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AM Radio Design Log

Discussion in 'Electronic Design' started by M. Hamed, Jul 16, 2013.

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  1. M. Hamed

    M. Hamed Guest

    #1
    ---

    Ok, now that I got some encouragement, I am going to start here. This is inspired by fitness forums where people post a training log to chart their progress and get guidance (Needless to say I no longer follow these fitness forums since I was bitten by the radio bug).

    Similarly, this is a design log of me building an AM radio receiver and most of the steps I have to go through. Some of that stuff will be laughable to plenty of people here due to my extreme electronics inexperience. I am anelectronics and RF newbie (I have posted about that before), and I am trying to learn.

    There will be another one later for FM radio. The goal is not AM or FM radio. It's the amplifiers, oscillators, mixers, noise figures, detectors, etc etc. I have chosen AM and FM because they are guaranteed to be always there, and the goal seems to be pretty well defined. Amateur bands will come later. I only got my license about a week ago and I don't have experience operating. With amateur bands you also really need a transmitter most of the time and transmitters are still further down my list.

    With respect to AM radio, I am fully aware of Ron Quan's popular book (and own it), but I promised myself I will not touch that book until I have gotten my hands dirty enough with personal experimentation. I want to commit some serious errors before I look into a ready-built project book :)

    ------------------------------------------------------------
    First step is to understand about Mixers. I was told these are the most complicated parts of the radio and have very specific requirement. At this point I don't have any spec for the signals that will be at the mixer ports (Jeff Liebermann are you staring at me?)

    So I thought I'd first build a two tone module with a variable output levelthen build the mixer and find the best performance the mixer can give. From this I can go backward to the Antenna and try amplifying the signal coming from the Antenna to the level established by my experiments with the Mixer.
    ---------------------------------------------------------------
    Two Tone generator:

    For the AM band build two oscillators one at 550 KHz, another at 1600KHz. Sum those two signals then using an attenuator, control the level going intothe Mixer.

    My choice of oscillator was arbitrary. Just pick the first oscillator design from "Experimental Methods" which happened to be a JFET Hartley oscillator. Seems like an overkill for a 2MHz oscillator but I don't care at this point. I'm sure I'll learn a thing or two.

    I decided to choose L much larger than C presumably because this improves the Q. I picked a value of C around 600pF. 1. Because I found some in my junk box 2. Because I found variable caps online in the pF range which makes it easier for me to change the design to use variable caps in case I wanted to build a VFO.

    @580pF + 550KHz -> L = 144uH. This was worked out to about 17 turns on anFT50-43 toroid

    @595pF + 1600KHz -> L = 16uH. This was worked out to about 15 turns on anFT50-61 toroid

    Still not sure whether the relatively small number of turns would hurt the Q or not.

    I have a few Toroids on order from eBay.
    ------------------------------------------------------------

    Summing amplifier:

    Once the two oscillators are working (hopefully) I would like to combine them together to form something resembling the AM band. For AM I should then probably insert a filter before the mixer.

    I looked into ways of combining signal and I found the concept of the Hybrid Combiner. I feel this is probably more usable at higher frequency.

    I decided I should be Ok just using an OpAmp. With an OpAmp I could sum thetwo signals and possibly easily control the output impedance seen by the filter or mixer. I could also amplify or attenuate the signal coming from the oscillator.

    I am convinced that I can use the same approach even when I start working with the FM band. I suspect that nowadays using an OpAmp to sum two signals in the 100 MHz range should be a piece of cake for modern OpAmps provided you pick the right OpAmp with good frequency response.

    ------------------------------------------------------------
    Takeaway:

    - Generate two tones using some arbitrary Hartley Oscillator
    - Choices of L and C may not be appropriate. Unsure about whether 15 turns on a toroid should be good for Q and oscillation.
    - Sum two signals using an OpAmp
     
  2. M. Hamed

    M. Hamed Guest

    Two resistors to sum voltages, wow, why didn't I think of that! It seems to me for this to work I'll need to make sure the input impedance of the next stage is much higher.
    Thanks for the tips on L's and C's. Now to actually figure it out using LTSpice, wouldn't I need to know the unloaded Q of the inductors for the simulation to be any useful?
     
