Now that the transponder design is at a usable point, it’s time for an update on the decoder:

I designed an amplifier/line driver for the receiving loop, and it’s working quite nicely. I had originally considered just using a TV-style 300-to-75-ohm transformer to interface the loop to a length of RG6 cable, but the ones I found had a low frequency cutoff of 5 MHz, which is a bit too close to the transponder carrier frequency, and of course the voltage from the loop is reduced by the transformer, which didn’t appeal to me.

The obvious solution was to make an active amplifier. The requirements were:
balanced (differential) input, to reject any common-mode noise on the loop;
present a controlled impedance to the loop, thereby setting a reasonable Q and bandwidth;
present a 75 ohm output impedance to match the coaxial cable connecting the amplifier with the rest of the system;
derive power from the coaxial cable, so no separate power supply is needed;
and add some gain.

The design uses discrete bipolar transistors, 2N3904 and 2N3906, as they are cheap, easily available, and rugged enough to withstand the numerous ESD (ElectroStatic Discharge) events that will probably occur. These have a transition frequency of about 250 to 300 MHz, which is plenty, but not so high that any special care must be taken in the circuit layout. The gain is about 16 dB, and the bandwidth is sufficient to reproduce the signal from the loop with minimal distortion (I sold my RF generator a while back, so I can’t make any direct measurement of the bandwidth). Current drain is about 10.5 mA, drawn from the center conductor of the coax, which is terminated at the far end with 75 ohms to +5V, giving a supply voltage at the amplifier of about 4.17V.

Right now there is no automatic gain control, but it could easily be added if necessary. Initial experiments indicate that it might not be needed, since overloading the amplifier still seemed to preserve the zero-crossings of the signal.

The output level is a few tenths of a volt, which will require translation to digital levels before it is fed to the phase detector. A bit of hysteresis will help eliminate random noise.

I’ll post some pictures and scope shots when I get a chance.