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Antenna Simulation in LTspice

Discussion in 'Electronic Design' started by rickman, Feb 28, 2013.

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  1. rickman

    rickman Guest

    I am working on a simulation for a loop antenna in LTspice and I can't
    figure out why the signal strength features are what they are. The
    model uses a pair of loosely coupled inductors to model the transmitter
    and antenna loop with a separate pair of tightly coupled inductors to
    model the coupling transformer. A cap on the primary circuit is the
    tuning cap and a cap on the secondary is parasitic effects of the
    circuit board leading to the inputs on the IC.

    There is a resonance near the frequency I would expect, but it is not so
    close actually. I can't figure why it is about 5% off. There is a
    second resonance fairly high up that I can't figure at all. None of the
    component values seem to combine appropriately to produce this peak.
    When looking at the tuning capacitor voltage there is an anti-resonance
    that is exactly at the frequency corresponding to the secondary
    resonance with the transformer and the parasitic capacitance. That
    makes sense to me, but it is pretty much the only part that jibes with
    what I can figure out.

    I have uploaded a zip file with the schematic and a measurement file.

    http://arius.com/temp/Antenna_trans_LTspice.zip
     
  2. Tim Williams

    Tim Williams Guest

    I expect if you reflect the CT secondary stuff (don't forget Lsec) back to
    the primary, your answer will appear. Offhand I can't reason out which
    sum of L and C makes the resonance, but it's a four pole series-parallel
    resonant circuit, analysis should lay it bare.

    Tim
     
  3. rickman

    rickman Guest

    Are you sure about this? When you say to reflect the secondary back to
    the primary that means the primary inductance would be doubled? I
    believe the coupling of the two coils means they are one and the same
    for the purposes of the circuit analysis, no?

    You can't reason which sum of L and C makes the resonance and I can't
    either. The calculation is off by about 5% and I can't explain that. I
    can explain a null at about 290 kHz. That is the resonance of the
    secondary with the secondary capacitance. I can't explain the other
    peak at 363 kHz at all. A higher frequency would imply a smaller L
    and/or C. How do you combine them to produce that? Consider the two
    caps to be in series???
     
  4. Tim Williams

    Tim Williams Guest

    Sure. If you bring the 10p over to the primary, it looks like 10p * (30m
    / 5u), or whatever the ratio was (I don't have it in front of me now), in
    parallel with the primary. (I misspoke earlier, you can safely ignore Ls,
    because k = 1. There's no flux which is not common to both windings.)

    Inductors effectively in parallel also increase the expected resonant
    frequency. If you have this,

    .. L1
    .. +-----UUU--+------+------+
    .. | + | | |
    .. ( Vsrc ) === C > R 3 L2
    .. | - | > 3
    .. | | | |
    .. +----------+------+------+
    .. _|_ GND

    You might expect the resonant frequency is L2 + C, but it's actually (L1
    || L2) = Leq. If L1 is not substantially larger than L2, the resonant
    frequency will be pulled higher.

    Incidentally, don't forget to include loss components. I didn't see any
    explict R on the schematic. I didn't check if you set the LTSpice default
    parasitic ESR (cap), or DCR or EPR (coil) on the components. Besides
    parasitic losses, your signal is going *somewhere*, and that "where"
    consumes power!

    The actual transmitter is most certainly not a perfect current source
    inductor, nor is the receiver lossless. This simulation has no expression
    for radiation in any direction that's not directly between the two
    antennas: if all the power transmitted by the current source is reflected
    back, even though it's through a 0.1% coupling coefficient, it has to go
    somewhere. If it's coming back out the antenna, and it's not being burned
    in the "transformer", it's coming back into the transmitter. This is at
    odds with reality, where a 100% reflective antenna doesn't magically smoke
    a distant transmitter, it simply reflects 99.9% back into space. The
    transmitter hardly knows.

