Q. Is a filter a reciprocal device? Is it possible to interchange between input and output ports?

A. Yes. Filter response is unaffected as long as the source and load impedances are the specified value.

Q. I intend to use an MCL 50Ω filter in a 75Ω system. What might be the consequences?

A. The impedance mismatch will change the response. Pass-band ripple, stop-band rejection, and VSWR will be affected to a degree which depends upon the particular filter design.

Q. Does MCL offer surface-mount filters?

A. We offer surface-mount filters and are continually developing new filters to meet customer requirements. If your requirements are not met by the filters listed, please call our applications engineers.

Q. How does temperature influence filter performance?

A. Our filters are designed to operate from -55 to 100°C and meet our specifications over this full temperature range. There may be slight variations from room temperature performance.

Q. Can I cascade a low-pass filter with a high-pass filter to achieve band-pass performance’?

A. Yes, if their pass-bands overlap enough to avoid interaction of their skirt responses, which could increase the combined insertion loss. As a guide, select the two filters so that the resulting bandpass has at least 5% bandwidth at 1 dB insertion loss.

Q. How can I calculate a filter’s rejection in the stop-band?

A. Refer to the CAPD page in this handbook which shows data or a graph for a filter in the same series as the filter you need. If the cutoffs are not the same, scale all frequencies in the CAPD by the ratio of the cutoff frequency of your filter to the cutoff frequency of the filters whose data are listed.

Q. I have a 130 MHz crystal oscillator and I would like to reduce the harmonics at its output. Which 50Ω filter series would you recommend?

A. In order to avoid frequency pulling of the oscillator and the generation of internal products, it is recommended that a matched broad-band impedance be connected to the oscillator output. Although, Mini-Circuits’ constant impedance filters would satisfy this criteria, they would not provide sufficient selectivity to substantially reduce the harmonics of the oscillator’s fundamental frequency output signal.

On the other hand, the low-pass filter series that Mini-Circuits offers, would provide excellent selectivity and would substantially reduce the harmonics of the fundamental. The problem, however, is that the filter impedance is highly reflective at the harmonic frequencies. This will cause the harmonic signals to be reflected back into the oscillator. The solution is to insert at least a 6 dB attenuator (12 dB return loss) between the oscillator and filter. The fixed attenuator section of this handbook offers a wide choice for your selection.

Q. I have a need to increase the selectivity performance of my system. I had chosen a PLP-100 low-pass filter. However, I now need 30 dB of attenuation at 146 MHz rather than the 20 dB specified. How should I proceed?

A. First determine your pass-band requirements, that is, the maximum attenuation allowed within your band width required. Let us assume your maximum allowable pass-band insertion loss is 2 dB. Then referring to the data pages for low-pass filters, choose the filter model that provides 30 dB attenuation at 146 MHz and has the widest pass-band for an insertion loss of 2 dB or less. If the pass-band is too narrow for the filter chosen, then you can use and alternative selection process. Reconfigure your system to use two low-pass filters. The insertion loss in the pass-band of the combination will be additive. Therefore, two PLP-100 low-pass filters will provide less than 2 dB of attenuation and can provide 40 dB of insertion loss at 146 MHz. If less pass-band insertion loss is desired, then the second low-pass filter can be chosen, such that the attenuation at 146 MHz is at least 10 dB. Please keep in mind that the low-pass filter VSWR outside the pass-band is highly reflective. Therefore, each of the two filters should be embedded in the system so that each one sees a good match. This ensures that the rejections are additive.

Q. My system requires a low-noise, 10 dB amplifier at 300 MHz with a bandwidth of ±10 MHz. The amplifier I have available has a frequency response from 200 to 400 MHz. I am very concerned about amplifying an undesired signal at 350 MHz. Should I select a band-pass filter or a low-pass filter to provide rejection at 350 MHz?

A. If there are no undesired signals between 200 to 290 MHz then a low-pass filter is recommended. The reasons are: (1) The low-pass filter has half as many reactive components as the band-pass filter. Therefore, it would cost much less than a band-pass filter. And (2) for the same rejection at 350 MHz, the low-pass filter would inherently have less dissipative loss in the desired pass-band. This is especially important because of the low-noise requirement.

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