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I.F. Ceramic Filter

Discussion in 'CB Radio Modifications' started by TheRealPorkchop, Aug 17, 2018.

  1. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    I’ve watched a pile of videos from Lester, I know you guys have too. I notice that in a lot of his videos he mentions changing a filter, the I.F. filter.

    I don’t know a lot about this so I’m hoping one of you can enlighten me and educate me on it. Why does he do that?

    Is it only for transmit or only for receive?

    Not really knowing what to look for or how to search for an answer, I come up with a 10.75 ceramic filter being listed for FM. Why would anybody care about FM on an export chassis radio in the states?



    So why does he do that, besides money, what’s the reason he’d tell you that it needed to be changed? I assume it’s for rejection of bleedover from other channels, is that why?
     

  2. Martian

    Martian Active Member

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    Legitimate.
    Only for receive. But for AM as well.

    He is adding / replacing a filter in the radio to improve receiver performance.
    It will help with bleedover and noise. He narrows the receiver which for someone that wants to talk on AM is a good idea.

    One of the few real receiver improvements that actually involves doing real work unlike the "Fine Tune" and 2SC2999 stuff that doesn't get you anything but a lighter wallet.

    There is a radio that he does or did that had a spot for the second filter that he populates. Most of them he is replacing the existing filters with narrower filters.
     
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Many of the issues around reception these days, is getting the signal into the radio and being able to amplify it.

    The way CB radio works, is to use a process similar toy hetrodyning. at least it's how the process is done...

    It's also why you see service manuals with charts showing PLL, channel, and IF frequencies the radio uses.

    Take 27MHz, and you have several Xtals that you can use to obtain it - directly, or use a method where the PLL can generate a known frequency and keep it accurate - mix in a known signal it can make, into the signal - and when the two mixed or combined, you have the main CB frequency, the PLL's own frequency, and "images" - one above higher, in frequency - which is the ADDED result of the mixing, another is LOWER than both the CB frequency,and the PLL's frequency - it's an image.

    To save bandwidth of discussion, Uniden, Cobra, Tandy - others took to use the LOWER set of frequencies to use and design circuits with a lot of gain So they use heterodyne to grab a signal (or channel - a narrow range of frequencies) by mixing in one, to obtains products or mix's of the main CB "channel" - and then they can design amplifiers with high gain around them.

    So, if you look at the service manual - let's say - Channel 19. 27.185MHz, IF - 16.945MHz - ok, what does that mean?

    It means the PLL can generate an IF frequency, and if you look further into the service manual - you'll see the PLL uses a 10.240MHz xtal as a time base reference - this is important to know. For now you have 3 frequencies you can design amplifiers around and for to obtain good signal using heterodyne and image.

    Ok, here's' an accelerated way to see this...

    27.185MHz = PLL will send to the 1st FET a signal of 16.490MHz, leaving two workable images, one at 43~44MHz (not too many transistor parts work well that high without serious impedance and matching input admittance issues) so we take the low-road and use methods to obtain the other LOWER image...or one close to 10.695MHz.

    We can use 10.695 for the DERIVED IMAGE - the one we wish to extract.

    So here's where your Crystal Filter selection comes into play.

    You can mix into the FET (1st IF) and obtain the 10.695MHz (or 10.7MHz if so desired) using you 10.695MHz Xtal - then amplify it really well.

    But we're not done yet!

    We can now use the PLLs' 10.240 Xtal timebase as a means again to Heterodyne the 10.695MHz down to 455kHz by mixing in 10.240 MHz directly from the timebase Xtal. It can be amplified so that it can be used not just for the PLL but for the Radios' 2nd IF stage to obtain 455kHz image and amplify that even more and better than at 10.695MHz.

    Then 455kHz can be your AM Detector stuff you get for receive - AM - but you can do this same process for FM but then FM doesn't need the 2nd IF - you can use the 10.695, and 10.240 and DISCRIMINATE against the signal to obtain FM that is internally referenced a 450kHz local oscillator signal and get audio out that way.

    Hope the above can show others how a CB with a simple 10.240 Xtal -can generate an IF signal in a freerunning oscillation around 16.400~17MHz that's kept in check by the PLL referencing it to the 10.240 Xtal time base it has. Then obtain 10.695MHz and then use the PLL's 10.240 clock against that - to obtain 455kHz and then detect that signal as your AM channel station...

    So instead of making a xtal vibrate by applying power, you can send power thru it, as a signal - and it will only pass the ones (signals) closest to it's resonate frequency.


    :+> Andy <+:
     
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  4. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Damn Andy, can you send me some of the water from your house? If somebody doesn’t have an understanding with your answers, they just missed the bus.

    Most of that I already had a general understanding of, now I have a better understanding. So my next question is, how do you determine what the current frequency is and how do you pick one to replace it with? It is really worth it?

    Martian, thanks for your response as well. What’s the fine tune method?
     
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  5. Martian

    Martian Active Member

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    He does the long answers I do the short ones :)

    It's not really a matter of the frequency since the first and second IF is pretty standard at 10.7ish Mhz and 455Khz respectively.
    The trick here is how wide the passband is on the filter.
    There are other variables to consider but keeping it simple.
    Here is a datasheet for review:
    https://www.mouser.com/datasheet/2/122/mono crystal filters-309914.pdf
    @Handy Andy can drop in the long answer.
    He likes to do those :p


    Now, the "fine tune" is just a gentleman that makes outlandish claims not supported by science. Butt end of a joke.
     
