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Amateur Radio Info & Exams - Practical Circuits 2 - Filters & Power Supplies

Filters & Matching Networks

Given capacitors pass high frequencies and impede low ones; and inductors pass low frequencies, and impede high ones; it is possible to use them in filters for audio and radio frequencies. They can also match the output impedance of valve / tube amplifiers to a 50 ohm or similar feedline.

Fairly simple filters are the low pass and the high pass filters. Bandpass filters pass a range of frequencies, and band-stop block a certain range. A notch-filter is a narrow (high-Q) band-stop filter.

Shape Factor

A filter might have 3 kHz bandwidth at 3 dB below the centre pass frequency. How well it rejects a strong signal a few more kHz away is down to the steepness of the filter edges. One way to assess this is the shape factor. If the bandwidth is 6 kHz at 30 dB down, then the shape factor at 30/3 dB is 2:1.

Power Supplies

There are several options for building a power supply to supply amateur equipment, including transceivers and accessories, typically from the mains supply.

The first is termed a linear supply, consisting of a heavy iron lamination and copper transformer, a diode bridge, and filter capacitor(s). If the circuit supplied is happy to run from a range of voltages, no regulation is needed.

If regulation is needed, and the load is only an amp, or several amps, the simplest option is a three terminal regulator, such as the 78xx series. In addition to the TO-220 lower current versions, there are also higher current TO-3 versions. The other option is to use a Zener or other voltage reference and a series pass transistor. For larger supplies chunky TO-3P or TO-218, and similar packages; or several in parallel, would be ideal. The exam terms these linear regulators.

A variation is a regulator using a pass transistor, with a 78xx or similar regulator as the reference.

The alternative is a switching power supply. In these the mains supply is rectified, and filtered by capacitors typically rated at 400 volts. After this an oscillator drives one or more high voltage transistors, switching current into a physically small transformer, in some cases with a a ferrite core. After this, diodes with a fast response rectify the output. A feedback system typically uses an optocoupler to signal if the voltage is high or low, so that the duty cycle can be adjusted to maintain the desired output voltage over a range of loads.

A shunt regulator loads the unregulated supply down to the desired voltage.

While the Technician paper included a simple supply, shunt regulated by a Zener diode, such arrangements are normally used within a device such as a transceiver, to supply specific low current stages with clean, regulated power. That said, some dodgy cordless 'phones shunt the unregulated supply, meaning that if you replace the supply with a battery based UPS circuit, the regulator will overheat.

All high power supplies require heatsinks on the power transisters or ICs, and some include cooling fans, as do some large amplifiers. The energy lost as heat is however larger in linear units.

There are also oddball arrangements, such as ferro-resonant supplies.

Solar charge controllers

Unless you are closely monitoring the battery voltage, say during a field day operation, you must use a charge controller if you have a station powered by a battery charged via a solar panel. This prevents the battery being damaged by over-charging. The complexity of these varies.

Relevant Questions

These are the actual questions from the Extra licence exam pool, as published by the NCVEC.

How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network's input and output?
A. Two inductors are in series between the input and output, and a capacitor is connected between the two inductors and ground
B. Two capacitors are in series between the input and output, and an inductor is connected between the two capacitors and ground
C. An inductor is connected between the input and ground, another inductor is connected between the output and ground, and a capacitor is connected between the input and output
D. A capacitor is connected between the input and ground, another capacitor is connected between the output and ground, and an inductor is connected between input and output

A low pass filter uses capacitors to shunt high frequencies to ground, and a series inductor to block or reduce these; while low frequencies are not shunted greatly, nor reduced significantly by the inductor. The Pi configuration has elements (caps) to ground at the input and output, and a single series element (inductor), in the shape of the letter pi, π, answer D.

Which of the following is a property of a T-network with series capacitors and a parallel shunt inductor?
A. It is a low-pass filter
B. It is a band-pass filter
C. It is a high-pass filter
D. It is a notch filter

The series capacitors pass high frequencies, and block low ones, while the inductor shunts low frequencies to ground, while doing little to high frequency signals, so it is a high-pass filter, answer C.

What advantage does a series-L Pi-L-network have over a series-L Pi-network for impedance matching between the final amplifier of a vacuum-tube transmitter and an antenna?
A. Greater harmonic suppression
B. Higher efficiency
C. Does not require a capacitor
D. Greater transformation range

A filter with additional elements has greater harmonic suppression, answer A.

