According to this data, we needed to change our capacitance in the antenna. 

With this value, the capacitance needed for the antenna should have been

Write Output: Modulation Frequency Close Up

The modulation envelope is very clean and is at the correct frequency input into the transceiver.  This shows that the connection debugging and removal of parts around the modulation pin was successful.  However, a retest with the above calculated capacitance gave the same carrier problems. 

Using a spectral analyzer, the coil was tuned to 125 kHz.  The inductance was found to be 4.5 mH.  The resonance frequency of the coil changed when plugged into the transceiver circuit.  When it was retuned, the transceiver was ringing with large amounts of decays and frequencies overlapping each other.

Varying Close ups of Transceiver Circuit Ringing

It was concluded that using higher inductance values in the antenna coil causes a great deal of undesirable decays in the circuitry.  Lower value coils were then tested for use.  It was also decided to use round coils for cleaner signals. 

The inductance of a round coil is:

Coil 2

A small coil was wrapped around a 28 mm plastic core with 32 turns.  This coil was tested with great results on frequency sweeps using the function generator and an oscilloscope.  The inductance was found to be around 40 µH.  This coil was used in the tag circuit.

Coil 2 Testing Data 1 [ML2]

Approximate resonance frequency = 168.4 kHz

Approximate Inductance = 40.6 uH

Approximate Capacitance Needed for 125 kHz = 36.35 nF

Coil 2 Testing Data 2 [ML2]

Approximate resonance frequency = 133.35 kHz

Approximate Inductance = 39.2 uH

Approximate Capacitance Needed for 125 kHz = 41.35 nF

These values were averaged to L= 39.9 µH and C= 38.84 nF.  These values were used as starting values for tuning the coil with the transceiver circuit.

The coil was put into a tag circuit to be tuned.  The capacitance needed for 125 kHz was 40.6 nF for the 39.93 µH coil.

125 kHz Carrier Close-up

2 kHz Modulation Envelope Close-up

Tag Circuit with Coil 2

Coil 3

A coil was ordered online with a value of 2.8 mH.  The capacitance for a 125 kHz resonance with the labeled value is 578.98 pF.  This coil had the same problem as the first coil in testing with frequency sweeps and gave inconclusive results.  Due to these results, it was decided that coil selection for the circuitry would remain in the µH range.

Coil 3 Data [ML3]

There are no remarkable peaks or consistent variance in voltage.  This is with a capacitance less than 2 pF away from the value calculated from the factory tested value of inductance of the coil.

Coil 4

A small coil was wrapped around a 28 mm plastic core with 52 turns.  This coil was tested with great results on frequency sweeps using the function generator and an oscilloscope.  The inductance was found to be around 150 µH.

Coil 4 Data [ML4]

The inductance of the coil from this data was calculated to be 154 µH.  The needed capacitance would then be 10.47 nF.

Coil 5

A small coil was wrapped around a 16 mm plastic core with 52 turns.  This coil was tested with great results on frequency sweeps using the function generator and an oscilloscope.  The inductance was found to be around 50 µH.  This coil was used in the reader circuit.

Coil 5 Data [ML5]

During this test, the circuit resonance frequency was 138.9 kHz.  The calculated inductance was 50 µH.  The capacitance for 125 kHz resonance was approximately 32.42 nF.  This was used as a starting value when tuning the antenna in the transceiver circuit.

The coil was put into a tag circuit to be tuned.  The capacitance needed for 125 kHz was 31.04 nF for the 52.23 µH coil.

125 kHz Carrier Close-up

2 kHz Modulation Envelope Close-up

Reader Circuit with Coil 5