- --- In loopantennas@yahoogroups.com, "jpopelish" <jpopelish@...> wrote:
>

I've been doing some LTspice simulations on this. I think I need to add some passive circuitry to apply a DC bias to put the differential input on the edge of triggering, so I included this in the simulation along with the parasitic capacitance of the chip input. The antenna has a loop inductance of about 90 uH. In LTspice the transformer turns ratio is indicated by attaching the corresponding inductance on the secondary (L1*N^2). I'm not sure how a real transformer works in this regard, inductance of primary and secondary.

> --- In loopantennas@yahoogroups.com, "redsp" wrote:

>

> > You've given me something to think about. What about the inductance of the secondary? How does that impact the primary circuit? Wouldn't that affect the tuning of the primary?

> (snip)

>

> According to the data sheet, the 200 turn winding has at least 80mH inductance. That means a single turn primary will have only 1/40,000th of that or 2uH inductance. This gives you an idea of the core they used, since you know roughly the size, and that it produces about 2uHy with one turn.

>

> But you can add as many turns as you wish to the primary to raise the primary inductance and lower the turns ratio.

>

> You can also tune the inductance with capacitance. This might work best with a series capacitor between the loop and the primary, and a parallel capacitance across the secondary of the transformer.

At very high turns ratios the parasitic capacitance creates a resonance with a null when looking at the tuning capacitance on the antenna. If that gets too close in frequency to the peak for the signal, it greatly disrupts the Q and the bandwidth making it very twitchy to tuning. Using the secondary capacitance alone to tune the response seems to drop the output voltage significantly with readings of -dB rather than some +60 to +70 dB when tuned with the primary capacitor. I'm not familiar with analysis of this circuit to understand why a null is created on the primary cap, but not the output...

The optimum balance seems to be a turns ratio around 25 to 33. In this range the difference in tuned output is in the 1 dB ballpark, but if the secondary capacitance drifts 1 pF the offtuning creates anywhere from -4 to -6 dB attenuation. Oddly at the higher turns ratios where the tuned signal is strongest the attenuation is such that the offtuned signal is weaker than at lower turns ratios. I'm not sure how realistic is a 1 pF drift with temperature, etc. of the inductor, PCB traces and the IC input.

Even odder is that if I add some capacitance to the secondary side in an attempt to swamp out the parasitic capacitance, the circuit has a higher Q and becomes *more* sensitive to that same 1 pF change!!!

I think I am a bit concerned with the capacitance of the secondary coil. If it has much capacitance I will have to run the coil with an even lower turns ratio.

No wonder they call it *complex* math.

Rick - <jpopelish@...> wrote:
> redsp wrote:

(snip)

> > I don't fully understand the field coil circuit. Why did

I added a second loop inductor to the schematic, loaded only with 1meg and also coupled to both the field coil and the antenna coil by the same 0.001 K factor, and it looks like a field coil inductance of about 80mHy produces a 0db signal across that calibration inductor at 60 kHz. It also shows about a +-0.2 db signal swing as the antenna circuit passes through resonance, so this gives you an idea of how approximate the field coil is at representing a far field. Perhaps it would be a bit better to lower the K factor to 0.0001 and raise the field coil inductance to 800mHy. (a 0.02db coupling of the antenna back into the field coil.)

> > you pick 1 mH for the radiator L4? I have no idea how to

> > include that into the calculations and I don't know if

> > this changes the signal strength value, not that it

> > necessarily is a meaningful number anyway. But in my

> > original circuit I assume the generator is the "untuned"

> > voltage the antenna would receive and so the signal

> > strength at Vout would be a realistic estimate of the

> > level at the receiver input. I will try the other two

> > methods of stimulating the circuit.

>

> The field coil inductance and driving current source are

> completely arbitrary. They both function only as a signal

> scale. It would probably be a good idea to pick a field

> coil inductance and current so that a plain loop inductance,

> possibly terminated with a meg ohm resistor or some such,

> produced a 0db output at 60kHz. Then all other signal

> strengths would be equivalent to gains.

--

Regards,

John Popelish