Whats the formula to figure out this LC circuit? - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: December 04, 2016, 04:08:08 am
content: I built this LC filter from part I found in an old computer monitor.There is a good write up on how it works but they dont tell you the formula.http://www.dxing.com/tnotes/tnote06.pdfIt calls for 365uf for the capacitor and 200mh or 220 for the inductor. Basically it is high pass filter that blocks out AM broadcast band for an SDR radio. On one end is ANT>Filter> balun> then coax to radio. Since I didnt have the parts I wanted to see how close I could get. Its series 72mhenry wire wound cylinder type inductor (not the toroid but many turns on what looks a spool of thread)Antenna IN OUT>72mH inductor>100pf ceramic disk>100pf>100pf>45pf>10pf>Ground (355pf)It works kind of I guess the AM broadcast band is just one really noisy high noise floor static, but the offending station (1200kHz religious) still comes in on other stations like the 5.0MHz time signal and just about anything else, like before. So what frequency is this thing blocking ? Its the circuit on the left.

author: jonovid
author_rank: Super Contributor
timestamp: December 04, 2016, 04:15:58 am
content: your schematicalso may need to know, the input -output loading ., Q & Hz ,

author: nugglix
author_rank: Regular Contributor
timestamp: December 04, 2016, 05:53:08 am
content: Also the difference betweenmilliandmicromight be interesting.And I bet one can find "the formula" by searching for "lc tuned circuit".´

author: bson
author_rank: Unknown
timestamp: December 04, 2016, 06:29:43 am
content: Looks like impedance matching devices. The impedance of the the shunt in parallel with the load is ideally the conjugate of the source impedance.

author: Simon
author_rank: Unknown
timestamp: December 04, 2016, 08:18:25 am
content: Series, parallel turned circuits, be careful the formulas are different for each version and there ar exact formulas and simplified aproximation formulas that work under most circumstances.https://en.wikipedia.org/wiki/LC_circuit

author: raspberrypi
author_rank: Frequent Contributor
timestamp: December 04, 2016, 10:12:46 am
content: So that's why it wasn't on the write up.I did some searching and this is what I found:So guesstimating says the long wire antenna is about 450 Ohms as its 10' off the ground and unterminated, open end; no resistor.Frequency tgt is 1800MHz and down, so its a range, or is more like its centered in the middle of the band say at 1100Hz (that would be perfect) and it decreases by so many decibels as you get further away?Q = 2? x Energy Stored / Energy Lost Per CycleBut still whats the formula?

author: rstofer
author_rank: Super Contributor
timestamp: December 04, 2016, 03:28:29 pm
content: You can get the resonant frequency from the Wikihttps://en.wikipedia.org/wiki/LC_circuit

author: snarkysparky
author_rank: Frequent Contributor
timestamp: December 04, 2016, 05:19:56 pm
content: series LC circuit is open ( high impedance ) to all frequencies butw = 1/sqrt(L*C) in rad/secThat is for ideal components.Real components will spread out the notch depending on the circuit QThe values you gave 0.2 mh and 365 uF give 117 rad/sec or 18.6 hz .I can't think of any use for such a notch filter

author: raspberrypi
author_rank: Frequent Contributor
timestamp: December 04, 2016, 11:25:42 pm
content: Explains why it doesn't work so well. So the lower the inductance the higher the frequency (in this exact case?) Or would it be more like a peak at 18mhz and let the AMBCB pass through with minimal attenuation? Or is it more like a bunch a little peaks harmonicly spread out? (assuming we are looking at a db vs frq chart)My math gave me a strange number that's obviously wrong. What happens if the capacitance goes lower?

author: IanB
author_rank: Super Contributor
timestamp: December 04, 2016, 11:34:37 pm
content: Quote from: raspberrypi on December 04, 2016, 11:25:42 pmsnarkysparky didn't say 18 MHz, they said 18Hz. One is a million times different than the other.

author: G0HZU
author_rank: Super Contributor
timestamp: December 04, 2016, 11:44:31 pm
content: The Figure 2B circuit is drawn incorrectly if the aim is to make a classic parallel trap filter at 1200kHz. So this can't work as described.

author: KerryW
author_rank: Regular Contributor
timestamp: December 05, 2016, 12:03:54 am
content: A 365uF variable cap would be as big as a house! What you have is a 365pF cap. It adjusts from ~65pF to ~365pF.f = 1/(2pi*sqrt(L*C))With a 200uH inductor, the range is ~590KHz@365pF to ~1400KHz@65pF. It should work just fine at 1200KHz once you adjust it correctly.

author: G0HZU
author_rank: Super Contributor
timestamp: December 05, 2016, 12:49:42 am
content: Just to clarify, the acceptor circuit on the left (Fig 2A) is OK to use as a trap but the one on the right (Fig 2B) is drawn incorrectly in case anyone is tempted to try it.The other thing to consider is the LC ratio and also the unloaded Q of the components used. It's easily possible to make a 200uH inductor with an unloaded Q down of just 2 at 1200kHz and it's also possible to make a high performance 200uH inductor with a Q of about 400 at 1200kHz. So the circuit on the left can still perform (very) poorly if the inductor has a very low quality factor and the receiver input circuit has a lowish impedance. The notch it produces could be very shallow. Probably best to aim for an inductor with an unloaded Q of >100 but a lot depends on your receiver input circuitry.

