| Electronics > Projects, Designs, and Technical Stuff |
| RF design and layout [questions/feedback] |
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| Yansi:
Antenna is rarely close to 50ohms, but that is not as problematic, as it is connected to the amplifier input. What is more problem, is that RTL SDR is really anything but 50ohms. Mind you, the tuner was designed for 75ohm environment and the available shit-documentation for R820T2 tuner (if it even is the case with your dongle!) is everything but specific about input impedance of that tuner. Output impedance of that integrated amplifier rarely is close to 50ohm, especially at these low frequencies. So plug in the correct model of the amplifier in the RFSIM, and check again. Not only for port impedance variance, but also do a component value spread simulation. Your supplied values of +-2% capacitors and +-5% inductors is not very realistic I think. Also, including some real Q values of inductors helps. //EDIT: You should think also about swapping the filter first, amp second. Otherwise strong local signals will overload the amplifier, distort, mix, and the filter then not helps much, as out of band signals will now become in-band due to inherent non-linearities of the overloaded amplifier. |
| hendorog:
Is that the only wavelength which can hit the LNA - What is preventing higher frequencies getting in? This is outside my experience, but something else to bear in mind is your filter will be highly reflective for some frequencies and that might cause issues with the amp. |
| Yansi:
Yes, checking the amplifier stability is also important. ADL5544 does not seem to be one of the most stable ones, especially, when poking in the low frequency band. The datasheet even suggests adding external network to make it stable. The filter shall be probably replaced with a diplexer, to become wideband 50ohm termination for the amplifier, and the other side also. Input of the amplifier shall at least be low pass filtered to get the highest frequency crap out. |
| hendorog:
Yep, I wonder if it would be good enough to just move the filter to the input side. Edit: Good page about via spacing here: https://www.edn.com/electronics-blogs/the-practicing-instrumentation-engineer/4406491/Via-spacing-on-high-performance-PCBs |
| Nitrousoxide:
It should also be noted that the noise figure of the total system will increase if you place the filter before the amplifier (I just thought I would mention it). It is good practice to place the amplifier as close as possible to the antennae since even transmission lines diminish the noise factor. The output of the amplifier can be matched for maximum gain as it has no bearing on the noise performance of the amplifier. However, the input matching does affect noise performance. I would recommend looking at the datasheet and through the use of a Smith chart, plot circles for the desired noise figure and constant gain. It would be then possible to find the maximum possible gain for the desired noise figure (by the intersection of circles), and thus the required matching impedance which can then be realized by lumped or distributed elements (for sub 500MHz you can use lumped). And yes! As previously mentioned you must check stability. Check Rollet's conditions to give you a good understanding. But, be careful to compute these with the operational s parameters (correct bias currents/voltages). If the system is unconditionally stable, then there is nothing to worry about. If not, then certain regions of impedance matching should be avoided. |
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