How does an impedance-matching circuit transform a complex impedance to a resistive impedance?
A. It introduces negative resistance to cancel the resistive part of impedance
B. It introduces transconductance to cancel the reactive part of impedance
C. It cancels the reactive part of the impedance and changes the resistive part to a desired value
D. Network resistances are substituted for load resistances and reactances are matched to the resistances

The reactive elements cancel the reactive part of the impedance, and and changes the resistive part to a desired value, answer C.

Which filter type is described as having ripple in the passband and a sharp cutoff?
A. A Butterworth filter
B. An active LC filter
C. A passive op-amp filter
D. A Chebyshev filter

This is the Chebyshev filter, answer D.

What are the distinguishing features of an elliptical filter?
A. Gradual passband rolloff with minimal stop band ripple
B. Extremely flat response over its pass band with gradually rounded stop band corners
C. Extremely sharp cutoff with one or more notches in the stop band
D. Gradual passband rolloff with extreme stop band ripple

These have an extremely sharp cutoff, with one or more notches in the stop band C.

Sharp cutoff often has the cost of ripple or other irregular response either side of the cutoff.

Which describes a Pi-L-network used for matching a vacuum tube final amplifier to a 50 ohm unbalanced output?
A. A Phase Inverter Load network
B. A Pi-network with an additional series inductor on the output
C. A network with only three discrete parts
D. A matching network in which all components are isolated from ground

This is a Pi-network with an extra series inductor on the output, to improve the match to the 50 ohm antenna terminal, answer B.

Which of the following factors has the greatest effect on the bandwidth and response shape of a crystal ladder filter?
A. The relative frequencies of the individual crystals
B. The DC voltage applied to the quartz crystal
C. The gain of the RF stage preceding the filter
D. The amplitude of the signals passing through the filter

This is the relative frequencies of the individual crystals, answer A.

What is a crystal lattice filter?
A. A power supply filter made with interlaced quartz crystals
B. An audio filter made with four quartz crystals that resonate at 1kHz intervals
C. A filter with wide bandwidth and shallow skirts made using quartz crystals
D. A filter with narrow bandwidth and steep skirts made using quartz crystals

Crystal filters, think high-Q, narrow bandwidth, and steep skirts, answer D.

Which of the following filters would be the best choice for use in a 2 meter repeater duplexer?
A. A crystal filter
B. A cavity filter
C. A DSP filter
D. An L-C filter

Cavity filters are a common choice for duplexers at VHF frequencies, answer B.

Which of the following describes a receiving filter's ability to reject signals occupying an adjacent channel?
A. Passband ripple
B. Phase response
C. Shape factor
D. Noise factor

This is shape factor, answer C.

A good shape factor means that adjacent channels are rejected.

What is one advantage of a Pi-matching network over an L-matching network consisting of a single inductor and a single capacitor?
A. The Q of Pi-networks can be controlled
B. L-networks cannot perform impedance transformation
C. Pi-networks are more stable
D. Pi-networks are designed for balanced input and output

The Q of Pi-networks can be varied depending on the component values chosen, answer A.

How does a linear electronic voltage regulator work?
A. It has a ramp voltage as its output
B. It eliminates the need for a pass transistor
C. The control element duty cycle is proportional to the line or load conditions
D. The conduction of a control element is varied to maintain a constant output voltage

The conduction of the control element (internal series pass transistor) is controlled so the output voltage is constant, answer D.

What is one characteristic of a switching electronic voltage regulator?
A. The resistance of a control element is varied in direct proportion to the line voltage or load current
B. It is generally less efficient than a linear regulator
C. The controlled device’s duty cycle is changed to produce a constant average output voltage
D. It gives a ramp voltage at its output

The transistor or FET has its duty cycle changed to produce a constant output voltage, answer C.

What device is typically used as a stable reference voltage in a linear voltage regulator?
A. A Zener diode
B. A tunnel diode
D. A varactor diode

A Zener diode, or similar reference diode is used, answer A.

Which of the following types of linear voltage regulator usually make the most efficient use of the primary power source?
A. A series current source
B. A series regulator
C. A shunt regulator
D. A shunt current source

The 78xx IC regulators, and discrete regulators as in figure E7-2, are termed series regulators, and are fairly efficient, as the required current is only the load current, plus that needed for the reference component, answer B.

Which of the following types of linear voltage regulator places a constant load on the unregulated voltage source?
A. A constant current source
B. A series regulator
C. A shunt current source
D. A shunt regulator

They are trying to describe is a shunt regulator, answer D, where an extra load reduces the output voltage to the desired level. A Zener (possibly a stud-mount one) is a typical load element.

