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Looking for resistor value and capacitor value for passive low pass filter remove RF

Discussion in 'General Electronics Discussion' started by dietermoreno, Apr 25, 2013.

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  1. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Looking for resistor value and capacitor value for low pass filter to remove RF.

    The application is removing undesired untuned RF from the crystal set before it reaches the amplifier it is connected to so that the amplifier it is connected to doesn't demodulate undesired untuned RF.

    Only audio frequencies are to pass to the amplifer.

    I found several schematics of crystal sets, but I don't understand them because they do not label which part of the circuit is the passive low pass filter to remove undesired RF. It must be implied, but for noobs like me I don't understand.

    I found several schematics of crystal sets that do label what part of the circuit is the passive low pass filter to remove undesired RF, but they don't specify the resistor value and the capacitor value.

    I want to know what resistor value and capacitor value would be good to buy for a few pennies on Amazon to come with my shipment of me ordering a new fuse for my PA amplifier.

    Yes I am still using the pencil lead for the other resistor and using a home made inductor and home made vari cap to be true to the original crystal set, but I think the resistor value and capacitor value need to be more precise for the low pass filter.

    I used this RC filter calculator to find the values of 1k resistor and 0.008 micro F capacitor for 20,000 Hz. Are these good values to choose?
     
    Last edited: Apr 25, 2013
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Crystal sets are very basic, which is why there's always been a justified fascination with them. The RC filter is placed post detector.

    Post your schematic.

    Chris
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK

    Untuned RF doesn't even make it to the detector. That is the entire purpose of the tuner. How well it does that is called selectivity. It's not too good for a simple crystal radio, so you often hear other strong and/or adjacent (in frequency) stations in the background.

    That's fair enough. You want to remove any RF after the detector.

    It is typically just a capacitor to ground and relies on the impedance of the earpiece (or frequently another resistor) to set the cutoff frequency.

    Because audio frequency and RF are so far apart, the values are not critical. So there's not just one value that works. Also the value chosen depends on the impedance of the source signal and the load (earpiece). Calculation of an exact cutoff frequency isn't straightforward.

    Yes I am still using the pencil lead for the other resistor and using a home made inductor and home made vari cap to be true to the original crystal set, but I think the resistor value and capacitor value need to be more precise for the low pass filter.

    I used this RC filter calculator to find the values of 1k resistor and 0.008 micro F capacitor for 20,000 Hz. Are these good values to choose?[/QUOTE]

    I'd probably start with something around 1nF (0.001uF) and not even bother with the resistor. So your calculations are in the ballpark.

    Anything between 10nF (0.01uF) and 0.1nF (0.0001uF) would probably be OK, so getting an exact value is likely unimportant. Because capacitors are so cheap, maybe you should get 5 or so that span the range 10nF 4.7nF 1nF 470pF, 100pF
     
  4. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Low pass filter schematic found on Google image search: [​IMG]

    My schematics that I drew in Paint attached.

    I want to make it receive FM, since I am already bored of hearing talk radio broadcasts on my guitar amp ocassionally.

    I already have tested the inductive antenna simply connected to my guitar amp with no crystal set and it does make the stations picked up sound louder than the mains hum in addition to now it is picking up 2 FM stations, 105.5 MHZ which is 5 miles away from my house in McHenry, IL and much quieter it is picking up 95.1 MHZ WIIL which is 15 miles away from my house in Union Grove, WI. My house is in Island Lake, IL. The loudest station heard is Star 105.5 and slightly quieter is heard 780 KHZ WBBM which is 15 miles away from my house in Itasca, IL. I am hearing these stations very quietly with my ear pressed to the guitar amp speaker when the guitar amp is connected to my inductive antenna with no crystal set connected. When I tried at night and used Audacity to filter the audio, I even picked up the only classical station in Chicago, which is already heard in the YouTube video I uploaded, which this station is on the northwest side of Chicago 40 miles away from my house. More stations are picked up at night than in the day.

    I think adding the dipole directional antenna will help to pick up WIIL over star 105.5 if I point the ski poles perpendicular to the north east to receive the electric wave of the transmitter. The inductive antenna is already receiving the magnetic wave of the transmitter.



    Oh okay, so a resistor is not needed for the low pass filter because the load it is connected to IS the resistor. So if the load it is connected to is a guitar amp that has a 16 ohm speaker output, is 16 ohms the load to use in the RC filter calculation?

    Or the RC filter could also just use the resistance of the 100k resistor (pencil lead), maybe.
     

    Attached Files:

    Last edited: Apr 26, 2013
  5. davenn

    davenn Moderator

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    yup OK ... but a standard crystal set isnt really designed to receive FM, it will but not efficiently

    Thats the way it often works, particularly 500 kc to 30 MHz which uses the F layer in the Ionosphere. Day/night doesnt have quite so much effect on FM band and higher because the propagation mode is quite different
    Weather can have a much greater effect on VHF/UHF propagation. It is also affected by the much more whimsical E layer

    the standard diople antenna picks up the EM wave in total. what inductive antenna ?

