glovisol's FM notch filter
Hello,
I would implement it: https://www.sdrplay.com/community/viewtopic.php?f=5&t=4317&sid=075461ec1cf8a599c0cd76b15c681002
The final boxed project looks different to what the L / C values above though:
Since that forum is now defunct, can anyone comment on the different number of turns used vs written ? Even better if the cap / inductance values can be inferred. Or perhaps staying away from this otherwise tempting design is the better idea.
Thank you
Comments
My experience of building filters like this, is that the values provided by the various on-line calculators for self-supporting air spaced inductors wound from wire, is that they are not that accurate.
Even when you have wound them, and tweaked than so that their measured values are correct, you will need to adjust them again once they are installed in the circuit, in order to achieve the required response curve.
To do this you ideally you need a VNA, or Spectrum Analyser and noise source, so that you can bend the coils in real time and observe the effect.
SMD components can produce reasonably repeatable filters, as their construction and tolerances are much less affected by external factors.
However, if you don't have access to the test gear required to set it up, then there are plenty of cheap kits, or ready built, FM Band Stop filters being sold on AliExpress, starting at less than $5 USD, which is pretty much just the component cost if you were to buy the parts alone.
https://www.aliexpress.com/item/1005007644615468.html
Regards,
Martin
The thing with the aliexpress filter is that it cuts into the air band and even beyond.
I shall try my luck at the glovisol filter, SMD components.
Thanks
How far does the Chinese filter cut into the VHF airband ?
The ones I have tried seemed to be acceptable.
The frequency range 108-118MHz is used for navigation aids rather than voice comms, that helps provide a bit of additional separation.
For example, the slightly more expensive type sold by RTL-SDR.com has -3dB points at 75 and 120MHz.
Incidentally, IMHO, unfortunately the RSP-1 wasn't that good, and suffered from several "issues". In fact, it's one of the few items of radio kit, out of the many I've bought over the years, that I actually sent back for a refund. By contrast, I think SDR Play resolved a lot of the problems that users had commented on when they released the updated the RSP-1A, which is much better, and I continue to own one of those.
If you are building the filter yourself as a one-off, I'd suggest winding your own coils, as you will need to adjust them to get the required response. SMD parts are good once you have a working design, as they make builds more repeatable. But for the initial build you would still need to optimise the component values, in order to compensate for the construction method, PCB layout, stray coupling etc.
Be sure to let us know how you get on with your project.
Regards,
Martin