HamSandwich

Fresh cut ham data, daily.

Study › Amateur Extra › E7C

E7C

PRACTICAL CIRCUITS

Filters and matching networks: types of networks; types of filters; filter applications; filter characteristics; impedance matching

Drill results are kept in this browser. Log in to keep them on your account and get a study plan.

E7C011 of 11

How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network's input and output?

Why A Pi-network is named for the shape of the Greek letter pi: two vertical legs to ground with a horizontal element bridging them. For a low-pass version, the shunt legs must be capacitors (low reactance to high frequencies, shorting them to ground) and the series element must be an inductor (high reactance to high frequencies, blocking them from reaching the output). Low frequencies pass straight through the coil while the capacitors look like open circuits, so the network passes the low end and attenuates the high end. This is the classic tube amplifier output tank arrangement used for both harmonic suppression and impedance matching.
Watch out Two inductors in series with a capacitor to ground is also a low-pass filter, but it forms a T-network, not a Pi; the two series capacitors with a shunt inductor is a high-pass T.
Pi = two legs down, one bar across. Low-pass Pi: caps are the legs to ground, coil is the bar.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C022 of 11

What is the frequency response of a T-network with series capacitors and a shunt inductor?

Why In a ladder filter the series elements decide what gets through and the shunt elements decide what gets dumped to ground. Capacitive reactance (Xc = 1/(2πfC)) falls as frequency rises, so series capacitors pass high frequencies and block low ones, while the shunt inductor's reactance (XL = 2πfL) is small at low frequencies and shorts them to ground. Both actions favor the high end, so this T-network is a high-pass filter.
Watch out Low-pass is the mirror image network: series inductors with a shunt capacitor. Band-pass and notch responses require resonant (L and C together) branches, which a single-reactance-per-branch T-network does not have.
Series C and shunt L = highs pass. Flip the parts (series L, shunt C) and you get low-pass.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C033 of 11

What is the purpose of adding an inductor to a Pi-network to create a Pi-L-network?

Why A Pi-network is already a low-pass filter, but adding a series inductor (plus the load capacitance) tacks an extra L-section onto the output, so the signal passes through two cascaded low-pass sections instead of one. Each additional reactive section increases the rolloff slope above the cutoff frequency, so harmonics of the transmit frequency are attenuated much more strongly. That is why classic tube transmitter tank circuits used the Pi-L configuration for final amplifier output.
Watch out Greater transformation range sounds plausible because the extra section does help step down to low impedances in stages, but the recognized purpose in the pool is the added attenuation of harmonics. Efficiency actually drops slightly, since every added component has some loss.
Pi-L = Pi plus one more L-section = one more low-pass stage = cleaner harmonics.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C044 of 11

How does an impedance-matching circuit transform a complex impedance to a resistive impedance?

Why A complex load looks like a resistance in series (or parallel) with a reactance. A matching network uses reactive components to present an equal and opposite reactance, canceling the load's X so the net reactance is zero, and at the same time it transforms the remaining resistive part up or down to the value the source wants to see (typically 50 ohms). That is the definition of a conjugate match: if the load is R + jX, the network makes the source see R - jX plus the transformation, leaving pure resistance. Because the parts are ideally lossless inductors and capacitors, no power is burned doing this.
Watch out The idea that reactive currents are dissipated in matched resistances is wrong because a proper matching network stores and returns that energy in L and C rather than turning it into heat; dissipating it would defeat the purpose. Negative resistance and transconductance are active-device concepts and have no role in a passive L, T or pi network.
Match = cancel the X, transform the R. Two jobs, both done with lossless L and C.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C055 of 11

Which filter type has ripple in the passband and a sharp cutoff?

Why A Chebyshev filter trades a flat passband for a steeper transition: it deliberately allows equal-amplitude ripple across the passband (0.1 dB, 0.5 dB, 1 dB designs are common) and in return gets a much sharper cutoff than a Butterworth of the same order. The more ripple you tolerate, the steeper the skirt becomes. That makes it the usual choice when you need to reject a nearby signal, such as harmonic filtering, and a little passband variation does not matter.
Watch out The Butterworth is the opposite design goal: maximally flat, no passband ripple, but a gentler rolloff. The other two choices are contradictory labels, since an op-amp filter is active, not passive.
Chebyshev = Choppy passband, sharp Cutoff. Butterworth = smooth as butter, but slow rolloff.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C066 of 11

What are the characteristics of an elliptical filter?