    Yes sort of

    NO !! the speaker is the load for the amplifier, NOT anything connected to the amplifiers input

    Just do what Steve said, connect a small value capacitor across the output of the receiver

    And finally ..... there's just so many errors and misinformation in those 2 attached files, I'm not even going to make a start in trying to correct them. I suggest VERY STRONGLY that you NEVER look at either of them again ... OK

    Dave
     
    Last edited: Apr 26, 2013
  6. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Well I got the idea that an inductive antenna would work good with an external aerial dipole for FM crystal set from this article: http://www.somerset.net/arm/reprints/fm_crystal_set_1.jpg

    I thought the inductive antenna in that article looks too small so I thought I would make it better by making it bigger. "big is beautiful" when it comes to antennas for crystal sets, I think but I might be wrong. big antenna might only work for AM, I'm not sure (of course it will pick up an FM station 5 miles away, but not very well).

    So I got the idea that any random coiled wire around a ferro magnetic metal like steel and including guitar pickups would do as an inductive antenna.

    Okay I'll pretend those two images that I attached don't exist and we will proceed forward with the low pass filter.

    So okay so your saying that no resistor is needed in the low pass filter because the load that the crystal set already has from the 100 k resistor and even the load of the phono cable connected to the amplifier is enough load?
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    After looking at those schematics (?) I think I need to wrap my head with duct tape to prevent it from exploding! :eek::eek:

    Chris
     
  8. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Okay now I think I should add a high pass filter to remove mains hum as well.

    I'm not sure what would be a good cut off frequency. Mains hum is at 60Hz, but it has an awful lot of anoying harmonics that by trial and error in recording the mains hum into Audacity I found that the mains harmonics are the loudest thing to listen to until at least 1,000 Hz high pass filter applied in Audacity then I can faintly here the local FM station 5 miles away with the anouncer saying "Star 105.5 McHenry County's variety." No other stations are heard (at least non that were heard over the mains harmonics, the booming news anouncer's voices from AM were probably removed with a 1,000 Hz high pass filter). This is what is heard with the inductive antenna connected to the guitar amp and the guitar amp is recorded into Audacity.

    So would a 1,000 Hz high pass filter before the detector work to remove mains harmonics?

    Since it would be put before the detector, it would not effect audio, it would only effect mains inductance pickup.

    I wish this forum allowed audio file attachments, but I think you probably know what I mean when I say anoying mains harmonics at much higher frequencies than 60 Hz.

    P.S.: lol the forum is calling me a senior member now.
     
    Last edited: Apr 26, 2013
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Honestly I don't see how anyone can give you any, and I do mean any design advice without you supplying a cogent schematic. So far what we've seen is rather like trying to read Pigeon English. I mean those schematics are positively weird stuff. It's a toss-up as to which is more weird, this one..
    https://www.electronicspoint.com/at...r-remove-rf-fm-crystal-reciever-schematic.jpg or this one https://www.electronicspoint.com/at...ole-directional-antenna-inductive-antenna.jpg but I'm giving the latter the incomprehensible award. Somehow the whole concept of what constitutes a Crystal Radio is lost in all of this strange stuff.

    I realize that I may sound like I'm berating you but that's not my intention. I'm just confused. :confused: The design features of Crystal Radios don't usually, ... correction,.. has never done that, .... Ever! :D

    Chris
     
    Last edited: Apr 26, 2013
  10. dietermoreno

    dietermoreno

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    Dec 30, 2012
    I'm sorry if I sound noobish, but so are you saying that crystal receivers never have a high pass filter to remove mains hum, just it is something to avoid picking up rather than something to remove?
     
  11. davenn

    davenn Moderator

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    from distant memories, I dont recal hearing any significant mains hum on my crystal receivers


    Dave
     
    Last edited: Apr 27, 2013
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I don't recall saying that or even hinting at it, but as Dave said, I too have never experienced significant mains hum on a Crystal set. Like Dave it's been a Long time ago though. Maybe we had less mains hum in the 50s. :D

    Chris
     
  13. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Oh okay, so most likely I'm doing something castrophically wrong if all I hear is mains hum, like a wire is unused and becomes an unintentional antenna that picks up mains hum.

    Similar to the problem I noticed when I posted on this forum 5 months ago and found out that using a TRS cable instead of an instrument cable to connect guitar to guitar amp is a source of hum.

    I don't think there is anymore mains hum now than there was in the 50s. Your really that old and you know how to use online forums? *Gasp* Take that as a compliment.

    Hey now that I think about it, silly me, in the testing of my inductive antenna, I was using a TRS cable instead of an instrument cable to connect the inductive antenna to the amplifier. Problem detected.

