Very accurate modulation measurements along with very low internal noise enables the 8901A to characterize even high performance signal sources. Its detection system is configured for wideband recovery of the entire modulation spectrum so that highly precise measurements such as signal-to-noise or distortion can be made on the modulation signal. Modulation depth and deviation accuracy is generally ± 1% of reading. Residual AM noise in a 50 Hz to 3 kHz bandwidth is < 0.01% while FM noise is < 8 Hz for 1300 MHz carrier frequencies, decreasing linearity to < 1 Hz below 100 MHz. Because the AM and FM demodulators are independent and highly insensitive to each other and because the analyzer has very low residual AM and FM, accurate and incidental AM and FM measurements can be made.

Three detectors are available for depth and deviation measurements: positive peak, negative peak, and an average-responding detector with rms (sinewave) calibration. A PEAK HOLD function captures and displays the maximum peak modulation of a signal and is ideal for making transient measurements such as modulation limiting on mobile radios.

For measuring convenience, two high-pass (50 Hz and 300 Hz) and three low-pass (3 kHz, 5 kHz and > 20 kHz) post-detection filters for filtering the recovered modulation are included. The > 20 kHz Bessel filter minimizes overshoot on square wave modulation. This allows accurate measurement of signals which are digitally modulated, such as FSK. Four de-emphasis networks commonly used in FM systems (25, 50, 75, and 750 µs) are also provided.

A modulation output provides calibrated signal levels relative to the displayed modulation reading.

Modulation calibrators (Option 010) provide two precision modulation standards. One is an amplitude modulation signal whose depth is calibrated to better than 0.1% accuracy. The second standard is a frequency modulated signal with peak deviation calibrated to 0.1% accuracy. The 11715 AM/FM Test Source is necessary to fully test and calibrate other modulation parameters.

The 8901A Modulation Analyzer is more than just a high quality modulation meter. It also performs as a frequency counter and power meter. Resolution for the 8901A’s 150 kHz to 1300 MHz frequency counter is 10 Hz below 1000 MHz and 100 Hz above 1000 MHz. Sensitivity is -25 dBm (12 mV rms) below 650 MHz and -20 dBm (22 mV rms) above 650 or an optional time base is available with 1X10-9/day stability.

The 8901A uses a diode detection circuit to measure RF input power. This technique measures peak voltage and is calibrated from 1 mΩ to 1 Ω for sine wave inputs. The RF level measurement accuracy is ± 1.5 dB from 150 MHz to 1300 MHz.