I think it is Frequency Modulation with Phase modulation which it the most bandwidth efficient

BPSK=>

1.The BPSK stands for “Binary Phase-shift keying”.

2.DPSK is a not a method of BPSK, where there is no reference phase signal.

3.amplitude shift keying

4.lesser bandwidth more probability error

DPSK=>

1.The DPSK stands for “Differential phase-shift keying”.

2.DPSK is a method of BPSK, where there is no reference phase signal.

3.It is one type of phase modulation used to transmit data by altering the carrier wave’s phase.

4.greater bandwidth probability error less

It means the Phase anlge is not matching with control board phase angle or more capacitor connected in the furnace line. we have to Remove some tank capacitor.

Bandwidth does not change with frequency. Bandwidth defines (part of) how the response of a circuit changes with frequency. Other things that define how the response of a circuit changes with frequency are: phase shift, roll-off rate, linearity of the passband, etc. but bandwidth ignores these.

The advantage of a binary phase-shift keying is that within a given bandwidth, modulation of higher orders allow to carry higher rates.