    Oh wait, its not just that, its that I'm being an idiot and the "inductive antenna" includes humbucker guitar pickups, so the antenna signal just cancels itself out and all that is demodulated by the guitar amp is the mains hum picked up by the cable used to connect the guitar to the guitar amp.
     
    Last edited: Apr 27, 2013
  14. davenn

    davenn Moderator

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    Dieter

    final warning for this thread and if you ever want to make it to 200 posts you will listen

    PLEASE FORGET ABOUT GUITARS AND RADIO RECEIVERS. As you have been told several times before ... forget about guitars and radio receivers combined. If you wanna make music play the guitar. If you want to build a radio then use a proper radio receiver design OK !!

    IF I see radio reception and guitar mentioned in the same post/thread again I will immediately lock it. If you ignore that, I will ban you.

    Now, also as said before, if you really want to build a radio receiver AM or FM, we are all happy to help, but you have to play your part .... OK


    Dave
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    After Dave's last post I had to know what triggered his response. As wacky as this thread is I knew there had to more going on to set him off like that. So I navigated to one of your other topics and posted there. You may want to read it.

    https://www.electronicspoint.com/tu...es-acting-speakers-t259308p2.html#post1545309

    My guess is that Dave wants to preserve the integrity of EP. Too much weird stuff makes us look like a bunch of nimrods! :eek:

    I suspect that if I read through more of your older topics I'd find more episodes of 'Ripley's Believe It Or Not'! :rolleyes:

    Chris
     
  16. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Hhm, well I do suppose that there are better ways to build inductive antennas than to use guitars, considering that (1) its not meant to do that (its not meant to be an inductive antenna), (2) if it uses humbucker pickups its a waste of time (part of the "its not meant to do that"), (3) a wire on a guitar string hovering over a guitar pickup is a poor antenna connection (also part of the "its not meant to that"), and (4) the guitar uses the guitar body as ground and since the guitar body touches the ground, any antenna signal is grounded and the only signal that is picked up is mains hum in a ground loop (another part of the "its not meant to do that").


    So now I will build a proper indoor inductive antenna.

    Like this: [​IMG]

    Any random large non conductor will work to wind some stereo speaker wire around.

    Like from looking at my surroundings while I am typing this, I think an unused about 6 inch x 12 inch Amazon box will do to wind the wire around.

    The only concern I could think of is: do the number of turns on the inductive antenna effect the tuning, like if it has too many turns it can only receive AM rather than FM and if it doesn't have enough turns then it can't receive AM?

    When I Googled FM crystal set, the schematics I was shown said that it was very important that the inductive antenna was only one turn or else it would pick up AM stations.

    Like this: [​IMG]
    [​IMG]
    http://solomonsmusic.net/FM_CrystalRadio.html

    So If I'm only supposed to have one turn on the inductive antenna, then I imagine that I should use a thicker gauge of wire rather than stereo speaker wire. The schematic says to use #18 bare copper wire. I can buy some on Amazon with my next shipment for only $2.10 (the one that is called "artistic wire"). http://www.amazon.com/gp/offer-list...1&keywords=#18+bare+copper+wire&condition=new

    So then I guess the converse is true for making the connections, the connection wire gauge should be thinner like stereo speaker wire so that the connection wires don't act as inductive antennas.

    And what is a good way to prevent the antenna from grounding itself? The schematic has the radio built on top of a piece of wood, I'm not sure if that is important to prevent the antenna from grounding itself.
     
    Last edited: Apr 27, 2013
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    If you had posted this in the beginning of your thread instead of all the weird crap we'd be done with this thread by now. What the author did in his circuit is nothing like what you're attempting. The output of his FM Crystal set is High Z and is fed to a High Z input jack of an audio amplifier. There's no inductive pickups in the sense that we know them anywhere in his design.

    Note: Ignoring Dave will not win you points!

    Chris.
     
  18. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Your right! No huge inductive antenna shown in the schematic and no huge long wire antenna shown in the schematic, which is for a good reason! (which I talked about that reason in my last post)

    So from now on I will post the schematic I found first before attempting drawing my own schematic.

    So then yes, we are in agreement that my new inductive antenna design is the same as in the schematic and the schematic shows that it can be connected to a dipole aerial, just like I am building.

    So I will build the antenna first and stick the antenna wire inside my hi-Z guitar amp input to see if the antenna works (if it works as in hearing static instead of hearing mains hum).
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I certainly didn't agree to that.

    Your "inductive antenna" (I presume you mean the loop connected to coax) is an idea, NOT a design.

    A design would (at minimum) have measurements.

    Also, you are connecting a balanced load (the loop) to an unbalanced line (the coax) which is often a cause for effects like you have famously seen with guitars.

    This explains the issue.
     
  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Ground Control to Major Tom....

    Please be advised that our instruments indicate that your oxygen levels are dangerously low. Mission Command recommends initiating emergency protocol.

    I feel like I'm talking to myself! Yes, I do that occasionally but the conversations are more productive than this.

    Chris
     
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