  3. josephkk

    josephkk Guest

    Oh my goodness. Are you designing a fixed frequency receiver or a
    tunable receiver? Two oscillators at fixed frequencies does not give you
    a tunable receiver without some serious DSP or FPGA components.

    Are you designing a direct conversion receiver or a superhetrodyne
    receiver?

    There seems to be some serious misconceptions here.

    Please read a primer on basic receiver types and the principles that they
    work on.

    ?-)
     
  4. M. Hamed

    M. Hamed Guest

    Neither. The two fixed oscillators are not for the final receiver. They are just for testing purposes and experimenting with mixers and filters. The final receiver should be a superhet.
     
  5. Robert Baer

    Robert Baer Guest

    The way i understand his approach, is that initially, those questions
    are in the future of experimentation.
    I see it as: step 1 use 2 ARBITRARY frequencies and try various
    amplitudes, (linear) summing methods, and (nonlinear) multiplier/mixer
    methods. Document results, make notes one what works and how well it
    works/does not work.
    Fundamental groundwork for next step.
    I see absolutely NO "misconceptions here".
    I see a rather rigorous investigation of a well-known technology
    using scientific analysis.
    The approach is admirable.

    Boil no water without pot or fire.
     
  6. Phil Allison

    Phil Allison Guest

    "Robert Bore"
    ** I see nothing else but misconceptions - too many to count.

    ** You on drugs ??

    ** Hamed's approach is patently absurd - nothing more than the day dreams
    of a bored code monkey.



    .... Phil
     
  7. Glenn

    Glenn Guest

    Hi Don

    The inverting opamp is playing I->V converter. The inverting opamp will
    via the the negative feedback maintain the inverting input at zero volt
    (ideally).

    /Glenn
     
  8. josephkk

    josephkk Guest

    Thank you. I was a bit worried there. Design some Hartleys, some
    Colpitts, and a couple other types then. Each one has different valuable
    properties.

    ?-)
     
  9. M. Hamed

    M. Hamed Guest

    #2
    ---

    Toroids arrived. First target is the 550KHz oscillator using the first Hartley oscillator shown in Exp. Methods. Using a JFET, diode, 1MOhm resistor, L, and C.

    My toroid calculation were reasonably accurate verified using an LRC meter.About 16 turns of #22 wire. The tap is taken after 4 turns.

    Laid out everything on a perf board. Soldered everything to the board. The book recommends taking the tap about 20% from ground. I realized the way I soldered thing was not exactly this. The tap was taken about 12 turns from ground. After a little soldering mishap with a capacitor, I connected power, an oscilloscope, and voila! Output came out at 578 KHz. Not bad at all given that all components are fixed. It also seems the tap location didn't matter.

    Off to the second oscillator!
     
  10. josephkk

    josephkk Guest

    About 5 percent off in frequency, looks like parts tolerances. How is
    the wave shape for being clean? Can you digitize it and do a FFT?

    ?-)
     
  11. M. Hamed

    M. Hamed Guest

    #3
    ---
    Wired up the second oscillator. The inductor value should come to about 16 uH. Tried 5 turns on FT50-43 with a center tap but this didn't work. My guess here is that for a good Q, the number of turns must be higher.

    Switched to FT50-61 with 15 turns, and a tap at turn 4 and the second oscillator started working. I got about 1.5MHz, 0.1 MHz off the target but acceptable.

    On this one I had the smaller number of turns closer to ground. At first I thought it wouldn't matter but it seems if it's done this way the output atthe top of the tank is larger. Since I didn't want the second oscillator output to be much higher than the first, I reversed the toroid to have the small number of turns closer to the top.

    I then connected the two oscillators to the same power and ground points, and connected their outputs to two 10K resistors (per Jan's suggestion) and took the sum from the mid point.

    I know I should probably isolate the powers and grounds of both oscillatorsfrom each other. Probably similar to when you connect Analog and Digital circuits and have their grounds connected at only one point. I should probably add some capacitor bypassing to power. At this frequency I'm not sure ifthis is really needed and how much improvement I can get. Further experimentation is needed.