What is the purpose of Q1 in the circuit shown in Figure E7-2?
A. It provides negative feedback to improve regulation
B. It provides a constant load for the voltage source
C. It controls the current supplied to the load
D. It provides D1 with current

This increases the current supply capability of the circuit, answer C.

Descrete component series regulator.

What is the purpose of C2 in the circuit shown in Figure E7-2?
A. It bypasses rectifier output ripple around D1
B. It is a brute force filter for the output
C. To self-resonate at the hum frequency
D. To provide fixed DC bias for Q1

It bypasses ripple (hum) from the rectifier at D1, answer A.

What type of circuit is shown in Figure E7-2?
A. Switching voltage regulator
B. Grounded emitter amplifier
C. Linear voltage regulator
D. Monostable multivibrator

This is a linear voltage regulator, made from discrete components, answer C.

What is the main reason to use a charge controller with a solar power system?
A. Prevention of battery undercharge
B. Control of electrolyte levels during battery discharge
C. Prevention of battery damage due to overcharge
D. Matching of day and night charge rates

These prevent the battery being overcharged, and so damaged, answer C.

What is the primary reason that a high-frequency switching type high voltage power supply can be both less expensive and lighter in weight than a conventional power supply?
A. The inverter design does not require any output filtering
B. It uses a diode bridge rectifier for increased output
C. The high frequency inverter design uses much smaller transformers and filter components for an equivalent power output
D. It uses a large power factor compensation capacitor to create free power from the unused portion of the AC cycle

The mains voltage is rectified to high voltage DC, and this is then fed into a small transformer through chopping transistors, a circuit which can be termed an inverter. As the frequency is high, the transformer and filter capacitors can also be smaller. The smallness of these parts can reduce cost and weight, answer C.

What is the function of the pass transistor in a linear voltage regulator circuit?
A. Permits a wide range of output voltage settings
B. Provides a stable input impedance over a wide range of source voltage
C. Maintains nearly constant output impedance over a wide range of load current
D. Maintains nearly constant output voltage over a wide range of load current

It maintains as near to a constant output voltage as possible over a wide range of load currents, answer D.

What is the drop-out voltage of an analog voltage regulator?
A. Minimum input voltage for rated power dissipation
B. Maximum amount that the output voltage drops when the input voltage is varied over its specified range
C. Minimum input-to-output voltage required to maintain regulation
D. Maximum amount that the output voltage may decrease at rated load

If we have a basic 7805, we need to have at least 8 volts on the input, and we call this required overhead, or minimum input to output difference the drop-out voltage, in this case 3 volts. Answer C.

There are also specialist LDO (low drop-out) regulators, which can work with an input something like 40 milivolts above the output rail. They often have a lower maximum input voltage than a standard device, say 12 vs 35 volts.

What is the equation for calculating power dissipation by a series connected linear voltage regulator?
A. Input voltage multiplied by input current
B. Input voltage divided by output current
C. Voltage difference from input to output multiplied by output current
D. Output voltage multiplied by output current

Take the above example, and 8-5=3 volts, and if we were puling the full amp, the dissipation would be 3 watts; if we were dropping from 28 to 12 volts, at 0.75 amps, we have P = (28-12) x 0.75 = 16 x 0.75 = 12 watts, needing a fairly decent heat-sink. P = (VIN-VOUT) × IOUT, answer C.

What is the purpose of connecting equal-value resistors across power supply filter capacitors connected in series?
A. Equalize the voltage across each capacitor
B. Discharge the capacitors when voltage is removed
C. Provide a minimum load on the supply
D. All these choices are correct

These equalise the voltage across the capacitors. Move importantly, it helps prevent electric shock if it is a high voltage supply, and someone goes "fingerpoken" in the supply (as long as it has been turned for a few moments). They also provide a minimum load, to prevent the voltage floating too high. It is this all of these, answer D.

What is the purpose of a step-start circuit in a high voltage power supply?
A. To provide a dual-voltage output for reduced power applications
B. To compensate for variations of the incoming line voltage
C. To allow for remote control of the power supply
D. To allow the filter capacitors to charge gradually

Initially starting the transformer via resistors limits the inrush current, and allows the filter capacitors to charge more gently, answer D.

On to: Practical Circuits 3 - Modulation & DSP

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, June 2022.

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