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Non-Linear-Transmission-Line (NLTL) circuits not only make excellent sampling strobe generators, they also make very good comb generators. The most commonly used semiconductor device for comb generation has historically been the Step Recovery Diode (SRD). While SRD’s have proven very useful in this application, they do have limitations, most prominently the introduction of undesired phase noise. SRD devices create a fast edge and generate harmonics by sweeping stored minority carriers from the depletion region during the transition from forward to reverse bias with large reverse recovery currents. As a result, SRDs are subject to recombination noise as well as shot noise. These processes add timing jitter to the output pulse, which manifests itself in the frequency domain as additional phase noise above the baseline 20logN increase of the ideal frequency multiplier. This can add as much as 6 dBc/Hz to the output phase noise. A New Breed of Comb Generators NLTL’s on the other hand, employ a completely different physical mechanism for frequency multiplication, more closely related to Schottky varactor based frequency multipliers. Being basically passive (albeit non-linear) majority carrier devices, they do not suffer from recombination and shot noise, and consequently have much better phase noise performance.
NLTL based comb generators also offer several other advantages over narrow-band tuned SRD-based comb generators. Typically, SRD-based comb generators have less than a 5% input frequency range and are available only for specific frequencies. Since NLTL lines are inherently broadband, they can accommodate well over an octave of input frequency range in a single device.
Late spring of 2005 PSPL introduced a new line of comb generators based on NLTL circuits. These comb generators cover inputs ranging from 80MHz to 2GHz and outputs to 50GHz. The output harmonics have been measured for one of the 100-200MHz models and is shown above.
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