Notes on The Tektronix TDS 380 Oscilloscope
|The ARS Lab uses an HP TDS 380 scope, which includes the ability to examine the signals in the frequency domain though Fast Fourier Transforms ("FFT"). Because we are assessing low power equipment, we are able to feed the transmitted signal to a 20 dB 50 ohm attenuator, which is connected directly to the scope. The scope has excellent documentation capabilities, including the ability to print hard copies and to store waveforms on a diskette.
The HP TDS 380 has a dynamic range of about 50 dB. Although that dynamic range is appropriate for determining whether a low power transmitter meets current legal requirements, it is not adequate for critical examinations of spectral purity. Those would require either a good quality analog spectrum analyzer, or a digitized instrument with greater than 8 bit capability.
The graphic below is a hard copy of the TDS 380's display in FFT mode. We have added annotations in red. The signal being analyzed was from a K-2 transmitting at 10 watts on a frequency of 3.520 MHz. The even harmonics were below the scope's dynamic range (exactly what you would expected from a push-pull RF amplifier). The third and fifth harmonics were within the scope's dynamic range and have been documented.