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Subelement E4
AMATEUR PRACTICES
Section E4D
Receiver performance characteristics: blocking dynamic range; intermodulation and cross-modulation interference; 3rd order intercept; desensitization; preselector
What is meant by the blocking dynamic range of a receiver?
  • Correct Answer
    The difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression
  • The minimum difference in dB between the levels of two FM signals which will cause one signal to block the other
  • The difference in dB between the noise floor and the third order intercept point
  • The minimum difference in dB between two signals which produce third order intermodulation products greater than the noise floor

From "Radio Receiver Design" by Robert C. Dixon, 1998 (Marcel Drekker, Inc., New York; ISBN 0-8247-0161-5) p. 370, section 14.3, states

"Blocking dynamic range (BDR) is the difference, in dB, between the noise floor and a off-channel signal that causes 1 dB of gain compression in the receiver."

Dynamic blocking range is the difference between the weakest signal that can be perceived and the strongest signal that can be present without adversely effecting a weak signal. This “adverse effect” is about 1 dB of attenuation since this is the smallest change that can be heard by the human ear.

drichmond60

Hint: The difference between the noise floor and the incoming signal that is enough to recognize the intelligence in the signal.

-KE0IPR

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Which of the following describes two problems caused by poor dynamic range in a communications receiver?
  • Correct Answer
    Cross-modulation of the desired signal and desensitization from strong adjacent signals
  • Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband
  • Cross-modulation of the desired signal and insufficient audio power to operate the speaker
  • Oscillator instability and severe audio distortion of all but the strongest received signals

Memory aid: the two problems are cross modulation and desensitization. (KG5KOU)

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How can intermodulation interference between two repeaters occur?
  • When the repeaters are in close proximity and the signals cause feedback in the final amplifier of one or both transmitters
  • Correct Answer
    When the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters
  • When the signals from the transmitters are reflected out of phase from airplanes passing overhead
  • When the signals from the transmitters are reflected in phase from airplanes passing overhead

Mixing of signals to produce intermodulation products will occur in any non-linear electrical circuit. The devices in RF output stages are often run in non-linear modes for better efficiency. This means that they can also act as mixers, signals from other nearby transmitters will travel down the antenna feeds and mix with the signal being transmitted. One way to prevent this is to install circulators. These devices allow the transmitter output to reach the antenna but block signals traveling down the feedline from the antenna.


Hint: mix is in the correct answer.

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Which of the following may reduce or eliminate intermodulation interference in a repeater caused by another transmitter operating in close proximity?
  • A band-pass filter in the feed line between the transmitter and receiver
  • Correct Answer
    A properly terminated circulator at the output of the transmitter
  • A Class C final amplifier
  • A Class D final amplifier

Installing a "Circulator" will reduce or remove intermodulation interference .

The devices in RF output stages are often run in non-linear modes for better efficiency. This means that they can also act as mixers, signals from other nearby transmitters will travel down the antenna feeds and mix with the signal being transmitted. One way to prevent this is to install circulators. These devices allow the transmitter output to reach the antenna but block signals traveling down the feedline from the antenna.

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What transmitter frequencies would cause an intermodulation-product signal in a receiver tuned to 146.70 MHz when a nearby station transmits on 146.52 MHz?
  • Correct Answer
    146.34 MHz and 146.61 MHz
  • 146.88 MHz and 146.34 MHz
  • 146.10 MHz and 147.30 MHz
  • 173.35 MHz and 139.40 MHz

The intermodulation-product signal is \(f_{mod}=146.70\:\text{MHz}\). One of the modulation signals is \(f_1=146.52\:\text{MHz}\). What is the other modulation signal?

The modulation equations are \(f_{mod}=2f_1-f_2\) and \(f_{mod}=2f_2-f_1\). Solve the modulation equations for \(f_2\).

Solving the equation \(f_{mod}=2f_1-f_2\) for \(f_2\) yields: \(f_2=2f_1-f_{mod}\) and solving the equation \(f_{mod} = 2f_2-f_1\) for \(f_2\) yields: \(f_2 = \frac{f_1 + f_{mod}}{2}\).

There are many possibilities for the intermodulation (IM) products resulting from mixing in the output stages of adjacent transmitters and each one is represented by a different formula. The lower odd order products are often (not always) the strongest. Here are the formulae for third order products generated by two adjacent transmitters at frequencies \(A\) and \(B\):

  1. \(2A+B\)
  2. \(2A-B\)
  3. \(A+2B\)
  4. \(2B-A\)

We can immediately eliminate products due to 1) and 3) since they will give products far away from the received IM frequency specified in this question. Let us plug the numbers into 2) and 4) and set the IM result to \(146.70\:\text{MHz}\) as detected by the receiver. Assume that the known transmitter is on frequency \(A\) and that the “other” transmitter is on \(B\).

Using formula 2) we have \(2 \times 146.52 - B = 146.70 \:\text{MHz}\)

Rearrange to get \(B = -146.70 + (2 \times 146.52) = 146.34\:\text{MHz}\).

Using formula 4) we have \(2 \times B - 146.52 = 146.70 \:\text{MHz}\). Rearrange to get \(B = \frac{146.70+146.52}{2} = 146.61 \:\text{MHz}\).

This gives the frequencies as in the correct answer ( \(146.34\:\text{MHz}\) and \(146.61 \:\text{MHz}\))


Hint: The sum of the two digits past the decimal equals 7 in the clue and correct answer.