  6. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Kinda like a 900w 98vhp?

    I meant what size bandwidth should one pick. I been up almost 2 days now, I honestly don’t know what I’m saying at this point. It’s time for sleep. I just know I want to shut the receiver up in my radio some and of stopping some or all of the bleedover comes with it, that’s good too.
     
  7. Martian

    Martian Active Member

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    Honestly I don't know whats in these radios.
    I could pull one out real quick and toss it on a tracking gen and see but I thought they were pretty wide. like 7.5k wide.

    You don't want it to be too narrow obviously.
    Let's see if someone chimes in that knows more about it than I do before I get to yanking one out to measure.
     
  8. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Well my radio of choice is my S9, I’ve had that thing for almost 15 years and it’s hands down my favorite, especially the receiver in it. I have several others that I’d like if the receiver in them was like my S9. I’ve looked at the schematics for them but maybe I’m overlooking what the differences are.

    I’ve got a 98 that I’d like if the receiver was better. Even with an alignment, it just isn’t the S9. Maybe this filter thing isn’t what I’m looking for but I’d like to fiddle with it and see what I get.
     
  9. Martian

    Martian Active Member

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    Filters can make all the difference.

    More so than "tweaks" and "tunes" can.
    But good filters are also expensive.
     
  10. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    I take the RF gain on maybe S9 and turn it all the way down (off), counterclockwise. I turn the squelch the same, off. I then turn the RF gain up slightly, maybe the 7 or 8 o’clock position. There’s hardly any static and I ride all day with it like this in my dump truck, I can hear everything that I could if I cranked the gain up. If there’s something that’s really out there and I’m having trouble hearing it, then I turn up the gain a bit more.

    I can’t do that with any Galaxy I’ve ever owned and I can’t do it with the Strykers that I have either. On my Galaxy radios, once you hit about halfway counterclockwise, there goes the receive.

    I don’t care what they cost. If they make an actual improvement then I’m all for buying them
     
  11. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Each radio is set up to amplify one set frequency in each stage - so they have to look at the "image" they want to use and develop the strip for that particular frequency - coils, semis, Resistors, Caps n such to amplify just that "group" or bandwidth of frequency.

    So the biggest selection problem - is really with the 1st IF stage - you just have to look at what the Xtals frequency you want to use.

    Now with most, there are 10.695, or 10.7 - even 10.697. You already have that pretty much chosen by Galaxy.

    They've done that...

    Now this is not a guarantee - but if you knew what to do, you could replace one Cap, with a Xtal of a SLIGHLTY OFF FREQUENCY - hint 10.697 - and use that in line at say the NB input side of you Band Pass Filter...I'd try swapping out the 5pF at C59 or C58 and squeeze the xtal in there and observe what happens (you'll have to tweak) the coils that BPF in there to recover your losses...

    And so you know, caps are chosen because they don't take a lot of effort to pass a charge across them - in these cases 5pF is low charge - but it can be transferred readily.

    Xtals - take power to resonate, but the resulting resonation is strong and selective. Meaning it vibrates the most, and transfers most of its energy efficiently - at the resonate frequency and no where else.

    It was one of the reasons why I liked radios like the PC122 - not too much room in there for a lot of triple - bypass surgery and the IF messes in routing and their noise injection points (front panel switches are a good point I'm using here) - look at the PC122 - it's pretty straightforward.. Which meant it had a narrower selective range for the Clarifier to use - you didn't have multiple conversion sections - just two - AM to detect after the 2nd IF and the SSB with was more direction conversion than anything else.

    Once you unlocked the clarifier - you had a whole other world out there to ha ve fun with - you can beat down dead keys and heterodynes by zeroing in on the carriers and used their carrier to help modulate yours by beat frequency BFO-ing) to their center - USB or LSB can help too - because their carrier contained the decoding information - you simply sent audio on one side of the band for it and the others would hear you thru it.

    Ok, sorry for the derailment - had a memorable moment there...but referring to the Xtals - you can use two Xtals that resonate just above and just below the frequency of interest. In doing this, think of it like a Venn diagram - you have to circles that will intersect at the IF frequency you chose, and that overlap is your bandpass - the range of IF frequency (1.5kHz of IF bandwidth) that passes through between the resonate ranges of the two crystals. It's why Lou Franklins idea worked so well - Xtals - and the More Xtals - took and even narrowed down the "skirt" or Venn overlap even smaller, narrowing the response to within 1kHz resolution.

    Doing it this way has drawbacks, someone wanting to talk to you on a simple radio a few Kilohertz off, wouldn't be able to make the trip. They'd have to "slide in" to the bandpass of the PLLs' IF (remember 1st IF) of 16MHz within a kilohertz - in order to be picked up and heard by you - and then you'd have to trim your clarifier to their slightly off carrier signal.

    Another drawback is an attenuation factor - again Xtals take in energy to vibrate - it's a loss over the entire effort. So what you gain is selectivity may be lost in noise and weak signal due to the effort.

    It's a choice and stronger signals fare better in these cases than weaker ones - they'll lose out being part of the noise.

    A radio - talking about it to a receptive audience, there's nothing like it...
    :+> Andy <+:
     
  12. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Ah, I see what you saying, makes a little more sense now. Also makes me second guess if that’s really what I want to do but I definitely want to experiment.
     

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