On 02/24/2013 05:30 PM, redsp wrote:

(snip)

> 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.

(snip)

If you upload your simulation file (not raw file), I will

take a look and check out your simulation, as best I can.

Maybe I'll be able to suggest some improvement.

--

Regards,

John Popelish- <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