author: danadak
author_rank: Super Contributor
timestamp: December 05, 2016, 12:00:46 pm
content: 2a is series resonant and "traps" signals in a narrow range depending on Q of circuit.Those signals are diverted to ground.2b is parallel resonant and passes all signals within its narrow range, again depending on Q.The signals outside its bandwidth are diverted to ground.So the first rejects a narrow range of frequencies, the other passes a narrow range of frequencies.Use this to graph performance and evaluate bandwidth and rejection.http://www.ti.com/tool/filterpro&DCMP=hpa_amp_general&HQS=NotApplicable+OT+filterproYou can download and install for free.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: December 05, 2016, 12:16:05 pm
content: A variable cap in the 100's of pF range is not as big as a house.See attached 320 pF cap.Regards, Dana.

author: KerryW
author_rank: Regular Contributor
timestamp: December 05, 2016, 12:45:53 pm
content: Quote from: danadak on December 05, 2016, 12:16:05 pmDo you have a picture of a variable cap in the 100's of uF range?

author: danadak
author_rank: Super Contributor
timestamp: December 05, 2016, 08:16:56 pm
content: My apologies, need to get a new glass prescription again.Makes me wonder how soon we will have variables in the uF range, its all about dielectricswhen fighting volume/size. We have farads now in thumb size, maybe soon.....Regards, Dana.

author: Bud
author_rank: Super Contributor
timestamp: December 05, 2016, 08:53:04 pm
content: The guy continues trolling, and you guys bite it.@raspberrypi, how does it feel to get caught? Would you post back your Duga3 mom's backyard project that you deleted?

author: G0HZU
author_rank: Super Contributor
timestamp: December 05, 2016, 10:22:58 pm
content: QuoteI'm not biting... I joined the thread to point out that the fig 2B drawing is incorrect in the linked article and it should look like the one below. I'm just here to correct a mistake in an online article about series and parallel tuned trap filters in case any newbies get confused why the original fig 2B won't work as described in the article textFig 2B should be as below.

author: KerryW
author_rank: Regular Contributor
timestamp: December 05, 2016, 11:03:19 pm
content: Even if the OP isn't interested in the answers, others might be. As for pointing out Joe Carr's errors, it seems like a harmless hobby. I have his Handbook of Radio Communications (Tab books, 1984). It is over 1000 pages and it seems like there is an error on every other page! (He DID show a wavetrap correctly, though)

author: G0HZU
author_rank: Super Contributor
timestamp: December 05, 2016, 11:23:12 pm
content: QuoteWhen you word it like that it appears that you are disagreeing with me when I say that Fig 2B is drawn incorrectly.Here's his text.QuoteQuoteQuoteFig 2B does not behave as the description above. It doesn't reject ONE specific trap frequency. Joe Carr's original FiG 2B circuit is a preselector and not a trap. It is not shown as being placed in series with the antenna line either. So it fails to agree with the text.The amended version that I posted up DOES behave as the text above. It is a parallel resonant circuit placed in series with the antenna line so it fits the description. At resonance it will provide a high series impedance at 1200kHz (an open circuit at 1200kHz) and it provides a low impedance to other frequencies that are some way away from 1200kHz. So they get through to the receiver. Again, it fits the description in the text.Fig 2A has a series tuned circuit and it works by crowbarring at 1200kHz (like a short circuit to ground) and the parallel version should work as an open circuit at 1200kHz. But it can't do this unless you amend it to a trap circuit as below.A trap should be tuned to the frequency of the unwanted signal. If you tuned the original Fig2B circuit to the unwanted 1200kHz frequency then the unwanted 1200kHz frequency would get through to the receiver the strongest!This isn't exactly rocket science...

author: G0HZU
author_rank: Super Contributor
timestamp: December 06, 2016, 12:10:24 am
content: A classic band reject (notch, trap) filter would use both series and parallel tuned circuits as below. You can see the way the centre 'trap' is arranged. This isn't the same as the original Fig 2B but it does agree with my amended version. In this filter all three resonant circuits are resonant at the unwanted frequency.In practice, it is often difficult to realise components with decent unloaded Q for the centre (parallel resonant) section and so the best designs use a combination of series and parallel traps as per the circuit below

author: KerryW
author_rank: Regular Contributor
timestamp: December 06, 2016, 02:23:18 am
content: Sorry, shipmate, communicating is the hardest thing we ever do. What I meant was that he had it right in his book. He had 2 parallel resonant circuits in series with the signal and one series circuit to ground in between. I agree it was wrong on the website, even though he obviously knows better.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: December 12, 2016, 12:40:42 pm
content: "Trap for young players"I connected all my caps in series not parallel to add up capacitance. Thats not going to give me the correct value at all. I;m surprised no one caught this as how critical the internet can be. Time to fire up the hayco, the leads are getting really short too, but I have a another inductor I'm going to add in. Wish I had an easy hobby like stamps.