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Forum Index : Microcontroller and PC projects : low pass filter
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| Volhout Guru  Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
The 4.7mH inductor and 2.7nf capacitor are exactly thay. A notch at 44kHz. Volhout PicomiteVGA PETSCII ROBOTS |
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| ville56 Regular Member  Joined: 08/06/2022 Location: AustriaPosts: 96 |
ah, yes of course. Didn't take a look into the manual, was just following the thread. Though i think the combination of fixed L and fixed C is courageous for a resonant circuit if one thinks about the tolerances of these components. Regards, Gerald 73 de OE1HGA, Gerald |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
As adjustable coils are rare now and adjustable capacitors of more than a few pF are big it's not easy to make a simple variable filter. Yes, it can be done, but it's not simple. 5% tolerance (which is good for a ferrite core) is way better than your hearing response anyway. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| ville56 Regular Member Joined: 08/06/2022 Location: AustriaPosts: 96 |
this is correct ... my upper cutoff frequency is now a mere 10 to 11 kHz. It used to be over 19 kHz when i was about 20. The residual FM stereo pilot tone drove me crazy on hifi sets with badly filtered stereo decoders. 73 de OE1HGA, Gerald |
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Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
@Volhout, I'm sorry, my BS detector distracted me from thanking you. So thank you for confirming the my filter will work as I had hoped.  Bill Keep safe. Live long and prosper. |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2141 |
"As adjustable coils are rare now " Yes, it was possible to get ferrite pot cores with an adjustment slug in the middle. High Q LC notch filters can be very narrow requiring adjustment. Toroidal inductors can be adjusted by adding / removing turns. Clumsy but only needs doing once. |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
I looked at the 4.7mH inductor and 2.7nF components on the Jaycar site. The inductor (they call it a choke) has a rated resistance of 0.39 ohms. I believe that would make the Q of the circuit fairly high meaning that the notch will be steep and narrow. The accuracy of both components is rated at +/-10%. Given that the circuit, if constructed from these components, could be up to 20% out and the slope of the filter will be quite steep around it's resonant frequency, what would be the affect of the filter on the 44.1kHz signal? If the values are spot on the suggested values the resonant frequency of the tuned circuit is 44.677kHz. Over 500Hz out. If the capacitor was 10% low and the inductor was 10% high the frequency will be 44.9kHz. 800Hz error Maybe Volhout could do a simulation of both of those cases to see how much 44.1kHz remains. If there is a problem then maybe adding a resistor in series with the inductor will lower the Q and widen the notch to sufficiently cover the 44.1kHz. Just Thinking. "Life wasn't meant to be easy" Malcolm Frazer: former Australian prime minister Bill Edited 2023-03-29 15:52 by Turbo46 Keep safe. Live long and prosper. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
IIRC you can adjust the notch width (and the steepnes) by shunting the LC network with a resistor or adding a resistor in series with L. I never really was one for filter designing though. I much prefer to leave the Dark Arts to others. :) You can always pad the capacitor with smaller values. Start off low and add parallel trimmers until you get it right. However, the next inductor will be from a different batch and you'll have to start all over again... If you want to get accurate stock values then you will have to get hold of 1% or better parts. Because of the way tolerances work you will be very lucky to find a 1% part among a group of 5% parts as most will have been picked out on test. They will all be either high or low. Precision values come at a cost. Is it really worth it? Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
I think I said that. IF there is a problem we need a solution that will always work, not one that needs specialized equipment to tune. Notice the big 'IF'. Bill Keep safe. Live long and prosper. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
To cope with tolerances, the filter has changed from the original 1.5mH to 4.7mH. The inductors used are 10 ohm or higher. The Q is limitted bij the elleptic filter components, also the 220 ohm drive resistor. Look at the whole filter, not only 2 components, that are not the parts actually used. Volhout Edited 2023-03-29 18:43 by Volhout PicomiteVGA PETSCII ROBOTS |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Has this been mentioned before? Will the Jaycar filter work as needed with a 10R series resitor? Should a specific inductor be specified? Sorry I don't understand the "that are not the parts actually used" part of that statement. Having spent a lot of my working and hobby life in electronic design, testing and commissioning I quickly learnt that you cannot accept a simple statement like "this circuit will do this" without testing it and proving that it does what you want before including it in a final product. Then testing it again. Just because the first prototype works doesn't mean the second one will. Been there several times. If it will be incorporated into many units that will go into service you need to KNOW that it will work reliably with all of the possible component variations. I'm sure that you feel the the same. When I looked at your circuit with a view to using it the questions that I raised in the previous post came up and I sill have no proof that the 44.1kHz is reduced to a level acceptable to those who worry about the safety of their tweeters and MOSFETs. Yes that is said with 'tongue in cheek' in case your BS detector starts beeping. I have not seen proof that your filter will sufficiently reduce the 44.1kHz nor even that it is necessary to reduce it to such a low level that many seem to 'need' and I don't feel sufficiently motivated to arrange a test setup to test it. Bill Keep safe. Live long and prosper. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