Why An elliptic (Cauer) filter trades ripple in both the passband and stopband for the fastest possible transition from pass to stop. Those stopband transmission zeros show up as deep notches just above (or below) the cutoff frequency, which is what makes the cutoff so steep for a given number of poles. It is the filter of choice when you must reject a signal very close in frequency to one you want to keep.
Watch out The description of an extremely flat passband with gradually rounded stopband corners is the Butterworth (maximally flat) response, and the gradual-rolloff answers do not describe any sharp-cutoff design at all.
Elliptic = Extreme edge plus notches. Butterworth = flat and Bland, Chebyshev = ripple in the passband.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C077 of 11

Which describes a Pi-L network?

Why A standard pi network is shunt capacitor, series inductor, shunt capacitor. A Pi-L adds one more series inductor after the output shunt capacitor, so the circuit becomes a pi section feeding an L section. That extra element gives a steeper rolloff above cutoff and better harmonic attenuation, which is why Pi-L tank circuits are favored in tube power amplifiers, and it also lets the transformation be done in two smaller steps for a higher loaded Q at the tube end.
Watch out The three-discrete-parts description fits the plain pi network itself; the Pi-L has four reactive elements, and both shunt capacitors are referenced to ground, so nothing is floating.
Pi plus L: pi network first, then an L section. Four parts, extra series coil at the output for extra harmonic rejection.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C088 of 11

Which of the following is most frequently used as a band-pass or notch filter in VHF and UHF transceivers?

Why A helical filter uses a helical resonator, a coil wound inside a shielded cavity, which behaves like a high-Q quarter-wave resonant line in a much smaller package than a straight coaxial cavity. That high Q at VHF and UHF gives sharp band-pass or notch skirts, so helical resonators are the standard front-end preselector and duplexer element in VHF/UHF radios. At those frequencies the physical coil size is small enough to be practical, which is why the technique dominates there rather than at HF.
Watch out The Sallen-Key filter is an op-amp active filter used at audio frequencies, and a swinging choke is a power supply inductor whose inductance varies with load current, not an RF selectivity device. The finite impulse response filter is a digital DSP structure, used after the signal has been sampled.
Helix in a can = high-Q VHF/UHF band-pass. Sallen-Key is audio, swinging choke is power supply, FIR is DSP.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C099 of 11

What is a crystal lattice filter?

Why A crystal lattice filter is a bandpass filter built from quartz crystals wired in a lattice (bridge) configuration, where pairs of crystals are offset slightly in frequency to set the passband. The very high Q of quartz gives a narrow passband with steep skirts, which is why these filters are the standard IF filter in SSB and CW receivers and transmitters. Quartz elements handle only tiny amounts of power, so the filter is placed in low-level signal stages, never in a high-current path.
Watch out The power supply choice is the trap: quartz crystals are RF resonators that would be destroyed by supply currents, and supply filtering is done with capacitors, inductors and regulators. The audio filter description with crystals spaced 1 kHz apart is invented wording, not a real device.
Quartz is delicate: lattice filters sit in the low-level IF chain, not in the power supply.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C1010 of 11

Which of the following filters is used in a 2-meter band repeater duplexer?

Why A repeater transmits and receives simultaneously on frequencies typically 600 kHz apart at 2 meters, so the duplexer must pass one frequency while rejecting the other by 80 dB or more just a fraction of a percent away. Only a resonant cavity, a tuned quarter-wave or half-wave section of coaxial or waveguide-like metal chamber, has the high Q and power handling needed to do that at 146 MHz. Several cavities are cascaded, often notch-tuned, to isolate the transmitter from the receiver front end.
Watch out Crystal filters have very high Q but are made for low-level IF frequencies in the kHz to tens of MHz range and cannot handle repeater transmitter power; DSP filters work on baseband or IF digital samples, not on the antenna feedline.
Repeater duplexer = big metal cans on the wall. High power plus razor-sharp VHF selectivity means cavity.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7C1111 of 11

Which of the following measures a filter's ability to reject signals in adjacent channels?

Why Shape factor is the ratio of a filter's bandwidth measured far down the skirts to its bandwidth near the top, commonly the -60 dB bandwidth divided by the -6 dB bandwidth. A ratio close to 1:1 means nearly vertical skirts, so a strong signal just outside the passband is attenuated hard while the desired signal passes. That steepness is exactly what determines how well the filter rejects an adjacent channel.
Watch out Passband ripple is tempting, but it describes amplitude variation on the desired signals inside the passband, not attenuation of signals outside it; noise factor is a receiver noise measure, not a selectivity measure.
Shape factor = -60 dB BW / -6 dB BW. Closer to 1 means steeper skirts and better adjacent-channel rejection.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
Yaesu FT-65R VHF/UHF Dual Band HandheldSponsored · View on Amazon →MFJ-259D HF/VHF Antenna AnalyzerSponsored · View on Amazon →
← E7B All groups E7D →