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E4B

AMATEUR PRACTICES

Measurement technique and limitations: instrument accuracy and performance limitations; probes; techniques to minimize errors; measurement of Q; instrument calibration; S parameters; vector network analyzers; RF signals

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E4B011 of 11

Which of the following factors most affects the accuracy of a frequency counter?

Why A frequency counter works by counting input cycles during a precisely defined gate interval, then scaling the count to hertz. That gate interval comes from a crystal oscillator time base, so any error in the time base translates directly, proportionally, into frequency error. This is why counter specs are quoted in parts per million of the time base, and why high-end counters use oven-controlled or GPS-disciplined references.
Watch out The decade dividers and logic are digital, they either count correctly or fail outright, so they do not introduce a small proportional error; the input attenuator only affects sensitivity and level handling, not the count.
A counter is just a stopwatch plus a tally: if the clock is off by 1 ppm, every reading is off by 1 ppm.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B022 of 11

What is the significance of voltmeter sensitivity expressed in ohms per volt?

Why Ohms per volt is just the reciprocal of the current needed for full-scale deflection of the meter movement, so a 50 microamp movement gives 1/0.00005 = 20,000 ohms per volt. Multiply that figure by the full-scale voltage of the range you selected and you get the total resistance the voltmeter presents to the circuit. For example, a 20,000 ohms/volt meter on the 10 V range looks like 200,000 ohms. The higher the ohms per volt, the less the meter loads the circuit and the less it disturbs the reading.
Watch out The choices about power drawn or shunt sizing confuse this with current-meter design; shunts set the range of an ammeter, while ohms per volt describes the loading resistance of a voltmeter.
Ohms per volt times volts full scale equals ohms of input resistance. Bigger ohms/volt means less circuit loading.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B033 of 11

Which S parameter is equivalent to forward gain?

Why S parameters are written as S(out,in): the first subscript is the port where the signal comes out and the second is the port where it went in. Forward gain means drive port 1 and measure what emerges at port 2, so it is S21. The reflection terms S11 and S22 have matching subscripts because the signal leaves the same port it entered.
Watch out S12 is the same path run backwards, signal in at port 2 and out at port 1, which describes reverse transmission or isolation, not forward gain.
Read S(out,in): forward means in at 1, out at 2, so 2 then 1 = S21. Matching digits = reflection.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B044 of 11

Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?

Why S parameters are written as S(receiving port)(driving port), so the first subscript is where the signal is measured and the second is where it was fed in. S11 means drive port 1 and measure what comes back out of port 1, which is the input reflection coefficient, and that is what a VNA converts into return loss or VSWR at the input. S22 is the same idea at the output port, while the mixed subscripts describe signal passing from one port to the other.
Watch out S21 is the forward transmission term, the gain or insertion loss from input to output, and S12 is the reverse transmission or isolation; neither describes reflected energy at a port.
Matching subscripts mean reflection: S11 in, S22 out. Mismatched subscripts mean transmission through the device.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B055 of 11

What three test loads are used to calibrate an RF vector network analyzer?

Why A vector network analyzer measures both magnitude and phase, so its calibration must remove the errors (directivity, source match, frequency response) in the test port and cables. The standard one-port calibration is SOL: short, open, and load. A short gives a reflection coefficient of -1, an open gives +1, and a precision 50 ohm load gives 0, three known points that let the analyzer solve for its error terms at every frequency.
Watch out Swapping the 50 ohm load for a resonant circuit fails because a resonant circuit's reflection coefficient varies with frequency and is not a known reference; the calibration needs a broadband, precisely known termination.
VNA calibration = SOL: Short, Open, Load (50 ohms). Reflection coefficients -1, +1, 0.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B066 of 11

How much power is being absorbed by the load when a directional power meter connected between a transmitter and a terminating load reads 100 watts forward power and 25 watts reflected power?

Why A directional wattmeter measures forward and reflected power separately. The net power actually delivered to and absorbed by the load is the forward power minus the reflected power, so 100 W - 25 W = 75 W. The reflected 25 W travels back toward the transmitter rather than being dissipated in the load.
Watch out The choice of 125 watts comes from adding the two readings, but reflected power is not additional power delivered; it is power returning from the load. The 100 watt choice ignores the reflection entirely.
Net power = forward minus reflected. Subtract, never add.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B077 of 11

What do the subscripts of S parameters represent?