    In this example, if you set R very large, you'll see ever more voltage on
    the output, and ever more current draw from Vsrc. You can mitigate this
    by increasing L1 still further, but the point is, if the source and load
    (R) aren't matched in some fashion, the power will reflect back to the
    transmitter and cause problems (in this case, power reflected back
    in-phase causes excessive current draw; in the CCS case, reflected power
    in-phase causes minimal voltage generation and little power transmission).

    Power is always coming and going somewhere, and if you happen to forget
    this fact, it'll reflect back and zap you in the butt sooner or later!

    Tim
     
  5. legg

    legg Guest

    <snip>

    Pulling out the old reactance paper, there are a couple of expected
    interactions using the values present:

    Around 50KHz (89.42uH+48uH) with 50.42nF (L3+L1) with C1
    Around 290KHz (89.42uH+48uH) with 6.25nF (L3+L1) with C2*N^2
    Around 360KHz 48uH with 6.25nF L1 with C2*N^2
    nL1/nL2=N=25

    The mid-resonance is a dip or rejection.

    What's the issue?

    RL
     
  6. rickman

    rickman Guest

    Reflecting the capacitance through the transformer changes it by the
    square of the turns ratio assuming the coupling coefficient is
    sufficiently high. I am simulating K at 1.

    This is also true for the inductance, but in the opposite manner. So
    going from the 25 turn side to the 1 turn side, the effective
    capacitance is multiplied by 625 and the effective inductance (or
    resistance) is divided by 625. In fact, in LTspice you indicate the
    turns ratio by setting the inductance of the two coils by this ratio.

    I see now that the reflected secondary capacitance is in parallel with
    the primary, rather than in parallel with the primary capacitor. That
    explains a lot... I'll have to hit the books to see how to calculate
    this new arrangement. I found a very similar circuit in the Radiotron
    Designer's Handbook. In section 4.6(iv)E on page 152 they show a
    series-parallel combination that only differs in the placement of the
    resistance in the parallel circuit. It need to be placed inline with
    the inductor... or is placing it parallel correct since this is the
    reflected resistance of the secondary? I'll have to cogitate on that a
    bit. I'm thinking it would be properly placed inline with the capacitor
    in the reflection since it is essentially inline in the secondary.
    Either way I expect it will have little impact on the resonant frequency
    and I can just toss all the resistances simplifying the math.

    I do see one thing immediately. The null in Vcap I see is explained by
    the parallel resonance of the secondary cap with the secondary inductor.
    If you reflect that cap back to the primary in parallel with the
    primary inductor (resonating at the same frequency) it explains the null
    in the capacitor C1 voltage I see. C2' (reflected) and L1 make a
    parallel resonance with a high impedance dropping the primary cap
    current and voltage to a null. This null is calculated accurately.

    What I need to do is change the impedance equation from Radiotron to one
    indicating the voltage at Vout relative to the input signal. I think I
    can do that by treating the circuit as a voltage divider taking the
    ratio of the impedance at the input versus the impedance at the primary
    coil. No?

    I see, L1 and L2 are in parallel because the impedance of Vsrc is very
    low. That is not the circuit I am simulating however. The loop of the
    antenna and the loop of the inductor are in series along with the
    primary capacitor. I'm not sure what the resistor is intended to
    represent, perhaps transformer losses? The resistance of L1 was added
    to the simulation model along with the resistance of the secondary coil
    which you have not shown... I think. It seems to me you have left out
    the tuning capacitor on the primary.

    Interesting point. My primary goal with this is to simulate the
    resonance of the tuning so I can understand how to best tune the
    circuit. In many of the simulations I run the Q ends up being high
    enough that a very small drift in the parasitic capacitance on the
    secondary detunes the antenna and drops the signal level. It sounds
    like there are other losses that will bring the Q much lower.

    I would also like to have some idea of the signal strength to expect. My
    understanding is that the radiation resistance of loop antennas is
    pretty low. So not much energy will be radiated out. No?

    You make it sound as if in the simulation, even with a small coupling
    coefficient all the energy from antenna inductor will still couple back
    into the transmitter inductor regardless of the K value. Do I
    misunderstand you? It seems to result in the opposite, minimizing this
    back coupling. Or are you saying that the simulation needs to simulate
    the radiation resistance to show radiated losses?

