General Description
R&S
®
CMW-KM645
9
User Manual 1173.9740.02 ─ 04
Fig. 3-1: Connecting an RF transmitter to the instrument
The measurement is performed in free run mode; no trigger signal is needed.
3.2 Measuring an FM Signal
After connecting your FM signal source to the R&S
CMW as shown in section
, you have to adjust the following analyzer settings to the properties of the ana-
lyzed signal:
●
The analyzer "Frequency" (RF carrier frequency)
●
The "Expected Nominal Power" and (optional) a "User Margin" and "External
Attenuation". Recommended values: "Expected Nominal Power" = nominal power
of the RF carrier. FM signals are transmitted at constant power (constant-envelope
signals), so the default "User Margin" of 0 dB should be sufficient.
With the settings described above, the R&S
CMW is able to receive and demodulate
the FM stereo signal and obtain the essential measurement results.
The following measurements require additional settings:
●
The demodulated left and right AF channel signals can be measured with different
filter settings in the AF analyzer path ("Deemphasis Filter", "Lowpass Filter", "High-
pass Filter", "Weighting Filter", "Bandpass Variable"). See section
.
●
To determine the Total Harmonic Distortion (THD), the Signal to Noise and Distor-
tion Ratio (THD + N / SINAD), and the Signal to Noise Ratio (SNR), two fixed, sine
wave audio tones are transmitted over the left and right audio channels ("THD (N)
Freq. Left" and "THD (N) Freq. Right"). See description of "THD / SINAD / SNR
measurements" in
chapter 3.5.3, "AF Measurement Results"
3.3 AF Filter Settings
The audio receive path of the R&S
CMW provides optional filter stages which affect the
AF results:
●
A deemphasis filter with a time constant of either 50
μ
s or 75
μ
s. A deemphasis is
designed to decrease the magnitude of the higher frequencies with respect to the
magnitude of the lower frequencies in order to improve the overall signal-to-noise
ratio.
Measuring an FM Signal
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