Why S parameters (scattering parameters) describe how RF energy is reflected from and transmitted through a network, and each one is labeled with two port numbers. The convention is S(out,in): the first subscript is the port where the response is measured and the second is the port where the signal is applied. So S11 is the reflection back out of port 1 when port 1 is driven (related to SWR and return loss), and S21 is what comes out of port 2 when port 1 is driven (gain or insertion loss).
Watch out The choices about timing, data quality, or frequency order sound like instrument bookkeeping, but S parameters are frequency-by-frequency ratios of wave amplitudes at specific ports, not a record of when or in what order measurements were taken.
S(out,in): second number is where you push, first is where you look. S11 = reflection, S21 = forward gain.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B088 of 11

Which of the following can be used to determine the Q of a series-tuned circuit?

Why Q is a measure of how sharply a resonant circuit selects frequencies, and it is defined as Q = f0 / BW, where BW is the width of the response between the half-power (-3 dB) points. So if you sweep the circuit and measure how wide the response is at those points, you can compute Q directly. A narrow response means high Q; a broad one means the circuit is heavily loaded or lossy.
Watch out The resonant frequency alone tells you nothing about Q, since two circuits can resonate at the same frequency with wildly different bandwidths; you need both f0 and BW. And at resonance the inductive and capacitive reactances are equal by definition, so their ratio is always 1.
Q = f0 / BW. Narrow skirt equals high Q. Resonant frequency alone is only half the equation.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B099 of 11

Which of the following can be measured by a two-port vector network analyzer?

Why A vector network analyzer sweeps a known signal into one port and measures the magnitude and phase of what is reflected and what comes out the other port, giving the S parameters S11, S21, S12 and S22 versus frequency. S21 is exactly the transmission response of a two-port device, so a filter's insertion loss and passband shape across frequency fall right out of the measurement. The 'vector' part means phase is captured too, so group delay and complex impedance come along with it.
Watch out Phase noise and pulse rise time are time-domain or noise-spectrum measurements that call for a spectrum analyzer or oscilloscope, and forward power in a live transmitter line is a wattmeter job; a VNA measures ratios using its own low-level test signal, not a transmitter's output power.
VNA = S parameters = response versus frequency. S21 is the filter's frequency response.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B1010 of 11

Which of the following methods measures intermodulation distortion in an SSB transmitter?

Why IMD is tested with a standard two-tone test: feed two audio tones of equal amplitude at non-harmonically related frequencies (classically something like 700 Hz and 1900 Hz) into the microphone input, and look at the RF output on a spectrum analyzer. Because an SSB transmitter is a linear amplifier chain, any nonlinearity mixes the two tones and creates extra products at 2f1-f2, 2f2-f1, and so on, which show up as sidebands spaced around the two wanted tones. The tones must not be harmonically related so the distortion products land on distinct frequencies instead of hiding on top of each other or on harmonics.
Watch out Injecting two RF signals is the way you test a receiver or amplifier front end for IMD, not how you modulate an SSB transmitter, whose input is audio. A peak reading wattmeter only shows total power and cannot separate the distortion products from the wanted tones.
Two AUDIO tones in, spectrum analyzer on the RF out, and keep the tones non-harmonic so the products stand alone.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E4B1111 of 11

Which of the following can be measured with a vector network analyzer?

Why A vector network analyzer sends a known signal into a port and measures both the magnitude and the phase of the waves reflected back and transmitted through, giving the full set of S parameters. S11 and S22 are the input and output reflection coefficients, and since reflection coefficient and impedance are directly related (Z = Z0(1+Γ)/(1-Γ)), the VNA can display input impedance, output impedance, or Γ itself from the same measurement. S21 and S12 then give forward and reverse gain or loss, so essentially all the port characteristics of a network come from one sweep.
Watch out Picking just one item misses that all three are the same data presented differently; impedance is simply the reflection coefficient converted through the reference impedance, usually 50 ohms.
Vector = magnitude AND phase, so a VNA gets Γ and every impedance that follows from it.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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