In the operation of electronic systems and circuits, the basic function of a filter is to selectively pass, by frequency, desired signals and to suppress undesired signals. The amount of insertion loss and phase shift encountered by a signal passing through the filter is a function of the filter design. Similarly, the amount of rejection of an undesired signal is a function of the filter design.

Mini-Circuits' filters are passive, they contain inductors and capacitors. Three types of filters, low-pass, high-pass, and band-pass are available. Two different low-pass filters have been designed to (1) provide high rejection of undesired signals very close to the pass-band and (2) to provide a linear phase versus frequency characteristic across the pass-band frequency range. A linear phase characteristic is essential when passing a pulse waveform in order to preserve the pulse shape and avoid a distorted waveform. High-pass filters have similarly been designed to provide high rejection of undesired signals very close to the pass-band. Constant impedance band-pass filters have been designed to allow signals to pass within the passband and to be rejected outside of this band. However, these filters provide a matched 50-ohm impedance both within and outside the passband. This is a very important characteristic especially when intermodulation distortion and non-linear devices, such as mixers and oscillators are to be considered. Refer to the article "Constant Impedance IF Bandpass Filters Improve Performance" for more details.

Mini-Circuits' filters are available in a variety of packages and connector styles. Pin plug-in and surface-mount packages are available to accommodate both commercial and military applications. In addition to the catalog models described on the specification sheets, Mini-Circuits designs and manufactures filters to specific customer requirements. Consult our Applications department for your specific needs.

The basic filter designs offered by Mini-Circuits utilizes a modified butterworth or "maximally flat" design, a modified Bessel-Thomson or flat delay design, and an Elliptic function design. All filters are specified by their amplitude response, both in the passband and reject-band, also by their VSWR and phase characteristic where applicable. For convenience, the 20 dB and 40 dB reject-bands are specified. The cut-off frequency, fco, of each filter is also given. This frequency corresponds to the 3 dB insertion loss point of the filter response. This frequency point easily allows the filter response to be normalized to fco.

The CAPD data given in the handbook for each filter has a significant amount of data points to clearly describe the filter performance characteristic. The data curves show the overall filter performance curves. Any resonances that would be available can easily be observed. The data and curves present a very accurate description of each filter and may be used in conjunction with various software programs designed to analyze system performance.

When the bandwidth, f

where

Mini-Circuits' PBLP filter models utilize a Bessel-Thomson design to achieve the linear phase characteristic or flat time delay. This enables the transmission of various frequency components contained in a pulse waveform to be delayed by the same amount while traveling through the filter thus preserving the pulse wave shape.

**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 50-ohm filter in a 75-ohm 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 in this Handbook 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 passbands 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 percent bandwidth at 1 dB insertion loss.

**Q. How can I calculate a filter's rejection in the stopband? **** **

**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 50 ohm 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 fundamental
frequency output signal.

On the other hand, the low-pass filter series offered 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 passband 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 passband for an insertion loss of 2 dB or less. If
the passband 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 passband
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 passband 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 passband 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 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 bandpass 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 bandpass filter. Therefore, it would cost much less than a bandpass filter. And (2) for the same
rejection at 350 MHz, the low pass filter would inherently have less dissipative loss in the desired
passband. This is especially important because of the low noise requirement.