Q. How does the return loss at the output of a frequency doubler vary as input power changes?

A. Return loss is influenced by how hard the diodes are driven in their conducting state. As input power to the frequency doubler is reduced, the impedance will increase. It is possible, as it increases, to notice an important decrease in VSWR or return loss; as input power is further reduced, return loss will degrade. Thus return loss is a function of the input signal level fed to the frequency doubler.

Q. What is meant by the conversion loss of a frequency doubler?

A. The conversion loss is the difference in power at the output of the frequency doubler (the second harmonic) compared to the power applied to the input (fundamental) in dB. The conversion loss will vary as a function of the power and frequency applied to the input of the doubler.

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Q. What is the spectrum at the output of a frequency doubler?

A. Since the frequency doubler operates in a non-linear state, its output contains harmonics of the applied fundamental signal. The amount of odd-order harmonics is a function of the symmetry of the diodes when conducting at each half cycle of the input signal. Basically, all harmonics, including the fundamental, will appear at the output. Each even-order harmonic will appear approximately 12 dB lower than its previous even-order harmonic; for example, the 6th harmonic is about 12 dB lower than the 4th.

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Q. Can I use a frequency doubler as a quadrupler?

A. Yes, by making use of the fourth harmonic rather than second at the output. There are two disadvantages to consider, however. First, the fourth harmonic output level is 12 dB lower than the second. Also, the harmonics closest to the fourth (the third and fifth) must be reduced, this could complicate filter requirements.

Q. Suppose I need a tripler. Can I somehow use a frequency doubler?

A. Use a different type of frequency multiplier, such as a comb generator or a Class-C amplifier.

Q. Will there be a change in the amount of harmonics generated by the frequence doubler as input power is varied?

A. No. The harmonics will remain the same but the power level associated with each harmonic will change as input level is varied.

Q. What happens to the output of a frequency doubler when it is driven too hard?

A. Think of a frequency doubler as a mixer with the same frequency signal applied to both the RF and IF ports; sum and difference frequency signals will appear at the output. When excessive signal level is applied, the frequency doubler will operate in heavy saturation, and higher-order harmonics will be greater in amplitude relative to the desired second-harmonic. In addition, the second harmonic output will not increase in linear proportion to the input so that effective conversion loss would be higher. Make sure the input power to the frequency doubler is within the power range specified for optimal performance.

Q. Can the input waveform be other than a sine wave?

A. Yes, a frequency doubler will operate with various input wave shapes. However, a square wave would probably provide higher efficiency since it would drive the device harder into its conducting and non-conducting state.

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