    Actually, my goal was to build the receiver and I realized that my
    design would require the largest signal I could get from the antenna. I
    never realized I would end up having to learn quite so much about
    antenna design.

    I've been planning to create a PCB with lots of options so I can test a
    number of configurations. Nothing about the simulation makes me doubt
    the utility of this idea.

    One thing that continues to bug me is that nothing I have seen gives me
    a hint on how to factor in the distributed capacitance of the antenna
    shield. I am using RG6 with 16 pF/Ft and likely will end up with 100
    foot of coax total. At some point I'll just have to make some
    measurements and see what the real world does.
     
  7. Tim Williams

    Tim Williams Guest

    You'll be much better off simply using the conventional radio approach
    than trying to simulate everything, especially when circuit equivalents
    are nebulous like this.

    After all, if you can't quite tell what it *should* look like, how would
    you know if you could implement your model once you've found a
    satisfactory result?

    What kind of antenna are you looking at, loop? The first thing to know
    about a loop is, if it's a very small loop (I'm guessing, at this
    frequency, it is), its radiation resistance is very low, meaning, you can
    treat it as a nearly pure inductance (Q > 10 I think is typical), and its
    bandwidth (even with a matched load) will be correspondingly narrow.

    The nature of the incoming signal could be modeled as a voltage or current
    source; how doesn't really matter, because it isn't really either, it's a
    power source that couples in. Again, you don't have voltage without
    current and vice versa, it's all about power flow, and the matching that
    allows the power to flow.

    Since the loop is inductive, your first priority is to resonate it with a
    capacitor at the desired frequency. This will require a very precise
    value, and even for a single frequency, may require a variable capacitor
    to account for manufacturing tolerances. In the AM BCB, a Q of 10 gets
    you 50-160kHz bandwidth, so you only get a few channels for any given
    tuning position. And if the Q is higher, you get even fewer.

    Now that you've got a high Q resonant tank, you can do two things: couple
    into the voltage across the capacitor, or the current through the
    inductor. You need only a small fraction of either, because the Q is
    still going to be large. This can be arranged with a voltage divider
    (usually the capacitor is split into a huge hunk and a small variable
    part, e.g., 300pF variable + 10nF, output from across the 10nF), a
    transformer (a potential transformer across the cap, or a current
    transformer in series with the inductor), an inductive pickup (the big
    loop carries lots of volts, but you only need a few, so a much smaller
    loop can be placed inside the big loop), an impractically large inductor
    (like in my example circuit, which models radiation resistance as a
    parallel equivalent), etc. Whatever the case, you need to match
    transmission line impedance (e.g., 50 ohms) to radiation resistance
    (whichever series or parallel equivalent you have).

    Once you get the signal into a transmission line, with a reasonable match
    (Z ~= Z_line, or alternately, SWR ~= 1), you can do whatever you want with
    it. Put it into an amplifier (don't forget to match it, too), etc. Yes,
    you're going to have funny behavior at other frequencies, and if you're
    concerned about those frequencies, you'll have to choose the coupling
    circuit and adjustable (or selectable) components accordingly. But for
    the most part, you completely ignore any frequency that you aren't tuning
    for, usually enforcing that concept by inserting filters to reject any
    stragglers.

    Example: suppose you have a loop of 5uH and need to tune it to 500kHz. It
    has a reactance of 15.7 ohms. Suppose further it has Q = 20. The ESR
    (not counting DCR and skin effect) is X_L / Q, or 0.78 ohms; alternately,
    the EPR is X_L * Q, or 314 ohms. The capacitor required is 20.3nF. If we
    use a current transformer to match to a 50 ohm line, it needs an impedance
    ratio of 1:64, or a turns ratio of 1:8. If we use a voltage transformer,
    it's of course 8:1. (A capacitor divider is unsuitable for resonant
    impedances less than line impedance, since it can only divide the
    impedance down. If the inductance were a lot larger, it could be used.)
    To a rough approximation, a smaller inductive loop, of 1/8 diameter of the
    larger, I think, would also work.

