Unit 4. Receivers

Question 1

4–1. Receiver Fundamentals

15. Identifying basic receiver characteristics
16. Why is it necessary for the RF amplifier section to isolate the local oscillator from the antenna?

Answer 1

If we connect the antenna directly to the mixer (frequency converter stage), a part of the oscillator signal might be radiated into space. This could cause interference for other receivers.

Question 2

2. What is the difference between a mixer circuit and a converter circuit?

Answer 2

The mixer requires two input signals–the radio frequency and injection frequency. A converter has its own self-contained oscillator, and thus requires only one input signal—the radio frequency.

Question 3

3. Define “image frequency.”

Answer 3

It is an undesired signal when mixed with the oscillator’s frequency, produces the correct IF.

Question 4

16. Extracting and processing audio

1. Define demodulation.

Answer 4

The process of recovering intelligence from the modulated wave.

Question 5

2. Briefly state the requirements for any type of demodulation.

Answer 5

The demodulator input circuit must be sensitive to the modulation characteristics. Nonlinearity must be present to cause heterodyning. An RF carrier signal must be present. Filtering is required to select the desired intelligence signal and reject the unwanted carrier.

Question 6

3. What are the main drawbacks of using diode detectors?

Answer 6

Their main drawbacks are they provide no power gain and selectivity is affected by loading of the tank.

Question 7

4. What factor determines the amount of amplification needed in the audio amplifier circuit?

Answer 7

The amount of amplification necessary depends upon the type of output device used. If the output device consists of earphones, only one stage of amplification may be necessary, if a large speaker is used, much more amplification may be necessary.

Question 8

4–2. Very High Frequency/Ultrahigh Frequency Receivers

17. Capabilities and limitations of the AN/GRR–23/24
18. How many different channels can the AN/GRR–23 be tuned to if using the 25 kHz IF crystal filter? When using the 50 kHz IF crystal filter?

Answer 8

1,360; 680.

Question 9

2. What is the sensitivity of the AN/GRR–23/24 with a 3.0 μV signal modulated 30% ±5% at 1 kHz applied to antenna input from a 50-ohm source?

Answer 9

100 mW into a 600-ohm load with 10 dB signal-plus-noise-to-noise ratio.

Question 10

3. The AGC limits output decibel variations to what level when a 6 μV signal modulated 30% ±5% is increased to 1 V?

Answer 10

3dB.

Question 11

18. Theory of operations

1. Which module in the AN/GRR–23 and AN/GRR–24 performs the function of pre-selection?

Answer 11

The tunable filter, FL2.

Question 12

2. Which module determines overall receiver selectivity?

Answer 12

Crystal filter FL1.

Question 13

3. What happens to the DC portion of the detector output?

Answer 13

It is amplified and processed into the AGC voltage.

Question 14

4. In the UHF mixer/multiplier A2, to what frequency would Q1 be tuned if the oscillator synthesizer’s output were 67.0 MHz?

Answer 14

Q1 would be tuned to the fourth harmonic—268 MHz.

Question 15

5. In the UHF mixer/multiplier A2, what frequency will the output of transistor Q8 pass with an operating frequency of 321 MHz?

Answer 15

Regardless of the operating frequency, the output of transistor Q8 in the A2 module always passes the IF which is 20.6 MHz.

Question 16

6. In what section does most of the receiver’s amplification take place?

Answer 16

The IF section.

Question 17

7. What is the purpose of the receiver’s AGC circuit?

Answer 17

The AGC circuit provides a constant audio output with a varying RF signal strength.

Question 18

8. What is the purpose of the receiver’s squelch circuit?

Answer 18

To pass the audio if the RF at the antenna is of sufficient strength and block the audio output if the RF at the antenna is too weak.