    The circuit is here :
    https://www.dropbox.com/s/gt4pf8kw4hfqse8/2 Hartley Oscillators 550K 1600K.asc

    Only difference from my actual circuit is that I'm using MPF102 JFET, and I'm too lazy to add the model for it.
    ------------------------------------------------
    Some of the observation with this type of oscillator:

    - I do not fully understand the function of the diode and the small 68pF cap connected between the diode and the tank. Exp.Methods book says its purpose is to have the gate at an average negative voltage, but it's not clicking.
    - At about 4V the 550KHz starts oscillating. At this point the diode seems fixed at about 0 V and the power supply shows it's drawing milliamps of current. As I increase my supply voltage, the signal at the diode starts increasing and looking more sinusoidal while the sine wave center point starts moving more negative. This is not clicking yet and still mysterious.
    - Connecting my VC3165 cheap Chinese frequency counter to the output of anyof the two oscillator shifts frequency by a noticeable amount. For examplethe 1.5 MHz becomes 1.3 MHz. The spec claims a 1MOhm input impedance. Connecting my scope which also has 1MOhm input impedance doesn't do that. This is also mysterious to me. I'm going to try connecting the Scope probe to the frequency counter and see if that works. Maybe the probe gives better isolation from the counter's input circuit.
     
  12. M. Hamed

    M. Hamed Guest

    The wave looks very clean and nice. It would be nice if I had a spectrum analyzer to look at the spectral purity :)

    At the moment I have no way to digitize it and FFT it. I may be able to bring the circuit to work and do that. I have access to a high end scope there.
     
  13. M. Hamed

    M. Hamed Guest

    I forgot to mention that as this happens, the current drawn from the supplygoes less and less as I increase the voltage. A bit counter-intuitive.
     
  14. Tauno Voipio

    Tauno Voipio Guest


    The diode at the gate (or the gate-channel diode
    in a JFET) rectifies your oscillation signal amplitude
    and charges the series capacitor so that the gate will
    be negative. The larger the amplitude is, the more
    negative the gate gets, and the drain current gets smaller.
    This is the AGC which keeps the signal reasonably clean.

    The trick is from the tube era. A grid-dip meter
    functions by measuring the grid/gate voltage by
    measuring the current in the grid resistor.
     
  15. M. Hamed

    M. Hamed Guest

    On Saturday, July 20, 2013 9:32:25 AM UTC-7, Jan Panteltje wrote:

    <Snip>

    So you're essentially suggesting to convert to a Colpitts. I have built Colpitts oscillators before and they seem easier to understand. I thought I'd take a shot at this strange Hartley and the use of tapped inductors.

    My use of R5 was to test the effect of an external load to ground taken from the summation point. I wanted to see how low my load resistance can go.
    Couldn't find anything in the electrical spec related to input capacitance. It seems they don't specify it.
    Could you explain a bit more why? According to the book, L is at DC ground hence the average will always be zero.
     
  16. M. Hamed

    M. Hamed Guest

    Now I'm going crazy. My multimeter connected to the tank shows 100mV in the AC setting when my scope is showing 9V p-p. I thought the Ac setting measures RMS!!
     
  17. josephkk

    josephkk Guest

    When the circuit is operating properly it is desirable to keep the gate
    from going positive WRT the source as forward biasing the gate source
    junction is very bad news for circuit linearity and JFET operation. The
    cap charges up against the diode to accomplish this.
     
  18. Tauno Voipio

    Tauno Voipio Guest


    Jan, that is a Colpitts. Please do not leave the gate capacitor
    and resistor out. They provide the amplitude control, see
    my previous mail.
     
  19. Tauno Voipio

    Tauno Voipio Guest

    The multimeter AC measurement is not reliable
    on these frequencies, the calibration is for
    the line (50 / 60 Hz).
     
  20. M. Hamed

    M. Hamed Guest

    This sounds intuitive but I noticed things are not so linear.

    I used two probes on my scope to subtract source from gate voltage (VGS). There are 3 phases of operation:

    - Supply is less than about 2.9V. In this case there is no oscillation. Supply is drawing 20 mA
    - Supply is from about 2.9-3.9, oscillation starts and as you increase/decrease supply voltage, VGS will go more/less negative. Supply current also goes from 20mA to almost zero very rapidly. You see there is a very small window where this happens. Also there is some kind of hysteresis going so the tripping points will change depending on whether you go up or down.
    - As supply goes higher than about 3.9, current almost vanishes, and the change in the negativity of VGS changes very little as you increase your supply.

    This reminds me of CMOS inverter action.

    The circuit is much more complex than I thought and very ecucational.
     
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