That filter went through several iterative stages, Bill, with value changes. This was at the time that I was designing the "flat" PCB to go under a keyboard, which is partially the reason that it worked out as a passive filter rather than an active one (not a lot of space!). It also triggered the change from a physically larger inductor, and to RS as a supplier because they have cheaper p&p charges than Farnell. The RS-supplied inductor has always been specified as the DCR is important. In actual fact, it's a common part and is easily available - even by looking for the manufacturer's part number on ebay. Volhout did post some response curves, but I couldn't tell you now exactly which filter designs they were for. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Thanks Mick, sorry I don't know what DCR means but anyway I don't follow every post in TBS closely and must have missed or forgotten that one because it was of little interest to me. I was using the Picomite manual as my reference and that does not specify a particular brand or source of filter. Most Australians that live in a major city or some towns have access to a local Jaycar or Altronics store or reseller so it is most likely they will source their components from them where available. I have two Jaycar stores and one Altronics store plus an Altronics reseller within about 6k or so. I just wished to express my concerns about the performance of the filter. If it does depend heavily on particular components then that should be spelled out each time the design is reproduced. While I admire Volhout and his contributions I just have concerns about this particular filter design. Bill Keep safe. Live long and prosper. |
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| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 3816 |
DCR might be DC resistance, i.e. not AC. John |
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| ville56 Regular Member Joined: 08/06/2022 Location: AustriaPosts: 96 |
just soldered together what was at hand. Changed L to about 4 uH and the C parallel to it to 3,3 nF Styroflex. Closely hit 44 kHz resonant frequency. Output load is 47 kOhm. Input impedance is 50 Ohms into the 220 Ohm resistor. 33nF and 68nF as per circuit diagram. In the attached graph you can see the transfer function of the filter. The lines differ by 200pf in the resonant circuit which represents a tolerance of +6% (cap in/cap out).  line color red green frequency [kHz] 43,6 45,0 insertion loss [dB] 56,1 55,6 To me this means a difference in insertion loss of 0,5 dB at the frequency in question. Interpolated linearly this would mean about 1,5 dB for +20% tolerance. So the worst case (+/- 20% for L and C) could be 3 dB which in voltages means a factor of 1,4. If we assume 3,3V carrier amplitude then after -56dB we have about 6mV left. The worst case difference then would be 6mV to 8,4mV. I think this is not too bad for this simple filter and the as the notch is not very pronouned, component tolerances will not effect the characteristics much. Regards, Gerald 73 de OE1HGA, Gerald |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
DCR = DC Resistance, no prob. The 4.7mH 5% inductor is EPCOS B82144A2475J000 RS 191-1197 Mouser 871-B82144A2475J Farnell 517 914 I think it's also sold as a TDK component. Unfortunately I've no idea what Jaycar or Altronics may have as we don't have either of those here in the UK. :) The data sheet for the inductor is Here. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2141 |
If you want a deeper notch it will be narrower so tuning may be useful. It isn't difficult. Tuning a notch filter requires no special equipment at all. Just a Pico a resistor and a multimeter. (which could be replaced with a detector circuit and ADC.) PWM is your sig. gen. => resistor => C => L => Gnd. AC voltage across C & L will dip at resonance. Add or remove turns till it dips at 44.1kHz. |
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| ville56 Regular Member Joined: 08/06/2022 Location: AustriaPosts: 96 |
IMHO notch depth is not effected by tuning, at least not in this type of filter. It is given by component characteristics (values, Q, tan(delta)) of the LC resonant circuit and the damping on input and output. I think you only can tune the notch to better match the carrier frequency, but the results are rather marginally as can be seen from my last post. 73 de OE1HGA, Gerald |
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| ville56 Regular Member Joined: 08/06/2022 Location: AustriaPosts: 96 |
just out of curiosity i measured the Q of the LC circuit i was using. It is a mere Q of 86. I think this is rather mediocre for 44 kHz. Generator coupled with 22pF, Load is about 1 Mohm//30pF. Should not dampen too much. So the ferrite core inductors seem to be not the best choice for that. Pot cores may be a different story with the right ferrite mixture.  73 de OE1HGA, Gerald |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
Remember that the reduction in carrier by the LC filter is in addition to that made by the RC filter that precedes it. Also, the current selection of components is to allow for an output impedance that is unknown and could be 32R headphones or the 10k input to a volume control. It's a compromise, not an "ideal world" calculation. It was also designed to use standard component values so tuning won't be perfect. That doesn't prevent it from giving very acceptable results though. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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