Best way to pick up 8.2 MHz EAS signal

1. The ultimate limit of the sensitivity is set by the ambient RF noise. However it is unlikely that you are approaching this limit with the setup that you described; it seems to be self noise limited.
2. The amount of energy captured by the antenna is coarsely proportional to the volume of the ferrite. Without increasing the size of the ferrite in either dimension, you can only optimize the parameters of the winding, the input amplifier and the matching between the antenna and the amplifier for the best noise performance. Depending on the current situation, that optimization may or may not provide for the substantial improvement.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Thanks! This seems an useful rule of thumb indeed. Is there any reference on this so I can try to grasp the reasoning behind this?

In the current (very crude) configuration the tank is directly connected to the BE of an unbiased BFR92. Field strength is now sufficient to turn it on. Inductor reactance is aprox. 600R (12uH) and the inductor Q is approximately 120. Any guess on the input impedance of the unbiased transistor, so that I could try a better match?

Pere

Besides antenna/ferrite mods, what is the noise and impedance matching situation of your current front end? If it falls into the category of "not so great" you might want to consider a switched pre-amplifier. Switched in order to make the 10uA because you can't really get a good noise figure at quiescent currents this low.

The feature you need is nowadays called "wake-on-radio" or WOR. It turns on, sees if RF is there, and if not goes back to sleep.

The current configuration could not even be called a "front end" but is a direct connection of the tank to the base of an unbiased BFR92! I could design a superregenerative receiver as the front-end (however, going down to the few uA that are available would be hard) but first I would like to exploit the passive possibilities that might be available.

If, as Vladimir says, there is little optimization possible without increasing ferrite volume (which is not possible due to size constraints), perhaps there is a matching circuit that might do the job. I have no idea what the unbiased BE junction of the BFR92 looks like (nothing in datasheet), however I suspect it will not be easy to match to the tank. If it's smaller than the tank resistance at resonance, a small series capacitor might do the job, however there should also be a path for dc... Any comments?

Pere

bias up the base of the BRF92 to about 0.5V DC so it is closer to threshold...then it will take less RF to turn it on...

or add an impedance matching network to step up the RF voltage applied to the base...the base is a Hi Z when off...

or both

Mark

Yep. Without any bias it'll be deaf like a door knob. But DC-regulate that bias, i.e. via an emitter resistor. Else it'll drift all over the place with temperature.

I'd say "and" add such a network. Sans bias there isn't much hope here.

Way high, but descending as the signal increases. And that's probably as precise a number as you can put on the dissipative part of the impedance, although you could pin the capacitance down better I'm sure.

You should be able to significantly increase sensitivity by biasing the transistor to just under conduction; then the signal won't have to get up to 0.6V peak to turn things on.

Be careful of temperature effects if you do this -- that turn on temperature will vary.

So, group.

At 300kHz, 20 years ago, the 'best' way to do this was with a 1 meter wire whip, driving the gate of a FET in source-follower configuration. The thing acts like a capacitive probe, and gives you plenty of signal at atmospheric noise levels.

Could this be better done by capacitively coupling to the aether, instead of magnetically? Could the OP use a patch at each end of his PCB, going to the gate of a FET, J- or MOS-?

I dunno the answer -- it seems like it'd take a really hot little FET to be worthwhile, and atmospheric noise is way down at 8MHz from 300k. But on the other hand, that one-meter capacitive whip scales waaaay down when you go to 8MHz.

IME it really never made much of a difference whether a FET or BJT was used. Not at those frequencies. For really, really short antenna stubs it might though. But it's got to be biased, else you almost need a nearby lightning strike to see some signal.