Question E2E04:
Which of the following is characteristic of the FST4 mode?  

Choices:  
A: Four-tone Gaussian frequency shift keying  
B: Variable transmit/receive periods  
C: Seven different tone spacings  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

Correct answer: D

Explanation:
- A: Four-tone Gaussian frequency shift keying — FST4 uses 4-tone GFSK (4-GFSK). The Gaussian filter smooths symbol transitions to reduce keying sidebands and narrow the occupied bandwidth, which is helpful for LF/MF weak-signal work.
- B: Variable transmit/receive periods — FST4 offers selectable T/R periods (from short to several minutes), trading data rate for sensitivity and Doppler/frequency-drift tolerance.
- C: Seven different tone spacings — FST4 provides seven selectable tone spacings so you can match bandwidth and frequency stability to band conditions and station capability.
- Therefore, D (All these choices are correct) is correct.

Why the other choices are “incorrect”:
- In this question, none of A, B, or C is incorrect; each accurately describes a feature of FST4, which is why D is the right choice.

Useful formulas and rules of thumb:
- Bandwidth of (C)PFSK (rule of thumb, Carson-like):
  $$B \approx 2(\Delta f + R_s)$$
  where $\Delta f$ is tone spacing (Hz) and $R_s$ is symbol rate (baud). For 4-FSK, this is still a reasonable approximation; Gaussian filtering further reduces sidelobes.

- Orthogonality guideline for M-FSK:
  $$\Delta f \cdot T_s \approx 1$$
  where $T_s = 1/R_s$ is symbol duration. Narrower $\Delta f$ generally implies longer $T_s$.

- Frequency stability guideline:
  $$|\delta f| \lesssim 0.1\,\Delta f$$
  Keep frequency error well below tone spacing for reliable decodes at narrow spacings.

- Processing gain from longer T/R (coherent integration):
  $$G_{\text{proc}} \approx 10\log_{10}\!\left(\frac{T_2}{T_1}\right)\,\text{dB}$$
  Doubling integration time gives about +3 dB sensitivity, which is why longer FST4 periods can decode weaker signals.