    Tim
     
  8. rickman

    rickman Guest

    290 kHz matches the calculations you just gave. But 290 kHz is the null
    (or dip as you call it) from C2 and L2 (or L1 and C2 reflected with N^2).

    I thought I wasn't getting the 60 kHz resonance, but I was mistakenly
    adding the two capacitances together. So that is closer. Using L3+L1
    with C1 I get 60.46 kHz while it is measured at 60 dead on in
    simulation. That's nearly a 1% error.

    I solved the equations finally. I found some info on the impedance of
    series and parallel circuits. With that info I wrote the equation for
    the ratio of Vout/Vin and found the roots. Turns out it is not so bad.
    The equation is a fourth order, but it has no x^3 or x^1 terms and so
    is actually a quadratic of x^2. Solving the quadratic gives the exact
    figures for 60 kHz and 393 kHz peaks. Since this is from taking the
    square root of x^2, there are also solutions at the negative values... duh!

    Reflecting C2 through the transformer to create C2', the two nulls I
    found can be calculated by the resonance of L1 and C2' (290 kHz null on
    C1) or L1 with C1 and C2' (96500 Hz null on L3).
     
  9. rickman

    rickman Guest

    I don't know what you mean by the "conventional radio approach".

    I was simulating a specific circuit for a specific purpose. I got the
    answer I was looking for.

    Yes, I plan to use a shielded loop. I have found some contradictory
    info on the effectiveness of the "shield". One reference seems to have
    measurements that show it is primarily E-field coupled in the longer
    distance portion of the near-field.

    I am aware of the low radiation resistance and have not included that
    factor in my simulation. The Q of just the antenna loop is around 100
    as calculated from the ratio of reactance to resistance.

    A friend in a loop antenna Yahoo group suggested the use of the
    transformer coupling with a low k to model the signal reception.

    Yes, that is loop antenna 101 I think. It was when I added a coupling
    transformer with 100:1 turns ratio that I was told I needed to consider
    the parasitics. I have found it is not useful to go much above 25 or
    33:1 on the turns ratio. I am receiving a single frequency, 60 kHz.
    There is no need for a wide bandwidth. Ultimately, I prefer a Q of >
    100 for the higher gain. If it gets too high, the off tuning by
    variations (drift) in the parasitic capacitance affects the antenna gain
    appreciably.

    Transmission line? What transmission line? The antenna is directly
    connected to the receiver which has a very high input impedance. Why do
    I need to consider radiation resistance? I have not read that anywhere.

    I'm not familiar with the concept of voltage transformer vs. current
    transformer. How do you mean that?

    How did you get the 1:64 impedance ratio and the 1:8 turns ratio? I
    don't follow that. Are you saying the line impedance should match the
    ESR? Why exactly would it need to match the ESR?
     
  10. Tim Williams

    Tim Williams Guest

    I trust this resource:
    http://vk1od.net/antenna/shieldedloop/
    He's got gobs of analytical articles.
    High Q isn't the goal, high radiation resistance is -- the bigger the
    loop, the better it couples with free space, until it's a wave length
    around.

    You can go ahead and make a teeny coil out of polished silver litz wire,
    and push the Q up into the hundreds, but all you'll see is internal
    resistance, hardly anything attributable to actual radiation. Since the
    losses dominate over radiation, it makes a crappy antenna. But you know
    that from looking at it -- it's a tiny lump, of course it's not going to
    see the outside world.

    It is true, however, that a small coil, with low losses, will have low
    noise. AM radios rely on this, which is how they get away with tiny hunks
    of ferrite for picking up radio.

