Wideband traveling wave tubes (TWTs) can fall short of an application’s power requirements due to low gain and low RF output power at the lowest frequencies. For example, an ultra-wideband TWT may have 60 dB gain at its mid-band frequencies, but only 10-20 dB at the low end. Fortunately, harmonic injection can be used to produce a new generation of ultra-wideband microwave power modules (MPMs) with twice the low-end RF output power of standalone ultra-wideband TWTs.
Using harmonic injection to improve a TWT’s output power is not new; design engineers have employed these techniques since the mid-1970s. However, using harmonic injection on an ultra-wideband TWT is a relatively new concept. In this case, the objective is to cancel the naturally-generated second (and/or third) harmonic within the TWT, and thus recover the available beam power back into the low-end fundamental frequencies. This can be achieved by driving the TWT input with an additional signal that is two times that of the fundamental drive frequency. The doubled signal is injected into the TWT at a preset amplitude and phase (relative to the fundamental signal) so that it cancels with the naturally produced second or third harmonic generated by the TWT.
In his technical paper, dB Control Senior Engineer Dave Pfaff offers some practical advice for harmonic injection. Read his paper here.