Lowpass RF Filter Design or Build Problem - Page 1
Forum section: electronics / rf-microwave

author: GinKumo
author_rank: Newbie
timestamp: January 19, 2021, 02:50:07 am
content: I tried to design and build small form factor (SMD 0603 parts) low-pass filters using rf-tools.com lc filter design tool. After building and testing two attempts (one set to stop at 148MHz (i.e. for low power 2M operation) and another at 11MHz (Filtering 10MHz OCXO, or 30M operation), I am perplexed at why the filters perform poorly. As one example the 2M filter has nearly 6dB attenuation in the 2M band (see CH1 LOGMAG) which is far worse than the design simulation. I realize that the simulations may be using too high of Q values for the inductors, so I re-modeled the same circuit in quicksmith with what seemed more realistic values, but still cannot get the simulated results to get close to as bad as what was built. Anyone have any ideas what I'm doing wrong here?Info on the 2M design is attached below. The 30M design (11MHz cutoff) turned out bad too, and possibly even worse. Using the same measurement setup on a 10MHz BPF I purchased looks fine, so I don't think it's the measurement setup....

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 19, 2021, 03:03:00 am
content: Don't you need to include the 50ohm resistors?e.g.,http://users.sussex.ac.uk/~pjly20/PCB.html

author: GinKumo
author_rank: Newbie
timestamp: January 19, 2021, 03:18:05 am
content: AFAIK, the 50-Ohm resistor shown is put in to model the place of an antenna (or other 50-Ohm input impedance RF circuit) and thus isn't part of the filter itself. I believe (but could be wrong) that the NanoVNA-V2 Port1(S2) input impedance is 50-Ohms so it should be Ok (I did SOLT calibration and am connecting the filter between two SMA cables). At least when I've measured other purchased filters I haven't had to add a 50-Ohm resistor and things looked as expected.

author: bob91343
author_rank: Super Contributor
timestamp: January 19, 2021, 06:11:43 am
content: I suspect the problem is that the cutoff frequency is too close to the operating frequency. My suggestion is to move the cutoff frequency up to around double the operating frequency.

author: Bud
author_rank: Super Contributor
timestamp: January 19, 2021, 07:03:30 am
content: Beside the operating frequency being too close to the cut off one, what type of inductors did you use? Use ceramic core wirewound inductors for RF filters, do not use ferrite core or chip multilayer inductors, they have losses and their inductance depends on frequency.

author: GinKumo
author_rank: Newbie
timestamp: January 19, 2021, 09:25:03 am
content: I may look into moving the cutoff frequency out, that is a good suggestion to make things function, but that doesn't explain why the built circuit is still so far off from simulation, even when I simulate with low Q values (i.e. 12 on the inductors). Maybe the core material as mentioned, but Murata's simulator for the inductor on their website shows the inductance holds well past 200MHz:LQG18HH68NJ00D Murata Electronics 0603 68nH +/-5%06035A470JAT2A AVX 50V 47pF C0G 0603 / Multilayer06035A220JAT2A AVX 50V 22pF C0G 0603 / Multilayerhttps://ds.murata.co.jp/simsurfing/rfinductor.html?partnumbers=%5B%22LQG18HH68NJ00%22%5D&oripartnumbers=%5B%22LQG18HH68NJ00D%22%5D&rgear=suaykx&rgearinfo=com

author: FenTiger
author_rank: Regular Contributor
timestamp: January 19, 2021, 12:42:17 pm
content: Try simulating your filter with some parasitic capacitance added. I've had similar experiences trying to build VHF filters, and found that my simulations start to match reality if I add 0.5pf or so to the capacitor values.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2021, 01:50:04 pm
content: Forgot to cut the trace under the left inductor.Tim

author: GinKumo
author_rank: Newbie
timestamp: January 19, 2021, 02:11:10 pm
content: Oh 😳, does the NanoVNA Testboard come with shorted pads?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2021, 02:17:51 pm
content: Heh... take a closer look at the silkscreen, and the copper under the soldermask.Tim

author: GinKumo
author_rank: Newbie
timestamp: January 19, 2021, 02:37:07 pm
content: Oh 😳, I think I see what you mean by looking at the picture I uploaded of my board... I will de-pop the left inductor and try cutting the trace... Thanks!

author: GinKumo
author_rank: Newbie
timestamp: January 19, 2021, 09:39:22 pm
content: Cutting the trace did the trick! Thanks again everyone for the insights/ideas, plus another set of eyes 😀 The circuit now behaves like simulations with more realistic Q values. Unfortunately I first designed the filter without that knowledge (low Q on inductors), so it a bit more lossy than desired, but still it's usable where I want. Next time I will take everyone's advice to heed and make an even better design...