Spectrum Monitoring Setup: SAM–NAM Calibration
Calibrating a SAM or NAM sweep is the only way to tie measurement to reality. Every value that follows, from the noise floor to the site’s interference profile, starts with a calibration signal and a properly adjusted offset. The sweep data is only as useful as the setup behind it.
SAM (Signal Activity Monitoring) and NAM (Noise Activity Monitoring) traces may look similar but are evaluated differently. SAM focuses on known signal peaks. NAM traces the raw noise energy beneath them. That distinction shapes how each is calibrated and where signal injection occurs.
Locking in the Plane of Measurement
Spectrum monitoring relies on a consistent reference point, typically the base of the receive antenna. Whether the analyzer is in a trailer or behind a multicoupler, calibration must account for everything between the antenna base and the analyzer input. That includes cable loss, preselector roll-off, amplifier gain, and any loss introduced by devices such as Iso-Ts in tower-top amplifiers (TTAs).
If the antenna connection is accessible, it becomes the calibration point. If not, the signal must be injected farther down the line, and the level adjusted to simulate what would be present at the antenna base. This makes sure the analyzer interprets incoming power as if it were measured from the correct location.
Injecting the Calibration Signal
A direct cable run makes calibration simple. The signal generator connects to the antenna end of the transmission line and is set to -60.0 dBm for SAM or -90.0 dBm for NAM. If extension cabling is required, its loss is added to the generator output. If injection occurs through a preselector or TTA test port, the signal level must increase to reflect gain and loss through the system.
For example, if the test port cable has 5 dB of loss and the Iso-T inside the TTA adds 30 dB, the generator must output -25.0 dBm to simulate -60.0 dBm at the antenna base. Jumper losses from the generator to the test port are also included. Generator levels often fall between -20 and -45 dBm, depending on the system layout.
NAM calibration requires even more control. All external noise must be blocked from the system. This means disconnecting the antenna or setting the TTA to terminated mode. If a TTA cannot be isolated or lacks a test port, NAM data should not be collected at that site.
SAM Calibration: Frequency Sweep and Offset Adjustment
With the signal injected, the technician selects the SAM-CAL-SIG sweep, typically found in Span Group 0000. The configuration is started, and the analyzer trace is observed. The injected signal, corrected for loss, should be clearly visible.
The amplitude offset is adjusted until the analyzer displays exactly -60.5 dBm. This tells the system how to convert internal measurements into field-accurate values. That offset is recorded and applied to all SAM spans, including the main collection group in Span Group 1000.
A valid calibration sweep should display two traces: the injected signal and system noise. Logging should only begin once both are steady and visible.
NAM Calibration: Noise Bandwidth Correction and Offset Application
NAM calibration focuses on noise power rather than discrete signals. Analyzer bandwidth affects these readings significantly. Most resolution bandwidth settings are nominal, but actual filter width tends to be wider, allowing more noise to pass through and inflating the measured value.
To account for this, NAM calibration targets a displayed level of -92.0 dBm even though the generator injects -90.0 dBm. This compensates for the wider filter shape and delivers a more accurate representation of the noise floor.
The technician selects the NAM-CAL-SIG sweep, usually in Span Group 5000, injects the signal with path loss accounted for, and adjusts the offset until the analyzer reads -92.0 dBm. That value is applied to Span Group 6000, which contains the active NAM collection spans. Calibration should run for five minutes to confirm reliable data capture.
Working Inside the Software
Most calibration problems occur in software. The span group dropdown resets to “All Span Types” whenever a sweep is selected. If the wrong group is active, offsets may be lost or applied incorrectly.
It is also common to forget to apply the same offset to both the calibration and collection spans. For SAM, the offset entered in Group 0000 must be repeated in Group 1000. For NAM, values from 5000 must carry over to 6000. Even small mismatches can skew results, especially in narrowband or high-gain applications.
Each configuration should produce two traces. If only one appears, or more than two are visible, the configuration should be stopped and reviewed before continuing.
Reruns, Edits, and Live Adjustments
Any change to analyzer settings, including RBW, VBW, channel spacing, detector type, or preamp status, invalidates the calibration. After any such change, the technician must return to the SAM-CAL-SIG and NAM-CAL-SIG spans, re-inject the signal, and apply a new offset.
A new database must also be created. Mixing sweep data from different configurations introduces baseline drift and makes the data unreliable. Once that happens, there is no way to correct it.
Final Pass with Trickle Feed
Before sweep data is finalized, the Trickle Feed process provides a final layer of correction. It compares the recorded traces against the known calibration levels and calculates a refined offset. For SAM, this value is -60.5 dBm. For NAM, it is -92.0 dBm.
These corrections should be applied only to the appropriate sweep group. Applying a NAM offset to SAM data, or the reverse, will distort the results and invalidate the entire data set.
Final Thoughts
Calibration doesn’t begin with software, it starts with understanding how signals move through the system and how losses affect what the analyzer sees. Every sweep depends on that foundation.
SAM and NAM offer different views of the spectrum, but both rely on accurate calibration. When done properly, these traces become dependable tools for site evaluation and interference analysis.
For engineers in the field or supporting remote projects, this procedure is the baseline that everything else builds on. It has to be done carefully and it has to be done right. Contact TX RX and speak to one of our engineers for more information.