Minimal Distortion Video Bandpass Filter

The present invention improves prior art filters by combining a pulse shaping element with an M-derived filter, a notch filter provided with damping at the cut-off frequency, and a d.c. blocking circuit to form a video bandpass filter which transforms a pulse input into a smooth sinusoidal output minimally affected by noise, distortion, and other spurious signals and harmonics.

The invention significantly reduces ringing in the circuit and provides damping which prevents over-notching at the cut-off frequency. In accordance with the present circuit, signal response at cut-off (e.g., 20 MHz) is attenuated, but enough signal is permitted to pass to preserve the response characteristics at the desired frequency (e.g., 10 MHz).

FIG. 1 illustrates the circuitry of the invention.

FIG. 2 is a graph comparing the response of the present bandpass filter (shown with solid line) and a prior art filter (shown with dashed line).

Referring to FIG. 1, a four-stage video bandpass filter 2 is shown. Input 4 to the filter 2 is rounded off, or smoothed, in a first stage comprised of a resistance R.sub.1, connected to ground. From the first stage, the signal enters an M-derived filter stage 6 comprised of two identical, in-series inductances L.sub.1 and L.sub.2 having a grounded LC series circuit, comprised of inductance L.sub.3 and capacitor C.sub.1, between inductances L.sub.1 and L.sub.2. The output of L.sub.2, which corresponds to the output of the M-derived filter stage 6, enters a series tuned notch filter stage 8. The notch filter stage 8 includes an RLC circuit to ground wherein a resistance R.sub.2 is in parallel to an inductance L.sub.4, both being in series with a capacitor C.sub.2. To block out d.c. portions and prevent them from affecting the sine wave output, a final stage comprising a capacitor C.sub.3 in series with an inductance L.sub.5 which feeds an output load R.sub.L is provided.

The frequency response achieved by combining the four stages and properly selecting the values of the components is much improved over filters of the prior art as shown in FIG. 2. For example, the filter 2 can be designed to pass frequencies at 10 MHz with a bandwidth of at least 10 MHz with some, but not much, response at 20 MHz and nearly no response at 30 MHz (at 30 MHz, the video signal would be contaminated with ringing and the like). Of particular significance in accomplishing these ends is resistance R.sub.2 which "smooths" the notching in stage three and serves to lower the Q of the second and third stages.

In essense, the stages act together to create a single desired output response. The first stage as shown in FIG. 2 smooths the input for subsequent filtering; stage two attenuates the 30 MHz portion of the signal as much as possible; stage three tunes to 20 MHz causing the 20 MHz trap not to notch too much, permitting a small response at 20 MHz; and stage four series tunes the filter to 10 MHz and blocks out d.c. signal components.

FIG. 2 shows the response of a prior art filter (in dashed line) and the present invention (in solid line) respectively. The response at 10 MHz remains essentially unaltered with greater attenuation at 20 MHz and 30 MHz with the present circuit.

Various other modifications, adaptations and alterations are of course possible in light of the above teachings. Therefore, it should be understood at this time that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.