handling and Noise Figure.
Most cheap LNA's don't have a sufficiently good enough strong signal handling performance, and they produce Intermodulation products, which typically show up around the MW broadcast band and either side of the 160m Amateur band.
The Intermodulation performance of an amplifier is usually specified by the second and third order intercept points that are typically measured at the amplifier output. This is a theoretical value where the Intermodulation products rise at a faster rate than the original signals, and if you plot all these levels against each other, you will find points at which they intersect. Once you have such a graph, you can then work backwards, to see what maximum level the input signals can be, before the Intermodulation products start to rise above the receiver noise floor.
I use a copy of the DXE-RPA1 pre-amplifier and have measured the IOP2 at +92dBm, and the IOP3 at +45dBm, and this is just about acceptable when connected to a decent antenna.
For comparison, a single PGA-103+ amplifier (still decent, and better than most) measured an IOP2 at +48dBm, and the IOP3 at +36dBm, but this does produce noticeable IMD products.
The Kiwi typically has a Noise Figure of 14dB at 30MHz, which is just about adequate. If you add an LNA with a better noise figure, that will improve the sensitivity on the upper HF bands. But every dB of attenuation, you place before the LNA, will degrade its Noise Figure by the same amount. Placing attenuation after the LNA, does not degrade the system NF to the same extent. There are various cascade Noise Figure calculators on the web, if you wish to investigate this for yourself.
Pre-amplifiers should ideally only have enough gain to overcome any system losses. As a simple test, if you connect a 50 ohm load in place of the antenna, and note the level of the noise floor at 30MHz, then connect the antenna back up, you should aim for a 6dB increase in the level of the noise floor. Any less than this and you are missing some signals, and any more than this, you are using up the KiWi's valuable dynamic range, which is not desirable.
Regards,
Martin
Attenuators before or after the pre-amp?
https://forum.kiwisdr.com/index.php?p=/discussion/comment/20293#Comment_20293Thanks, Martin.
I have a Noolec LaNA (LNA) and a PGA-103+ (AliExpress) and both appear similar in their characteristic and performance.
If however, I put the notch filters between the LNA and the RX - I get a lot of undesirable MW imaging right up in the 160 - 80mtr bands. The same applies with either LNA.
I purchased the Noolec AM filter and an FM filter off Ali.
I will have another tinker tonight.
Can you recommend any filters that you might use?
You can place attenuation ahead of an LNA to improve strong signal handling, but the amount of attenuation needs to be chosen carefully, to take into account the LNA gain and Noise Figure, in order to ensure that the overall system Noise Figure is not any worse than the basic receiver alone.
If you search though the forum you will find many posts on the subject of equalisers, filters and notches.
Here is one example
https://forum.kiwisdr.com/index.php?p=/discussion/comment/5723
For a more detailed analysis of how equalisation can be applied in order to reduce the noise contribution due to intermodulation effects without affecting the basic Signal to Noise performance, see if you can find a copy of this paper on-line.
https://digital-library.theiet.org/doi/10.1049/ree.1977.0028
Regards,
Martin
Just when I thought it was going to be simple, down the rabbit hole I go! ;-)
as my only OV threats sre several MWBC stations, I use a few simple notch filters for those stations before the LNA.
AM Overload Solved
https://forum.kiwisdr.com/index.php?p=/discussion/593/am-overload-solved#latest