    Of course, it doesn't hurt that AM stations are 50kW or so, to push over
    atmospheric noise.
    Ok, then you can merge the matching transformer, transmission line and
    receiver input transformer into one -- an even larger stepup into whatever
    impedance it's looking at (what's "very high", kohms? Mohms?) will get you
    that much more SNR.
    Current transformer measures current (its winding is in series), potential
    transformer measures voltage (in parallel).
    ESR (and Q) measured on the coil corresponds to radiation resistance
    (series equivalent) *plus* internal losses (also series equivalent). You
    can't separate the two components, so you can only get the best power
    match by the good old impedance theorem.

    ~1:64 is 50 ohm / 0.78 ohm, and N2/N1 = sqrt(Z2/Z1), or 8:1 turns ratio.

    Tim
     
  11. Tauno Voipio

    Tauno Voipio Guest

    Please note that high Q will destroy the modulation sidebands on
    the signal you're listening to.

    In aviation, there are non-directional beacons which are transmitting
    in a frequency around 300 kHz (1 km wavelength). The antennas cannot
    obviously be of efficient length (250 m / 800 ft), so they are short
    (20 m / 70 ft) force-tuned to the transmitting frequency. This creates
    so high Q that the identification modulation sidebands for the customary
    1050 Hz audio do not fit in, and the ID is modulated using 400 Hz audio.
     
  12. rickman

    rickman Guest

    I appreciate the advice from everyone, but much of it is not in the
    proper context and way off target. "High" Q is how high? Where are the
    modulation sidebands? My point is that I have already considered this.
    The modulation sidebands of this signal are on the order of low 10's
    of Hz. This signal is modulated at a 1 bit per second rate. I will be
    demodulating a 30 Hz sample rate. So a bandwidth of 100 Hz is plenty
    which corresponds to a Q of around 500.

    I said I was looking for a Q over 100, maybe I should have said a Q of a
    bit over 100. By the time it gets to 300 it is to peaky to hold a tune
    setting. That is the problem I am concerned with.

    Ok, but that is nothing like my application, receiving WWVB.
     
  13. rickman

    rickman Guest

    Yes, I've seen this page. Thanks.

    I'm not clear on why you keep referring to radiation resistance for a
    receiving antenna. Does this result in a larger received signal? I am
    concerned with maximizing the voltage at the input to the receiver.

    I have no idea why you are talking about Litz wire and tiny coils. I
    never said I was looking to maximize the Q. I said I wanted a Q of over
    100. I should have said, slightly over 100. A higher Q clearly does
    increase the voltage on the input in my simulations. Is there something
    wrong with my simulations?

    Yes, a higher stepup ratio gets larger signal up to a point. That point
    is determined by the parasitic capacitance of the receiver input. That
    capacitance is reflected back through the transformer and affects the
    antenna tuning. In my simulations it creates a filter with two resonances.

    Series and parallel with what? I'm not following this. I have trouble
    with series and parallel resonance, but I'm starting to get the concept.
    Sometimes it is hard to tell how a circuit is being stimulated.

    Internal losses of what? How do you determine the internal losses?

    Ok, so you were matching the hypothetical ESR to the hypothetical line
    impedance.
     
  14. Tauno Voipio

    Tauno Voipio Guest

    I'd still be wary of high Q. The antenna is, by definition, in close
    interaction with its surroundings, and a high-Q thing is quickly
    detuned.

    At those low frequencies, the atmospheric and other outside noise is
    far larger than the internal noise of an amplifier, so in my opinion,
    the way to go is a loop tuned to 60 kHz with as low Q as easily comes
    without extra attenuation and a good pre-amplifier. The preamp can
    then contain a tuned interstage tank for interference suppression.
     
  15. rickman

    rickman Guest

    I understand. But this is intended to be *very* low power and I haven't
    found an amp I can use that is in the low double digits uW power
    consumption range. I plan to use no amp and go straight to digital.
     
  16. Tim Williams

    Tim Williams Guest

    You're also not concerned about that -- you're concerned about maximizing
    SNR at the receiver.

    A Q of a million will get you gobs of "gain", but if it doesn't couple
    into free space, it's only the thermal noise of the loss generating that
    signal.

