19.3 Measurement Procedures

19.3 Measurement & Repair Procedures

Quick Answer: Sensitivity → 12 dB SINAD. Modulation-acceptance bandwidth → effective receiver bandwidth. CDMA power ≠ trustworthy on analog wattmeters. P25 scanner needs P25 decode. Wireless programming often infra-red. Wideband FM voice deviation 5.0 kHz. CMOS wrist strap < 100 kΩ. Desolder plated-through holes with a vacuum device. Cut ties flush with flush-cut diagonals. Ideal strip = thermal stripper. Hot-gas bonder = non-contact solder melt. Board repair priority: safety glasses.

Key topics 078 (Measurement Procedures) and 079 (Repair Procedures) move from “owning instruments” to using them correctly and fixing gear without destroying it.

Measurement procedures that show up on Element 3

Receiver sensitivity — 12 dB SINAD

Common method for determining the exact sensitivity specification of a receiver: measure the recovered audio for 12 dB of SINAD. SINAD is the ratio of Signal + Noise + Distortion to Noise + Distortion. The 12 dB SINAD point is the industry-standard quieting figure for land-mobile and many commercial receiver specs. Distractors (10 dB SINAD, 10 dB quieting, 25 dB quieting) are nearby numbers—do not swap them.

Workflow sketch:

1. Inject a standard modulated RF test signal (often 1 kHz tone, specified deviation).
2. Recover audio into a SINAD meter (Section 19.1: dB AF voltmeter + 1000 Hz notch).
3. Reduce RF level until the meter shows 12 dB SINAD.
4. That RF input level is the sensitivity figure for the procedure in use.

Modulation-acceptance bandwidth

A communications technician performs a modulation-acceptance bandwidth test to determine the effective bandwidth of a communications receiver. It is not primarily an audio-frequency-response curve, a CTCSS check, or a mere repeat of the 12 dB SINAD test—though SINAD metering gear may appear in the same procedure family.

Digital and modern-modulation measurement caveats

Why CDMA fools analog meters: CDMA and many digital formats are noise-like with high peak-to-average power ratios. Average-responding analog RF meters calibrated for CW/FM carriers mis-read the true power. Use instruments and modes rated for the emission (true-average/peak sensors, manufacturer digital power procedures).

P25: Project 25 digital voice is not intelligible on an analog-only scanner; decoding capability is required.

IR programming: many portables accept clone/program data over an optical IR port—no cable, but line-of-sight and clean optics matter.

Deviation: classic wideband land-mobile voice channels use ±5 kHz peak deviation; narrowband channels use smaller values (e.g., 2.5 kHz)—the pool’s “normal wideband” answer is 5.0 kHz.

Safe measurement of high RF and high voltage

GROL work includes ship transmitters, aircraft systems, HF linear amps, and radar-adjacent environments. Measurement skill without safety is career-limiting.

High RF

1. Prefer dummy-load testing over open-antenna radiation when aligning or troubleshooting transmitters.
2. Use directional wattmeters and proper attenuators rated for the power and frequency—never couple a sensitive spectrum analyzer directly to a PA output.
3. Keep body clear of open feedline ends and antenna elements under power; RF burns and induced currents are real at HF/VHF/UHF power levels.
4. Disable or isolate transmitters before connecting/disconnecting RF connectors under power.

High voltage

1. Capacitor banks and tube anode supplies can remain lethal after power-off—discharge with proper bleed tools, not a screwdriver “spark test.”
2. One-hand technique, insulated tools, and known ground references reduce across-the-chest shock paths.
3. Measure HV with rated probes and meters only; ordinary DMM leads are not 5 kV probes.
4. Interlocks exist for a reason—defeat only under controlled procedures with the equipment de-energized.

Component-level repair workflow

A disciplined repair flow beats shotgun part-swapping:

1. Symptom → system — confirm power, antenna/dummy, control settings, and external accessories first.
2. Block diagram isolation — is the fault in RX, TX, PSU, synthesizer, or control/CPU?
3. Instrument confirmation — meters, scope, SA, service monitor (19.1–19.2).
4. Component-level — schematics, bias voltages, RF levels stage-by-stage.
5. Replace / rework — ESD-safe, correct parts, correct soldering process.
6. Align / verify — manufacturer alignment points and performance tests.
7. Document — what failed, what was done, measurements before/after.

ESD and CMOS wrist straps

When soldering or working with CMOS electronics, a wrist strap must have less than 100,000 ohms of resistance to drain static safely. The strap is not “banned on TTL,” does not require a dedicated water-pipe ground as the defining rule, and works with anti-static mats when the system is designed correctly. Too little resistance can be a shock hazard from chassis voltage; too much fails to bleed charge—pool number is < 100 kΩ.

Desoldering plated-through holes

Preferred method of cleaning solder from plated-through circuit-board holes: use a vacuum device (desoldering pump / vacuum desoldering station). Dental picks damage barrels; >900 °F iron tips cook boards; air-jet devices are not the preferred Element 3 answer.

Wire ties, insulation, and hot-gas tools

Why thermal strippers win: they soften insulation without nicking copper the way knives and aggressive mechanical strippers can—critical on fine avionics and marine harness wires.

Hot gas bonder: delivers heated gas to reflow solder without a contact iron tip—useful on delicate SMD work when used per process controls.

Safety glasses: molten solder, clipped leads, and flux spatter make eye protection the top board-repair priority on the pool—not clever trace-bridging with solder blobs.

Soldering RF assemblies

RF repairs add constraints beyond ordinary DC boards:

1. Lead length and layout — excess lead inductance detunes VHF/UHF stages; dress replacements like the original.
2. Ground returns — broken ground vias or lifted ground planes create mysterious instability and spur problems.
3. Coax and connectors — reflow connector pins carefully; cold joints raise VSWR.
4. Heat management — ceramics, PTFE, and multilayer RF boards hate prolonged heat; use proper tip mass and flux, not brute temperature.
5. Shield cans — reseat shields after alignment; missing shields change both performance and EMI.

Alignment procedures overview

Alignment is not random slug-turning. Typical commercial flow:

1. Stable supplies and dummy load connected.
2. Service literature open—order of adjustments matters (LO, then IF, then RF; TX power last, etc.).
3. Reference instruments calibrated (counter, service monitor, SINAD meter).
4. Record as-found readings before changing anything.
5. Adjust only specified cores/trimmers to the specified indication.
6. As-left measurements and functional on-air or simulated checks.
7. Seal/paint adjustments if the OEM requires it after certification work.

Documentation

Good documentation is part of professional maintenance:

• Equipment ID, serial, location, date, technician.
• Symptom and operational impact.
• Measurements (power, frequency error, SINAD, deviation, VSWR).
• Parts replaced and alignment performed.
• Operational test results and open items.

Marine, aviation, and public-safety environments often require this paper trail for inspections and liability—not only for your own next visit.

Exam-day checklist (3-L 078–079)

1. Sensitivity → 12 dB SINAD; modulation-acceptance bandwidth → effective RX bandwidth.
2. CDMA ≠ accurate on analog power meters; P25 needs P25 decode; wireless program → IR.
3. Wideband FM voice deviation → 5.0 kHz.
4. CMOS wrist strap → < 100 kΩ; desolder PTH → vacuum; strip → thermal; ties → flush-cut flush.
5. Hot-gas bonder → non-contact solder melt; board work → safety glasses first.
6. High RF/HV: dummy loads, rated probes, discharge, attenuators; finish with alignment + documentation.

Next section covers installation codes, bonding/grounding, connectors, and systematic troubleshooting.