Waveform Generation Tutorial
dBm RMS Voltage Peak-to-Peak Voltage
-50.00 dBm 0.707 mVrms 2.00 mVpp
-56.02 dBm 0.354 mVrms 1.00 mVpp
For 75 Ω or 600 Ω loads, use the following conversions.
dBm (75 Ω) = dBm (50 Ω) – 1.76 dBm (600 Ω) = dBm (50 Ω) – 10.79
Signal Imperfections
For sine waves, common signal imperfections are easiest to describe and observe in the frequency domain, using a spec-
trum analyzer. Any output signal component with a frequency different from the fundamental (or "carrier") is con-
sidered to be distortion. Those imperfections can be categorized as harmonic distortion, non-harmonic spurious, or
phase noise, and they are specified in decibels relative to the carrier level, or "dBc."
Harmonic Distortion
Harmonic components occur at integer multiples of the fundamental frequency and are usually created by non-linear
components in the signal path. At low signal amplitudes, another possible source of harmonic distortion is the Sync sig-
nal, which is a square wave with many strong harmonic components that can couple into the main signal. Although
Sync is highly isolated from the instrument's main signal outputs, coupling can occur in external cabling. For best
results, use high-quality coaxial cables with double or triple shields. If Sync is not required, leave it unconnected or off.
Non-Harmonic Spurious
One source of non-harmonic spurious components (called "spurs") is the digital-to-analog converter (DAC) that con-
verts the digital waveform values into voltage. Non-linearity in this DAC gives rise to harmonics that can be higher than
the Nyquist frequency and will therefore be aliased to a lower frequency. For example, the fifth harmonic of 30 MHz
(150 MHz) could create a spur at 100 MHz.
Another source of non-harmonic spurs is the coupling of unrelated signal sources (such as the embedded controller’s
clocks) into the output signal. These spurs usually have constant amplitude and are most troublesome at signal ampli-
tudes below 100 mVpp. For optimal signal purity at low amplitudes, keep the instrument’s output level relatively high
and use an external attenuator.
Phase Noise
Phase noise results from small, instantaneous changes in the output frequency ("jitter"). On a spectrum analyzer, it
appears as a rise in the apparent noise floor near the frequency of the output signal. The phase noise specification rep-
resents the amplitudes of the noise in 1 Hz bands located 1 kHz, 10 kHz, and 100 kHz away from a 30-MHz sine wave.
Be aware that spectrum analyzers also have phase noise, so the levels you read may include analyzer phase noise.
Quantization Noise
Finite resolution in the waveform DAC causes voltage quantization errors. Assuming the errors are uniformly dis-
tributed over a range of ±0.5 least-significant bit, the equivalent noise level for standard waveforms is approximately -
95 dBc. At this level, other sources of noise in the instrument dominate. Quantization noise can be of concern, though,
148 Agilent 33500 Series Operating and Service Guide
To Next Page
Loading...
Need help?
General