    An antenna with high (expressed as ESR) radiation resistance might have a
    modest Q, but gives far better SNR because it couples to free space.

    Raw volts don't matter, you can always throw more amplifiers at it (as
    long as they don't corrupt the SNR also!).
    Oooh, capacitance! I like capacitance. Capacitance is easy to
    cancel...inductors are good at that. :)

    What's a nearby inductor working against that capacitance? The current
    transformer in your simulation, if its inductance can be controlled, would
    be an excellent candidate. The circuit effectively becomes a double tuned
    interstage transformer, like,

    http://www.jrmagnetics.com/rf/doubtune/doubccl_c.php
    This is two resonators coupled with a cap, but any coupling method will
    do. Capacitive, magnetic (putting the coils end-to-end) or
    electromagnetic (coils side-by-side) coupling does equally well; normal
    arrangements have them all in phase, so in practice, unshielded coils will
    need smaller coupling capacitance than designed, etc.

    If you line up that 10p resonance with the operating frequency, you should
    get gobs more gain. In fact, because the reactances cancel, the driven
    impedance will be much higher than you were expecting, and so will the
    gain. The CT might go from, say, 1:8 up to, who knows, 1:20? 1:100?

    The bandwidth of that coupling (not necessarily of the antenna itself, so
    they should be similar bandwidths) is determined by the coupling
    coefficient (in the coupled-inductors case, simply k) and Q of the
    components.

    If your receiver datasheet specifies an equivalent input circuit, you
    might be able to estimate the equivalent loss and optimize gain.

    Tim
     
  17. rickman

    rickman Guest

    SNR would be good, but I am concerned with maximizing the signal actually.

    I think you aren't reading what I am writing. I said I wanted a Q over
    100, not 1 million. I don't get why you keep talking in hyperbole.
    What you are describing is not even a tradeoff between signal strength
    and SNR. If there is no coupling, there is no signal.

    I have not found anything to indicate this produces a better receive
    antenna. I have a formula for the effective height of a loop antenna
    which is what determines the received signal strength at the antenna. It
    does not calculate the radiation resistance, it uses the coil parameters
    and the wire resistance. Is that a wrong formula?

    Maybe you didn't read my other posts. I am not using an amplifier. I
    am running the antenna and coupler output directly into a digital input.

    The receiver input is high impedance, approximately 10 MOhms with a low
    capacitance between the differential inputs of not more than 10 pF.

    Your description of what is happening is very terse and full of
    shortened terms that I don't understand. What do you mean "line up that
    10p resonance with the operating frequency"? I assume you are referring
    to the 10 pF input capacitance. How does this get "lined up" with
    anything?

    When you talk about reactances canceling, that sounds a lot like a tuned
    circuit at resonance. That is what I *am* doing and where this thread
    started. One problem with that is the lack of precision or stability of
    the parasitic capacitance. Any idea how to deal with that?

    Have you looked at the simulation data I had posted? I think you are
    describing exactly the circuit we are simulating which I believe is an
    accurate representation of the circuit I plan to build. Is that not
    correct?
     
  18. Jim Mueller

    Jim Mueller Guest

    <snip>

    An electric circuit consists of a source of power, a load, and something
    (like wires) connecting them. Transformers can be used if the source is
    providing alternating current. A voltage transformer is connected in
    parallel with the load so that the source, the transformer, and the load
    all see the same voltage. It can also be used to match a load to a
    source. A common example of a voltage transformer is the power
    transformer in a piece of equipment that changes the AC line voltage to
    whatever other voltages are required by the equipment.

    A current transformer, on the other hand, is connected in series with the
    load so that the source, load, and transformer all have the same current
    flowing through them. The most common use of a current transformer is to
    measure the current flowing into a load. A clamp-on ammeter is a common
    example.

