This article is written after I received my giveaway prize of LaNA, Flamingo+ FM, and HF Balun 1:9 from the amazing Nooelec!
Nooelec is a company that sells Software Defined Radio stuffs, their SMArt SDR seems to be the premium in RTL-SDR class. What I’m trying to review first is the Flamingo+ FM, since it’s the simplest out of them.
Basically, the Flamingo is a FM notch filter, it does as expected, blocking out FM broadcast signal. FM broadcast in my hometown is quite full, occupying the whole frequency band plan for FM broadcast from 87MHz to 108MHz. This band has a center frequency of 97.5MHz, which is exactly where the Flamingo+ is designed for.
As the brochure said, Flamingo+ FM is a 7 order notch filter that is centered on 97MHz. This can easily be realized by passive LC filters, with 7 pairs of inductors and capacitors connected in series or parallel. There is two ways to implement this filter, using the Pi configuration or T-configuration. You can find more information on WA4DSY on how the band-stop filter is configured. LC notch filter is a passive device so it does not require any Bias-Tee to operate. Calculating the components for every inductor and capacitor for the filter is done by following the Chebyshev/Butterworth/Bessel polynomial functions.
Using the Notch Filter
Using the Notch filter is simple, there is an input port and output port, technically, both of them can be used interchangeably, but the Flamingo+ FM design adds an ESD diode in the input to prevent voltage transients from the antenna. The antenna is connected to the Input port of Flamingo+ FM filter, then the output port is connected to the SDR directly via a SMA barrel. Getting the ports mixed up should be okay, but you will be losing the ESD protection on the antenna if you got them mixed up.
When to use this FM notch filter? This is dependent on the location of the SDR-based solution that we’re trying to deploy. This FM notch filter may be needed if there is a high powered FM-broadcast in close proximity with the SDR site.
If I had a NanoVNA, I would probably measure the S11 and S21 performance of the filter, since I have none, I might have to wait until I can get my hands on one of them.
In my next article, I would love to talk more about the performance of filter, but first, I forgot that the only Software Defined Radio I have in hand is the old MCX connector. I would have to wait for my order of NESDR SMArtee v2 to arrive. D’oh!
While I wait for the NESDR SMARtee v2 to arrive, I will be writing the plans for what I’m gonna do with the SMArtee, LaNA, and Flamingo+ FM.