The invention is a circuit for eliminating or reducing unwanted components from an optical output signal. The circuit employs a photodetector for converting a portion of the output signal to an electrical signal, and a feedback loop which passes only the second harmonic band or the frequency difference band for combining with the input biasing signal.

A broadband microwave recursive filter that provides sharp transitions in the frequency domain between adjacent stop and passbands comprising a signal input node; a signal output node; a filter circuit connected between the signal input node and the signal output node for providing a signal flow therebetween which has a predetermined frequency bandwidth characteristic; a microwave transistor circuit, with the microwave transistor circuit being band-limited to provide gain in only a restricted window of frequencies within the predetermined frequency bandwidth and connected for providing amplification to signals flowing in the filter circuit between the signal input node and the signal output node while suppressing out-of-window signals resulting from design approximations. The filter circuit includes a plurality of distributed feedback loop circuits, with each of the feedback loop circuits including the microwave transistor circuit therein, and wherein each of the feedback loop circuits has a different electrical length in relation to the other of the plurality of feedback loop circuits. Finally, the present filter design includes a passive filter connected in common to all of the plurality of distributed feedback loop circuits in the filter circuit for circuit for providing filter zeros on both sides of the restricted window of frequencies. In a preferred embodiment, the microwave transistor circuit includes an FET which is impedance-matched to obtain an approximately flat gain response across the window. It is preferred that the plurality of feedback loops all have amplitude weighting factors of the same sign.

A plurality of stages are connected in series for storing graphic pattern data. In order to repeatedly use the graphic pattern data previously stored in any stage, the data of the lowermost stage is fed back to an upstream stage via a feedback bus. Each stage includes a data selector and a stage register. The data selector is switched by a write pointer and a feedback pointer to select the data to be stored in the stage register. Hence, regardless of different lengths of graphic patterns, a mapping of the graphic patterns can be carried out uniformly at a high speed.

Process for the production of filters whose active filtering part is made up of a folded filter material web, preferably made from paper, which folded web is delimited in the process by side edges at which the folds terminate, and end edges at which a first or last fold is located, so that in this way filter units (49) are formed. The filter units (49) are produced in an automatized process in which the filter material web is creased and pushed together to form the folds. The spacings between the folds are maintained constant by means of a screw element with threads pushing down between the folds and forming spacing elements between these. The screw element is rotataed during forward movement of the folded web. Glue-coated strips are pressed onto the said side edges of the web, so that the ends of the folds are covered over by the strips. The blank, which is thus continuously produced and which comprises the folded material web and the strips glued onto its side edges, is cut off to form the filter units (49). Several such filter units (49) are assembled in a casing (69) to form a filter having a filter surface which corresponds to the combined surface of the units included.