Find out what a RLC is filtering - Page 1
Forum section: electronics / projects

author: kinap35
author_rank: Contributor
timestamp: January 09, 2025, 04:48:47 pm
content: Hi,I'm trying to understand some equipment schematics, but i fail to understand what some RLC based filters are filtering.For example, the filter in attachment (NOTincluding the FL501 LPF filter, only what is after it).I believe (but may be wrong), that:- R502 : 1kOhms- R500: 10 kOhms- LV501: adjustable 1.5mH (don't know min/max range of it)- C504: 39pFare all together making a filter.The transistor's base at the bottom is activated by an external signal, and i guess it activates the filter by connecting C504 to GND.Found this site:http://sim.okawa-denshi.jp/en/Fkeisan.htmBut even with 4 components only, this filter is beyond the basic designs. So i don't know how to understand what kind of filter is it, and what it is filter out.Could you please help to find out what this one is doing? And would you have some advices about how i can recognize and understand some others in other schematics (beside asking here!)?Thanks in advance!

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2025, 05:26:57 pm
content: Its a notch filter. Its precise behaviour depends on the complex output impedance of the preceding FLS01 low pass filter. Assuming FLS01's output impedance is purely resistive, expect a notch center frequency of about 65 kHz or a bit higher depending on adjustment of LV501 core.Incidentally, vintage schematics usually only showed capacitor values in μF or pF, often without any suffix, and capacitors from approx. 1nF to 1uf would be in decimal fractions of a μF. You could distinguish between μF and pF by context and also by anything over a couple of μF being a polarised capacitor. In parts lists microfarad was often abbreviated 'mF' and 'mmF' was micro-microfarad i.e. picofaradEdit: Thanks Tim for correcting my 'mmF' mixup!

author: TimFox
author_rank: Super Contributor
timestamp: January 09, 2025, 05:47:30 pm
content: Quote from: Ian.M on January 09, 2025, 05:26:57 pmI remember vintage notes that stated "capacitor values >= 1 are in mmf, values < 1 are in mf, unless polarized which are in mf.""mmf", pronounced "mickey-mikes", were micro-micro Farads (pF), and nF were not popular yet.To further confuse things, some old electronic stuff used "M" to mean 1000 (as in the Roman numeral), along with accounting applications back then.The only remaining use of M = 1000 that I am aware of is for large electrical wires quoted in "MCM", meaning 1000 circular mils area.I wish that the SI, when specifying prefixes for decimal factors, had allowed "u" for 10-6, in place of Greek "mu" for micro, completing the dominance of the Latin alphabet in metrology.

author: kinap35
author_rank: Contributor
timestamp: January 10, 2025, 12:14:31 am
content: Thank you Ian.MSorry for the wrong cap value.I saw the .asc file attached to your message, and found out it is an LTSpice file. A quick download later i can see a nice frequency curveIs it the best or simplest way to find out what a filter do?The specifications of the LPF filter on the left is not given. It is a Sony 1-235-478-11 with only "FILTER,HIGH PASS" specified. Not very useful!