Need some idea on building my first AM radio receiver

Discussion in 'The Projects Forum' started by tomshong, Jul 26, 2012.

  1. tomshong

    Thread Starter Member

    Oct 6, 2011
    Hi all,

    I've been trying to learn the basics of RF/Microwave systems on my own. I have the books and tools and I have the education, but I lacked the hands on experience.

    For a while now I've been playing around with amateur radio based on SDR that I purchased online, but I think I need to take a few steps back and work on something more basics, just to beef up on my fundamentals.

    I am thinking of building a simple AM receiver, capable of picking up local AM radio stations, that has the analog front end similar to SDR, but no software, no ADC, no DSP. Instead of process the signal in digital domain, I want to build an analog demodulator in place of the ADC/DSP block. But I want to build it in such a way, that later on I can still modify it into an SDR by replacing the discrete demodulator with an ADC/DSP block should I choose to.

    I see a lot of SDR's out there are using direct convesion topolegy with quadrature (RF slowing down to IF, but split into two signals, In-phase and Quadrature, they are identical but 90 degrees out of phase). The SDR's I've played with all have similar topolegy, so I could use those design as my reference when I design the RF front end for this AM radio.

    The part I am not so sure about is, since I have not fully understood the in's and out of how a signal is demodulated in I's and Q's, so I could use some pointer as to how to build a circuit that could demodulate the I's and Q's, in AM.

    Is what I am thinking of doing a possible feat? Has anyone attempted to build an AM radio based on direct conversion topolegy in quadrature? Or is there a design out there I can look at as a reference?
    Last edited: Jul 26, 2012
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Check AM radio kits on net; might be a source for front end parts which you could add to later? Got as far as Escondido over week end for grand daughter's wedding, oll the way to Irvine sometimes. Welcome to AAC. ' built a 5 tube AM, battery op, from back cover of RCA tube manual 1946.
  3. bertus


    Apr 5, 2008
  4. tomshong

    Thread Starter Member

    Oct 6, 2011
    Thanks guys.

    So I am starting to wonder if I should start with something more basic than what I posted earilier. I am thinking one of these

    I see it’s basically a darlington pair and a Common Emitter, and both’s got feedback.

    I don’t see where is the equivalent of doling mixing and filtering in this circuit, and I don’t see how the amplitude is demodulated here. Can someone elaborate ?

    I think, for my education value, I would like to build something with major RF building blocks in it (amplifier, filter, mixer), and I would be able to test each of the blocks out separately, and if possible, a system that has I and Q output so later if I decide to DSP it I can. Is there any projects like that out there someone can recommend?
    Last edited: Aug 6, 2012
  5. tomshong

    Thread Starter Member

    Oct 6, 2011
    While I was looking up information on the AM, I come across this FM receiver chip.

    Looking at the block diagram, it look like the pins on the chip are connected in-between various stages of the receiver.

    It got me to wonder, if it is feasible to measure things such as Dynamic Range, IIP3, NF, etc on various stages on this IC simply by taking measurement across those pins? Say if I want to measure the IIP3 of that amplifier on the lower right corner of the block diagram, between pin 8 and 9. Having that amplifier input connected to mixer output and the amplifier output connected to the IF filter input, would that pose some kind of loading effect on the measurement?

    Has anyone here worked with this chip in the past? I'd be interested to know my idea is feasible. It's hard to predict what will happen if one don't use a chip as intended. Block diagrams are representative of the function and not the implementation.

  6. vk6zgo

    Active Member

    Jul 21, 2012
    Well,it is a very basic Tuned Radio Frequency (TRF) receiver,
    The Darlington pair Q1 & Q2 are a RF amplifier with an untuned output which then passes to Q3 which is the demodulator.
    As the signal is AM,the sidebands & carrier are mixed in the demodulator
    (base emitter junction of Q3,I think).
    As there is demodulated audio on Q3 base,it also operates as an audio amplifier.
    Filtering is quite basic,probably there is still a bit of RF present at the audio output,but this won't pass through the fairly high inductance of the headphones.
  7. tomshong

    Thread Starter Member

    Oct 6, 2011
    I ran the LTspice simulation of this circuit, the file of which can be found here, saved under file name AM_Receiver.asc, I am not the originator of this file. Temp/tomshong/

    Looking at the Transient Response
    At the output of the Darlington Pair at the collector of Q2, it seems it clipped the top half of the modulated signal. How did that happen? With a VDC of 9v there should be plenty of head room for the signal to swing up?

    On the more fundamental level… why Darlington pair? That is, why do we need to have a current gain at this stage?
    How does the Q3, a Common Emitter with a feedback, work as a demodulator?

    Looking at the AC response.
    Following the transient response, somehow I was expecting to see a frequency response of only the signal signal at the output. Yet, it look like whatever resonant frequency between L and C and antenna is carried over to Q3 output. Can someone explain what is going on here?