  • \(146.70\) -> \(7+0=7\)
  • \(146.52\) -> \(5+2=7\)
  • \(146.34\) -> \(3+4=7\)
  • \(146.61\) -> \(6+1=7\)

Another hint: The midpoint between the two given frequencies is 146.61. It appears on only one answer.

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What is the term for unwanted signals generated by the mixing of two or more signals?
  • Amplifier desensitization
  • Neutralization
  • Adjacent channel interference
  • Correct Answer
    Intermodulation interference

Intermodulation or intermodulation interference is the amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities. The intermodulation between each frequency component will form additional signals at frequencies that are not just at harmonic frequencies of either, but also at the sum and difference frequencies of the original frequencies and at multiples of those sum and difference frequencies.

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Which describes the most significant effect of an off-frequency signal when it is causing cross-modulation interference to a desired signal?
  • A large increase in background noise
  • A reduction in apparent signal strength
  • The desired signal can no longer be heard
  • Correct Answer
    The off-frequency unwanted signal is heard in addition to the desired signal

Remember: off-frequency is in the question and the answer

A large increase in background noise and a reduction in apparent signal strength are both minor effects, but they do happen.

Hearing both transmissions is annoying, but sadly does happen if the off-frequency signal is powerful enough/is using a large bandwidth. This is the most significant effect.

The desired signal can no longer be heard: an off-frequency signal will not cause you to not hear the desired signal, but it can make it harder to understand.

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What causes intermodulation in an electronic circuit?
  • Too little gain
  • Lack of neutralization
  • Correct Answer
    Nonlinear circuits or devices
  • Positive feedback

According to Wikipedia: "Intermodulation is caused by non-linear behaviour of the signal processing being used." http://en.wikipedia.org/wiki/Intermodulation

Memorization: The question and the answer has the words Circuit and Circuits mentioned.

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What is the purpose of the preselector in a communications receiver?
  • To store often-used frequencies
  • To provide a range of AGC time constants
  • Correct Answer
    To increase rejection of unwanted signals
  • To allow selection of the optimum RF amplifier device

There are some cases where a preselector might help.

To increase rejection of unwanted signals would be one case of the use of a preselector in communications receiver.

-KE0IPR

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What does a third-order intercept level of 40 dBm mean with respect to receiver performance?
  • Signals less than 40 dBm will not generate audible third-order intermodulation products
  • The receiver can tolerate signals up to 40 dB above the noise floor without producing third-order intermodulation products
  • Correct Answer
    A pair of 40 dBm signals will theoretically generate a third-order intermodulation product with the same level as the input signals
  • A pair of 1 mW input signals will produce a third-order intermodulation product which is 40 dB stronger than the input signal

The third order intercept point (IP3) is one measure of the intermodulation (IM) performance of a receiver. It is a measure of the tolerance of the receiver to strong signals outside the passband. (drichmond60)


Hint: only the correct answer contains the word "theoretically" (KG5KOU)

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Why are third-order intermodulation products created within a receiver of particular interest compared to other products?
  • Correct Answer
    The third-order product of two signals which are in the band of interest is also likely to be within the band
  • The third-order intercept is much higher than other orders
  • Third-order products are an indication of poor image rejection
  • Third-order intermodulation produces three products for every input signal within the band of interest

A third-order product of two signals may be within the band of the receiver.

NOTE from team: This explanation could use more detail, please add some if you have it =]

Silly Hint: The most "likely" answer is the only option with the word "likely" in it :-]

See also: http://en.wikipedia.org/wiki/Third-order_intercept_point

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What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?
  • Correct Answer
    Desensitization
  • Quieting
  • Cross-modulation interference
  • Squelch gain rollback

The term for the reduction in receiver sensitivity caused by a strong signal near the received frequency is desensitization.

This is caused by RF energy from a strong signal near the receive frequency entering pass-band of the receiver. One way to reduce this is to utilize additional filtering to eliminate signals outside of the receiver's desired bandwidth.

In the case of a repeater, the receiver must be adequately isolated from its transmitter, if normal receiver performance is to be expected. Isolation (dB) is required to protect the receiver from transmitter spurious and noise radiation and receiver desensitization.


Tip: Reduction in sensitivity --> desensitization

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Which of the following can cause receiver desensitization?
  • Audio gain adjusted too low
  • Correct Answer
    Strong adjacent channel signals
  • Audio bias adjusted too high
  • Squelch gain misadjusted

An input signal can be strong enough that the receiver no longer responds linearly and its gain begins to drop. This reduction in gain due to the strong signal causes weaker signals to appear to fade. This reduction in gain is called gain compression or blocking. Blocking may be observed as desensitization or desense — the reduction in apparent strength of a desired signal caused by a nearby strong interfering signal.

ARRL Inc. (2015-06-08). The ARRL Extra Class License Manual (Kindle Locations 6209-6212). ARRL, the national association for Amateur Radio. Kindle Edition.

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Which of the following is a way to reduce the likelihood of receiver desensitization?
  • Correct Answer
    Decrease the RF bandwidth of the receiver
  • Raise the receiver IF frequency
  • Increase the receiver front end gain
  • Switch from fast AGC to slow AGC

Desensitization occurs when a receiver is unable to receive a weak signal because a stronger, unwanted signal at a nearby frequency is present. By DECREASING the RF bandwidth the unwanted signal is filtered out. Reduce= decrease WP4AES

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