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Subel. E4
14 questions
Subelement E4

AMATEUR PRACTICES

Section E4D

Receiver performance characteristics: dynamic range; intermodulation and cross-modulation interference; third-order intercept; desensitization; preselector; sensitivity; link margin

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 that will cause 1 dB of gain compression
  • The minimum difference in dB between the levels of two FM signals that 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.

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

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Which of the following describes problems caused by poor dynamic range in a receiver?

  • Correct Answer
    Spurious signals caused by cross modulation and desensitization from strong adjacent signals
  • Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband
  • Poor weak signal reception caused by insufficient local oscillator injection
  • Oscillator instability and severe audio distortion of all but the strongest received signals

Dynamic range means the amplitude range of input signals that can be properly processed by the receiver. Insufficient dynamic range for strong and weak signals means the strong signals are no longer treated linearly, creating modulation products that spill over into the desired signals.

Silly Hint: Spurious is BAD.

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What creates intermodulation interference between two repeaters in close proximity?

  • The output signals cause feedback in the final amplifier of one or both transmitters
  • Correct Answer
    The output signals mix in the final amplifier of one or both transmitters
  • The input frequencies are harmonically related
  • The output frequencies are harmonically related

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 is used to reduce or eliminate intermodulation interference in a repeater caused by a nearby transmitter?

  • A band-pass filter in the feed line between the transmitter and receiver
  • Correct Answer
    A properly terminated circulator at the output of the repeater’s transmitter
  • Utilizing a Class C final amplifier
  • Utilizing a Class D final amplifier

Installing a "Circulator" may reduce or remove intermodulation interference. When one port of a three-port circulator is terminated in a matched load, it acts as an isolator since a signal can travel in only one direction between the remaining ports.

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 insert a circulator configured as an isolator to block signals traveling down the feedline from the antenna from entering the RF output stages.

Hint: the answer is the only one that contains the word repeater

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What transmitter frequencies would create 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
  • 146.30 MHz and 146.90 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 the reduction in receiver sensitivity caused by a strong signal near the received frequency?

  • Reciprocal mixing
  • Quieting
  • Correct Answer
    Desensitization
  • Cross modulation interference

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 reduces the likelihood of receiver desensitization?

  • Correct Answer
    Insert attenuation before the first RF stage
  • Raise the receiver’s IF frequency
  • Increase the receiver’s 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

SILLY HINT: Question has "desensitization" which may be seen as to reduce. Answer has "Decrease" which is also to reduce.

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What causes intermodulation in an electronic circuit?

  • Negative feedback
  • 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 frequencies that are often used
  • To provide broadband attenuation before the first RF stage to prevent intermodulation
  • Correct Answer
    To increase the rejection of signals outside the band being received
  • 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.

Hint: preselect rhymes with reject.

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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 input signals will theoretically generate a third-order intermodulation product that has the same output amplitude as either of the input signals
  • A pair of 1 mW input signals will produce a third-order intermodulation product that 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 odd-order intermodulation products, created within a receiver, of particular interest compared to other products?

  • Correct Answer
    Odd-order products of two signals in the band being received are also likely to be within the band
  • Odd-order products are more likely to overload the IF filters
  • Odd-order products are an indication of poor image rejection
  • Odd-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.

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 link margin in a system with a transmit power level of 10 W (+40 dBm), a system antenna gain of 10 dBi, a cable loss of 3 dB, a path loss of 136 dB, a receiver minimum discernable signal of -103 dBm, and a required signal-to-noise ratio of 6 dB?

  • -8dB
  • -14dB
  • Correct Answer
    +8dB
  • +14dB
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What is the received signal level with a transmit power of 10 W (+40 dBm), a transmit antenna gain of 6 dBi, a receive antenna gain of 3 dBi, and a path loss of 100 dB?

  • Correct Answer
    -51 dBm
  • -54 dBm
  • -57 dBm
  • -60 dBm
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What power level does a receiver minimum discernible signal of -100 dBm represent?

  • 100 microwatts
  • 0.1 microwatt
  • 0.001 microwatts
  • Correct Answer
    0.1 picowatts
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