Cheap LCD Radios

Discussion in 'Wireless & RF Design' started by Art, Oct 26, 2013.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi Guys,

    There are some cheap AM/FM pocket radios with LCD screens telling the tuned
    frequency to the user, but without any PLL.
    They still have variable tuning capacitors.

    http://www.jaycar.com.au/productView.asp?ID=AR1741

    My question is how do they know the frequency they are tuned to?

    I thought maybe using the tuning capacitor,
    but other connections to it would interfere with the radio.
    Maybe there is a spare capacitor gang in the tuner.
    Also thought the value off the tuner would be a bit small to use a resistor
    to time the discharge of the tuning capacitor.

    Thanks, Art.
     
    Last edited: Nov 5, 2013
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Could be by the freq of the LO
     
  3. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi,
    Could you put that into words understood by people who's strongpoint is not analogue electronics? :D
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    We'll since it is not clear what your level of understanding is how about you asking some specific questions rather then me going into a full explanation of a superhetrodyne radio receiver.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    I would imagine that even an inexpensive radio with a digital readout would be using digital frequency synthesis.

    LO stands for local oscillator.

    The local oscillator is usually tuned to a frequency that is the radio frequency plus the intermediate frequency (IF).

    For example, if the IF is 455kHz and your radio station signal is 1010kHz the local oscillator will be set to 1010 + 455 = 1465 kHz.

    When the LO is mixed with the incoming RF you will get 1010 + 1465 = 2475kHz
    and 1010 - 1465 = -455kHz (oops, I suppose that should be 1465 - 1010 = 455kHz).

    The IF amplifiers are tuned to 455kHz thus rejecting the 2475kHz signal.
     
  6. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Yes, thanks :)
    What I meant with the question was, where would the interface would be made.

    One could directly sample with a micro digitally at KHz frequencies.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    With Direct Digital Synthesizers (DDS) the frequency selection and LO generation is done digitally. You select the station digitally. Hence there is no need to measure the LO frequency.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Yep, that's how they jump from station to station instantly, they just generate the frequency digitally, and already know what the station frequencies are. Some of the very cheap radios just have an up/down button pair, that jumps between stations.
     
  9. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    I was sure it had a tuning gang that had to be turned, and wondered how the LCD worked at the time.
    I'll sacrifice one and bring for show and tell next time. I should have looked harder.
    That will take a week for pay day hobby budget though.
    I got one of these a while back and wrecked it because it wasn't a proper PLL radio.
    If you could change channels with buttons, and station memory, I'd have kept it.
     
  10. TecknoTone

    New Member

    May 20, 2012
    21
    1
    Radios of this sort use an internal frequency counter which deduces the received frequency from that of the VFO.
     
  11. praveenchandra

    New Member

    Nov 5, 2013
    1
    0
    This topic is quite interesting to discussion. I think, It will be happening with the internal frequency counter. Generally, Frequency is defined as the number of events of a particular sort occurring in a set period of time. The accuracy of a frequency counter is strongly dependent on the stability of its time base.
     
  12. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    If the chip is a frequency counter then where is the clock coming from?
    The old fashioned way?
    The chip FM radio I made on protoboard with a radio in a chip,
    has enough inductors and capacitors outside the chip for tuned frequencies.

    I should go get one tomorrow, I think it's too dumb to jump between radio
    stations. If it was able to, it would be made that way, so I'm inclined to believe it's analogue.
    I can afford to wreck a cheap radio on the hobby budget.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    You can tap off the signal from the local oscillator (LO) using a FET.
    You need to know the intermediate frequency (IF) being used in the radio.
    Then you have to add/subtract (depending of the radio design) the IF to/from the LO frequency.

    If you are building your own frequency counter there is a way to add/subtract the IF.

    Here is a photo of my DX radio with a homemade frequency readout showing the IF offset that is preset into the counter.
    I can choose to add or subtract the IF offset.

    [​IMG]
     
  14. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Thanks MrChips, I will get onto you when I'm actually doing it.
    The frequency counter isn't hard, it might be interfacing to the radio,
    and/or buffering the clock without wrecking the clock signal.
    I do know the Intermediate Frequency.

    What has me a little puzzled since last posting is the frequency counter,
    even the one you have pictured, if not this radio, must be run by a chip
    significantly faster than 108MHz to be useful for FM,
    and I don't think there's a micro that fast in a cheap radio to dodge putting a PLL in it.

    Unless the higher frequency can be divided, at the cost of resolution,
    or there is a relative, but lower frequency available somewhere else.
    But that's not consistent with what you so far explained about a super-het.
     
  15. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Another project of mine has this:
    [​IMG]
    on the way, and then I think we are back in business.
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    My frequency counter was built many moons ago using standard TTL chips.
    You can get TTL counters such as 74F160 that will count up to 120MHz.
     
  17. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Ok, I want to do LCD graphics as well. I got a 128x64 display given to me.
    Lucky it's AM, I can count frequency that fast cheaply.
     
  18. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    The radio I linked has been discontinued by the retailer,
    so I got this one instead:
    http://www.jaycar.com.au/productView.asp?ID=AR1736
    The guy that sold it to me assured me it's more or less the same thing.
    It has a belt drive in it to make the tuning knob more sensitive
    because the tuning knob on the case can do 2.5 revolutions,
    but it's still an old fashioned tuning gang.
     
  19. MrChips

    Moderator

    Oct 2, 2009
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    Why exactly do you want to do this? That is, add frequency readout to an analog radio tuner when you can buy a radio with digital tuning and readout?

    I'm just curious.
     
  20. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    For the valve radio in my other thread, I want to interface to it,
    and count the frequency from it's clock.

    With this one I'm throwing it's parts straight in the bin as I remove them.
    Just a sec.. It's impressive. I've got a few for Photobucket. BRB.
     
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