Variable Capacitor Angst

Discussion in 'Wireless & RF Design' started by atldave, Aug 12, 2010.

  1. atldave

    Thread Starter New Member

    Jul 26, 2010
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    OK. I've been reading the ARRL Handbook (2008) and the "Experitmental Methods in RF Design" and sites around the web - including the links that Bertus posted at the top of the forum.

    Every receiver example circuit I've seen uses a variable capacitor.

    Aside from the fact that the electronics supply sites don't sell them anymore, I would prefer to build and learn about newer circuit designs - I would like to be able to build things from components from the "grab bag" parts offered.

    Any suggestions (books, sites)?
     
  2. bertus

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  3. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    If you want old style, they are on ebay. example http://www.californiarf.com/variable-capacitor/
     
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  4. atldave

    Thread Starter New Member

    Jul 26, 2010
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    That's the thing, variable capacitors are like tubes - they're for old radios - which unfortunately is what all the amateur radio technology materials (books, websites, etc..) cater too. The Handbook talks about the PLLs, VCOs and whatnot but all the example circuits are changing frequency by adjusting capacitance instead by voltage. And how would you, for instance, tune for the whole shortwave broadcast band using PLLs and VCO when the diode can only tune to a narrow band? Things like this are what's kind of getting me searching here....

    I'm in the middle of drinking out of a firehose and I feel like I'm drowning.
     
  5. bertus

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    Last edited: Aug 13, 2010
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    They're still available but pricey as heck, believe it or not the hams still like to build their own equipment and they're almost a must to have when working with antennas.

    I lucked into a big lot of the things on eBay many years ago, a girl's father who was a ham radio enthusiast, left behind a shack full of parts she hadn't a clue what were. Snce she wrote a very poor description and this was back when most people had to develop pictures then scan them (if they could even do that) then figure out how to manipulate and post them it took me a while and some letter exchanges to figure out what she had and amid all the stuff I scored about 40 air variables. I ended up classifying and selling about half of them for probably 50 times what I paid for the entire lot of stuff and kept the rest. Nowadays I wish I hadn't even sold a single one as they keep going up in value ($, not rating) but then again many were for things like 80 meter transmitters with huge plates spaced for transmitting tube voltages. Not as eloquent, and certainly without the "feel", on occasion I've had to fabricate a panel mount slug tuner and use it to tweak an inductor.

    Can't wait to get one of my pending RF projects together. Three element cubical quad antenna with a mast mount preamp operating in the commercial FM band. (88 - 108) The preamp promises around +20 dB of gain with less than 1 dB of noise, also has remotely tunable input, output and gain. The antenna itself should theoretically start out with +10 dB of gain as well. After figuring all the coordinates into things I pretty well have line of sight to the intended station, it's the distance that makes this station so hard to receive.
     
  7. atldave

    Thread Starter New Member

    Jul 26, 2010
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    I'm a newbie with regards to radio electronics and my electronics background is pretty basic. It makes it pretty hard to learn radio electronics when the lessons use outdated components - and it's not only variable capacitors it also includes the NE602!

    I'm one of those people that has to build things to really understand them. And I like to start with something the works first before experimenting.

    Anyway ...

    I've been digging through the links and reading about varicaps. I've been studying this.

    I also found a couple of old beat up boom boxes in the basement. One was all electronic tuning the other had a VC - WooHoo! I have one more boom box with a mechanical frequency display so I'm pretty sure that one may have a VC too. The CD player crapped out so I'm not too sad about ripping it apart.


    Now I understand why those guys showed up and asked about old radios when I was having a tag sale a couple of years ago.
     
  8. atldave

    Thread Starter New Member

    Jul 26, 2010
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  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    You are on the right track with your VariCap research. I understand what you are talking about with regard to the materials you are supposed to learn from being a generation or two behind. From my course in the 90's I have texts that are full of Vacuum Tube circuits.

    If necessary you can switch select parallel or series regular capacitors in and out your tuning circuits. That can make up for some of the problems of not having enough variable capacitor value for detail tuning. Switched tuning components are the best way to switch to widely separated frequency bands.

    The wiring of a switch for RF can be a pain so it is a possible solution but not necessarily easy. For some switching requirements you will want to use diodes in the tuner and your switch just selects the bias on the diodes so that they are selecting on or off for the tuning components. If you don't give the diode enough forward current it won't turn on completely and will become a reactive component. I am just warning you that for switching you have to stay out of the diodes non liner region. It is a balance. You want to stay out of the active bias region but if you go too far with bias current or it is driven with too much signal you can get the diode into the thermal region and that can be prone to causing oscillator drift.
     
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  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Thus the reason a lot of gear had a door on the top and coil sets mounted on an octal plug you could just change back and forth.

    RF wasn't and still isn't too difficult to design in if you're under 100 MHz, there are a handful of common rules and theories that apply and unpredictability is the name of the game. It isn't unusual for even the most experienced RF designer to have to make several changes to his work or especially parts layout for optimum performance.

    I rarely find myself having to deal with anything much over 1 GHz but I will do some occasional work in the 2.4 GHz band if I must. The truth is digital electronics is getting so fast that a good working knowledge of RF principles is almost a must nowadays, for instance having one PC Bd trace an inch longer than another is more than enough to cause pulses to arrive out of sync enough that a circuit won't work or work erratically. Forget square corners on PC Bd traces, line width and copper thickness all become factors and every trace also exhibits capabilities as an antenna that can couple to another trace not far away which hapens to hit around the same self-resonance.

    Designing the board for my FM preamp is on the list for today, half of it is totally SMD components so it's going to present a bit of fun.
     
  11. MMcLaren

    Well-Known Member

    Feb 14, 2010
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    Hi atldave,

    If you want to consider slightly more state-of-the-art technology then you might look at the AMQRP 1-60 MHz DDS-60 Kit which could be used for a receiver local oscillator and/or investigate Google search results for SDR (Software Defined Radio) which uses the sound card in your PC to perform signal demodulation functions.

    Cheerful regards, Mike, K8LH
     
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