simple filter questions - Page 1
Forum section: electronics / rf-microwave

author: chilternview
author_rank: Frequent Contributor
timestamp: January 19, 2023, 09:17:56 am
content: I needed a quick filter for a 10MHz sine source to get rid of 2nd/3rd harmonic distortion. So I tried a simple LC filter (see photo of schematic) with parts I found in the junk box. Simulation with Spice looked good (next photo - showing an insertion loss of about 6dB at 10MHz and second harmonic down at some -46dB.Then built the circuit on a bit of perf board as in 3rd photo and connected it up to a 4396B VNA set in network analyser mode with a 87511A transmission/reflection set. Set IF BW to 10KHz and averaging on, then did a reponse cal, the thru showed 0dB from 300KHz to 1GHz.Connected up the filter and got a response as shown in the last photo, and it's here I have the questions:1) The VNA is saying -50dB at 10MHz, where is this big loss coming from? Compared to the simulation there's ~43dB loss somewhere.2) Above about 30MHz the response increases to a peak at 70MHz then on up to about -12dB at 1GHz. Is this likely to be just parasitics from the through hole components?thanks!

author: yl3akb
author_rank: Regular Contributor
timestamp: January 19, 2023, 10:52:08 am
content: The fact that You get peak at 10 MHz probably shows that series LC resonates correctly at 10 MHz with parasitics not yet being significant even though 4.7uH seems huge.Shouldnt there be parallel LC at the middle section, not just shunt C? It seems a little bit unusual...Are grounds connected everywhere? In picture only right side ground seems connected on top side. If something is in the bottom as well, then should be okay.With such value of inductance I would not expect good response at higher frequencies, because its self resonant frequency probably is on order of few tens of MHz only, but it should work at least at near 10 MHz with reasonable loss.

author: chilternview
author_rank: Frequent Contributor
timestamp: January 19, 2023, 12:21:57 pm
content: Thanks - the design came from some online filter tool a while back. Yes the ground is connected everywhere (one is on underside).I measured the inductors and capacitors using the 4396B's equivalent circuit model to get their parasitics, and put that back into the simulation.It is a bit closer to measurement so I guess the parasitics are key.What I still can't understand is why the measured peak at 10MHz is so much lower than the simulation.

author: yl3akb
author_rank: Regular Contributor
timestamp: January 19, 2023, 12:49:16 pm
content: Okay. Are You sure there are no shorts and opens anywhere? Probably not, but it seems that something is wrong with physical assembly.Can You try more simple circuit, for example only single series L+C? It should resonate at same frequency.p.s. just out of curiosity - whats is self resonant frequency of your inductor, which you measured?

author: chilternview
author_rank: Frequent Contributor
timestamp: January 19, 2023, 01:41:55 pm
content: I've had great difficulty attaching .png files - eevblog doesn't like them for some reason. Says they are a security risk or something.Anyhow, the inductor resonates at 106MHz, The 330p capacitor at 32MHz, the 68p capacitor at 61MHz. I guess the 330p cap is causing the HF response as over 32MHz its turning inductive and is going to look like an open at high frequency.I have checked for shorts/opens - it is a very simple circuit, which is why I'm puzzled!

author: chilternview
author_rank: Frequent Contributor
timestamp: January 19, 2023, 02:04:11 pm
content: Aaargh.There was a short at the output SMA connector, which is why it was reading 50dB down...With that fixed, it seems to agree reasonably well with the simulation (including the parasitics)Thank you for reminding me to check connectivity again!!!

author: yl3akb
author_rank: Regular Contributor
timestamp: January 19, 2023, 02:29:06 pm
content: Great, looks good now!It is interesting that even 68 pF is resonating at only 68 MHz. My first intuition is that it should be >1GHz, but then again these are through hole not SMD...

author: chilternview
author_rank: Frequent Contributor
timestamp: January 19, 2023, 02:58:30 pm
content: Yes it is rather low, these were polystyrene caps - seem to have a lot of inductance.

author: MathWizard
author_rank: Super Contributor
timestamp: January 23, 2023, 01:25:14 am
content: Quote from: chilternview on January 19, 2023, 01:41:55 pmSo are you talking about the RLC model of each part with their parasitics, and measuring their self resonant frequencies right ?So that's not something my LCR meter measures, or a typical LCR meter is it? That's when people go for a VNA ? I've seen their price, but there's little pocket sized ones for ~$200, that do way better than no VNA. I do have a scope and signal gen, I guess I can use them to find out stuff like that too. Some day soon I'll get a pocket VNA

author: chilternview
author_rank: Frequent Contributor
timestamp: January 23, 2023, 12:43:28 pm
content: Yes I assume you are referring to the NanoVNA, I believe they can also display the resonant frequency of components.The HP VNA I have is a bit more sophisticated (and more expensive even though it's 20 years old!) and can compute the equivalent circuit and its values.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 23, 2023, 05:07:53 pm
content: I would rebuild the circuit and keep the lead lengths short. Make the circuit compact.

author: RoV
author_rank: Regular Contributor
timestamp: January 23, 2023, 05:26:37 pm
content: I was surprised by the bad performance of capacitors, especially the polystyrene ones which were commonly used for RF, while I would have pointed my finger against the inductors, that I expect to have very low Q in that shape. I would prefer iron powder (or low permeability ferrite) toroids in a filter application at this frequency.

author: chilternview
author_rank: Frequent Contributor
timestamp: January 23, 2023, 05:42:09 pm
content: Quote from: ahbushnell on January 23, 2023, 05:07:53 pmYes the 'real' version of this will be built using SMD inductors and C0G capacitors, with the intention of filtering a 10MHz square wave from an OXCO to drive a buffer amp. Initially it was breadboarded but gave even worse performance!