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Subelement E7
PRACTICAL CIRCUITS
Section E7C
Filters and matching networks: types of networks; types of filters; filter applications; filter characteristics; impedance matching; DSP filtering
How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network's input and output?
• Two inductors are in series between the input and output, and a capacitor is connected between the two inductors and ground
• Two capacitors are in series between the input and output, and an inductor is connected between the two capacitors and ground
• 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
• 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

Think of the symbol "Pi" ($\pi$). It's the same shape, with two lines down to the ground.

## For a Low Pass function, the two capacitors will lead the higher frequencies to ground while opposing that path to lower frequencies. The inductor leads the lower frequencies to the output while opposing that path to higher frequencies.

Also see Wikipedia article section and accompanying images: https://en.wikipedia.org/wiki/Antenna_tuner#Low-pass_π_network

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

A capacitor has a response that increases as frequency increases and an inductor has the opposite response, it decreases as frequency increases. In the circuit described the inductor is between the signal path and ground and the capacitor in the signal path.

So, the capacitor impedes the passage of low frequencies in the signal path and the inductor allows the passage of low frequencies to ground leaving the higher frequencies as the only ones that pass through the T-network described.

Hint: a Touchdown pass is thrown HIGH

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What advantage does a Pi-L-network have over a regular Pi-network for impedance matching between the final amplifier of a vacuum-tube transmitter and an antenna?
• Greater harmonic suppression
• Higher efficiency
• Lower losses
• Greater transformation range

One of the most common issues with transmitting into a multi-band antenna system is the creation of harmonic distortion that can cause cross interference with the outgoing signal. Using some sort of filter network just prior to the last stage of the amplification process can suppress harmonics within that particular frequency transmission. Of these many different Filter Networks the Pi-L (π) network is one of the most effective methods to suppress the harmonics in the final stage.

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

Hint: 4 "C's" Circuit + Complex = Cancels + Changes

kg5kou

The term “impedance matching” is rather straightforward. It’s simply defined as the process of making one impedance look like another. Frequently, it becomes necessary to match a load impedance to the source or internal impedance of a driving source. It’s crucial that the reactive components cancel each other. An example is the delivery of maximum power to an antenna. Impedances in radio-frequency transmitters must be matched to pass maximum power from stage to stage. Most impedance include inductances and capacitance that must also be factored into the matching process. Antenna impedance must equal the transmitter output impedance to receive maximum power.

-KE0IPR

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

Chebyshev filters are analog or digital filters with a steep roll off at the edge of their passband and a ripple within the passband or stopband.

You can easily rule out two answers because op amps are active and LC filters are passive.

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

An elliptic filter (also known as a Cauer filter, named after Wilhelm Cauer, or as a Zolotarev filter, after Yegor Zolotarev) is a signal processing filter with equalized ripple (equiripple) behavior in both the passband and the stopband. The amount of ripple in each band is independently adjustable, and no other filter of equal order can have a faster transition in gain between the passband and the stopband, for the given values of ripple (whether the ripple is equalized or not). Alternatively, one may give up the ability to adjust independently the passband and stopband ripple, and instead design a filter which is maximally insensitive to component variations.

As the ripple in the stopband approaches zero, the filter becomes a type I Chebyshev filter. As the ripple in the passband approaches zero, the filter becomes a type II Chebyshev filter and finally, as both ripple values approach zero, the filter becomes a Butterworth filter. - K4AGO

https://en.wikipedia.org/wiki/Elliptic_filter

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What kind of filter would you use to attenuate an interfering carrier signal while receiving an SSB transmission?
• A band-pass filter
• A notch filter
• A Pi-network filter
• An all-pass filter

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Which of the following factors has the greatest effect in helping determine the bandwidth and response shape of a crystal ladder filter?
• The relative frequencies of the individual crystals
• The DC voltage applied to the quartz crystal
• The gain of the RF stage preceding the filter
• The amplitude of the signals passing through the filter

Very high quality IF filters, called crystal ladder filters, can be constructed by using serial arrays of crystals.

https://en.wikipedia.org/wiki/Crystal_filter

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What is a Jones filter as used as part of an HF receiver IF stage?
• An automatic notch filter
• A variable bandwidth crystal lattice filter
• A special filter that emphasizes image responses
• A filter that removes impulse noise

Think "Indiana Jones and the Kingdom of the Crystal Skulls"

The Jones Filter is a special type of crystal lattice filter that allows for a variable bandwidth.

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

A cavity filter is the best choice for use in a 2 meter repeater duplexer because it has a very high Q, can handle high power and is mostly stable to temperature changes. It provides a "steep" notch to only pass the band of interest with little loss.

The other answers given are worse choices because:

LC filters suffer from less than ideal L and C behaviors of their components.

Crystal filters typically cannot handle higher power.

DSP filters ________________ (need an explanation!)_______________

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Which of the following is the common name for a filter network which is equivalent to two L-networks connected back-to-back with the two inductors in series and the capacitors in shunt at the input and output?
• Pi-L
• Cascode
• Omega
• Pi

Since a Pi network contains two capacitors in shunt at the input and output, and a series inductor, two L networks connected back-to-back would create that if the two inductor value are added together to create one inductor.

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

Hint:

Pi-L, L for inductance. So, its a Pi with an inductor. (jmsian)

Hint: It's the only answer that has "Pi" in it, which comes from the loose suggestion it looks like the pi symbol.

kg5kou, km4ecu

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What is one advantage of a Pi-matching network over an L-matching network consisting of a single inductor and a single capacitor?
• The Q of Pi-networks can be varied depending on the component values chosen
• L-networks cannot perform impedance transformation
• Pi-networks have fewer components
• Pi-networks are designed for balanced input and output

Just two elements! That seems simple enough. Do we always use these L-networks when constructing lossless matching networks?

Nope. L-networks have two major drawbacks:

1. They are narrow-band.
2. Capacitors and inductors are difficult to make at the upper microwave frequencies!

The disadvantage of the L circuit - it can match loads equal or less than 50 Ohm. If the L circuit is reversed it can match loads equal or higher than 50 Ohm. It can not match on both sides. For example If the load is changing from 35 to 100 Ohms a reversed L network will match only from 50 up to 100 Ohms and will not match from 35 to 50 Ohms.

So the answer: The Q of Pi-networks can be varied depending on the component values chosen

-KE0IPR

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Which mode is most affected by non-linear phase response in a receiver IF filter?
• Meteor scatter
• Single-Sideband voice
• Digital
• Video

Many digital modulation methods depend on the phase shift of the baseband signal to convey information.

If you have a filter in a receiver that has a different phase shift depending on frequency, then the decoder/demodulator may have a difficult time extracting the original and meaningful phase shift.

Modulations that have quadrature components (QAM etc) are susceptible. FSK is more robust.

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

A crystal lattice filter is A filter with narrow bandwidth and steep skirts made using quartz crystals.

Note: There is a slight difference in layout between crystal lattices and ladders. There are pairs of crystals within lattice networks. Resonance modes are paired with each crystal in the lattice that facilitate an intended bandpass envelope (shape) to pass.

As far as the "skirt" jargon - When viewed graphically, some filter transitions are said to resemble one or both sides of a woman's skirt, so sharp transitions are known as steep skirts.

-KE0IPR

Silly test tip: I just remembered this as, “Crystal likes to wear steep skirts.” -N5MAJ

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