19.5 Frequency Standards, SINAD Meters & EMI Hunting

19.5 Frequency Standards, SINAD Meters & EMI Hunting

Quick Answer: A counter measures the time between events, or the frequency, which is the reciprocal of the time. A frequency standard produces a highly accurate reference frequency; field technicians use a rubidium standard. A SINAD meter = multirange AF voltmeter in dB + sharp internal 1000 Hz bandstop filter. Hunt EMI with a portable AM receiver. An analog wattmeter is calibrated only at full scale — the upper 1/3 is the trustworthy part.

Sub-topic 3-L-075 (Test Equipment) is about choosing the right instrument, which is exactly the judgement an FCC-licensed maintainer is expected to have.

How a frequency counter works

How is a frequency counter used? To measure the time between events, or the frequency, which is the reciprocal of the time.

The answer states the duality precisely. f = 1/T, so an instrument that can measure an interval can report a frequency, and vice versa. In frequency mode the counter opens a gate for a precise interval and counts cycles; in period mode it counts reference-clock ticks between two edges of the input.

Period mode matters practically: at low frequencies, counting cycles in a one-second gate gives terrible resolution (a 10 Hz signal yields "10", with a resolution of 10%). Measuring the period instead and inverting gives many more significant digits. Good counters switch automatically.

Frequency standards

What is a frequency standard? A device used to produce a highly accurate reference frequency.

Which frequency standard is used as a time base standard by field technicians? A rubidium standard.

Section 7.5 established that a counter can never verify a tolerance tighter than its own time base. A rubidium standard is the practical resolution to that problem: portable, warms up in minutes, and holds accuracy orders of magnitude better than the frequency tolerances in Parts 80 and 87. GPS-disciplined oscillators serve the same purpose in modern practice, but the pool's keyed answer is rubidium.

The SINAD meter

Which of the following contains a multirange AF voltmeter calibrated in dB and a sharp, internal 1000 Hz bandstop filter, both used in conjunction with each other to perform quieting tests? A SINAD meter.

SINAD stands for Signal, Noise And Distortion — the ratio of everything at the audio output to everything except the wanted tone.

The measurement procedure explains the instrument's construction:

1. Feed the receiver an RF signal modulated with a 1000 Hz tone.
2. Measure the total audio output on the dB-calibrated AF voltmeter — that is signal + noise + distortion.
3. Switch in the 1000 Hz bandstop (notch) filter, removing the wanted tone.
4. Measure again — that is noise + distortion alone.
5. The difference in dB is the SINAD ratio.

Reduce the RF input until SINAD falls to 12 dB, and the RF level required is the receiver's standard sensitivity figure — the "12 dB SINAD" number quoted on every commercial receiver spec sheet (section 12.4). The two elements in the pool's question, the dB voltmeter and the 1000 Hz notch, are precisely the two things this procedure needs.

Hunting EMI

What equipment may be useful to track down EMI aboard a ship or aircraft? A portable AM receiver.

Counter-intuitive if you expect a spectrum analyser, and correct for a practical reason: AM detection is envelope detection, so it renders broadband noise audible. An arcing brush, a failing switching supply, a corroded bonding strap, a fluorescent ballast — each has a characteristic sound, and an experienced technician identifies the source by ear while walking the space with the receiver's own antenna as a direction finder.

An FM receiver would be poor at this: the capture effect and limiting deliberately suppress amplitude variation, which is exactly the information you need. That is why the answer is specifically a portable AM receiver, not just any handheld.

The analog wattmeter trap

On an analog wattmeter, what part of the scale is most accurate and how much does that accuracy extend to the rest of the reading scale? The accuracy is only at full scale, and that absolute number reading is carried through to the rest of the range. The upper 1/3 of the meter is the only truly calibrated part.

This is the sharpest practical trap in the sub-topic. An analog meter's accuracy is specified as a percentage of full scale, not a percentage of reading — so the absolute error is constant across the dial:

At the bottom of the scale the reading is nearly meaningless. The professional habit follows directly: choose a range that puts the reading in the upper third of the scale, and if a 25-watt transmitter reads near the left-hand stop on a 250-watt slug, fit the right slug before you record the number. The same logic applies to analog VOMs and to directional wattmeters with interchangeable elements.