    Historical examples of voltage and current transformers are the "picture
    tube brighteners" that were commonly used in TV sets to prolong the
    useful life of the CRT. There were two types, parallel and series. The
    parallel types were used in transformer operated TVs and consisted of a
    step-up transformer to raise the heater voltage of the CRT above normal
    to increase emission. The series type was used in sets with the tube
    heaters in series and consisted of a step-down transformer that raised
    the heater current above normal. Of course, raising either the voltage
    or the current also raised the other. These were, respectively, voltage
    and current transformers.

    A loop antenna is a distributed source with the voltage being generated
    along the length of the wire and also having a magnetic field so that it
    can be used as part of a transformer. This blurs the distinction between
    a current and voltage transformer.
     
  19. rickman

    rickman Guest

    Yes, I have done my homework on the WWVB signal. I am at the fringe of
    the 100 uV/m contour. I would very much like to see the signal on an
    oscilloscope when I test this. They have a receiver not far from here
    in Gaithersburg, MD and the signal is often strong during the day. So
    much so that I don't follow why they say there is this day/night signal
    strength fluctuation. It looks much more random to me.

    The WWVB signal is not truly on-off keying. I believe they use a 10 dB
    modulation factor for the AM signal. This is close to on-off I agree.
    But they also phase modulate the signal and I will be demodulating both
    to see which one works best in my design.

    The ADC in my design is truly one bit. It is an LVDS input on an FPGA.
    I looked at delta-sigma (or is it sigma-delta? ;) conversion and got
    code from the chip vendor for a simplistic implementation. I don't
    think I have the power budget for that and am using a much simpler 1 bit
    ADC at 4x the carrier rate. The bit stream is multiplied by quadrature
    carriers at 60 kHz and each stream summed for 1/30 of a second to
    implement what can be considered a DFT bin, a decimated FIR filter or a
    decimated down conversion; take your pick, they are all mathematically
    the same in this case because the sampling is synchronous to the carrier
    (or very close to synchronous).

    What comes out the other end of this processing gains nearly 40 dB in
    SNR. My simulations show a recoverable signal when it is more than 20
    dB below the noise.

    Of course, I have not tested this yet on a real signal. I want to run
    some tests on the antenna and coupling transformer to verify the
    simulation. Then I will start working with the FPGA to see if I can
    make the LVDS input do what I want. I have ideas on how to bend digital
    circuits to do my bidding. This LVDS input is why I want as large a
    signal as possible from the antenna. With the high impedance input on
    the chip I should be able to boost the signal pretty well with just
    passive devices and signal processing.

    The loop antenna is rather large. I would like to end up with something
    smaller. Once I get this working with a shielded loop antenna I will
    check out the ferrite core antennas. My understanding is that they
    don't produce as much signal.

    I'm not sure how you came up with 2 Hz for the bandwidth. In this case
    the bandwidth is not just twice the bit rate. I believe the stated
    "system" bandwidth is around 5 Hz (from a 1995 paper prior to addition
    of the phase modulation). Regardless, I am sampling at 30 Hz and if I
    expect to see significant changes in phase or amplitude within one
    sample time, I need an appropriate bandwidth.

    Even so, that is not the limiting factor. The limiting factor is the
    difficulty in holding tune with drift in passive component values. The
    Q can be raised by increasing the turns ratio on the transformer, but it
    becomes so sensitive to the parasitic capacitance that the sensitivity
    drops 10 dB with a 1 pF change.

    Thanks. I will take a look at that.

    I will be needing a time code simulator. I designed a commercial
    product that works with the IRIG-B time code which is similar. The
    functionality is not hard, it is just a matter of generating the data,
    encoding it into the modulation pattern, then impressing the carrier
    with the modulation. Working in an FPGA this sort of stuff is easy.

    The trouble is if you make the same mistake in both the generator and
    receiver they work just fine in simulation, but not with other
    equipment. lol

    I'll take a look at this link.

    I might look into that. Certainly it can't hurt to get more input.
     
  20. rickman

    rickman Guest

    Is this a current transformer or a voltage transformer?
    .--------. .--------.
    | | | |
    | C||C >
    VAC C||C > Load
    | C||C >
    | | | |
    `--------' `--------'
     
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