Low pass filter (RF and microwave)

01

Set the passband, cutoff and stopband

Mini-Circuits' SLP-100+ datasheet defines a low pass filter by four frequency points: passband DC to 98 MHz with loss under 1 dB, nominal cutoff of 108 MHz at 3 dB loss, a stopband from 146 to 189 MHz with loss over 20 dB, and from 189 to 400 MHz with loss over 40 dB. It quotes a 20 dB to 3 dB bandwidth ratio of 1.35 (typ.) as its attenuation rate. [1]

Editorial calculation: the 20 dB point at 146 MHz is 146 ÷ 108 ≈ 1.35 times the cutoff, which is the ratio Mini-Circuits states; if your nearest unwanted harmonic or signal sits closer to cutoff than that, ask for a filter with more sections or a different construction. [1]

UIY lists low pass filters with frequency ranges up to 20 GHz for military, space and commercial applications, and Yusntech describes its RF and microwave filters as covering DC to multi-GHz ranges for satellite ground stations, RF test equipment and defence communication systems. [2] [3]

Give the supplier the passband edge and maximum passband loss, the 3 dB cutoff, the stopband frequencies with required rejection, return loss or VSWR, impedance, input power (average and peak), construction (LC, cavity, microstrip, waveguide), connectors or mounting, size limits, operating temperature and quantity.

02

Check insertion loss, return loss, power and connectors

The SLP-100+ lists passband VSWR of 1.7:1 (typ.), a stopband VSWR of 18:1 (typ.), 50 Ω impedance, 0.5 W maximum RF input power, SMA connectors in a rugged shielded case, and operating and storage temperatures of −55 to +100 °C. Its performance-data table shows 0.58 dB insertion loss at 98 MHz rising to 5.98 dB at 120 MHz. [1]

Editorial note: the high stopband VSWR shows that a reflective low pass filter sends rejected power back towards the source rather than absorbing it, so check that a transmitter feeding the filter tolerates that reflection at its harmonic frequencies. [1]

03

Size stopband rejection against a spurious-emission limit

A low pass filter placed after a transmitter is specified by the spurious-emission limit the transmitter has to meet. The FCC's amateur-service rule 47 CFR 97.307 gives numeric limits: for transmitters installed after 1 January 2003 and operating below 30 MHz, the mean power of any spurious emission must be at least 43 dB below the mean power of the fundamental emission. Between 30 and 225 MHz the requirement is at least 60 dB; a transmitter of 25 W mean power or less instead has to keep spurious power supplied to the antenna transmission line at no more than 25 µW and at least 40 dB below the fundamental, but need not go below 10 µW. [4]

Editorial calculation: 25 µW is −16 dBm, so for a 25 W (44 dBm) transmitter the 25 µW cap corresponds to 60 dB below the fundamental, while the 40 dB floor alone would allow 2.5 mW. Subtract the harmonic level the transmitter produces without filtering from the required figure to get the stopband rejection to ask for at each harmonic frequency. [4]

Editorial comparison: the SLP-100+ quotes over 40 dB rejection from 189 to 400 MHz, so a single section of that filter meets the 40 dB floor at those frequencies but not, on its own, the 60 dB figure that 97.307(e) sets for higher-power transmitters in the 30–225 MHz range; the remainder has to come from the transmitter's own harmonic suppression or a second filter. Other services and countries set their own limits, so quote the rule that applies to your equipment. [4] [1]

04

Six listed low pass filter suppliers, from waveguide parts to ranges up to 20 GHz

Ask each supplier for measured insertion-loss and return-loss curves across passband and stopband, the power rating test conditions, and connector and mounting drawings. [1]

05

Resonant sections, housing, connectors and construction type

Construction decides how far up the filter can work: UIY lists low pass filters to 20 GHz, while the SLP-100+ is a 50 Ω coaxial part with a 108 MHz cutoff. [2] [1]

06

Cutoff, passband loss, stopband rejection, VSWR, power and connector for a low pass filter

Apart from UIY's 20 GHz range, the published values below are the SLP-100+'s; they show the form each field takes, and your own frequencies, rejection and power replace them. [1] [2]

07

Send the frequency, loss and power data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Passband: [MHz; max loss dB]
Cutoff: [MHz at 3 dB]
Stopband: [MHz; rejection dB]
Return loss: [dB or VSWR]
Impedance/power: [Ω; W avg; W peak]
Build: [LC; cavity; microstrip; waveguide]
Connectors/size: [type; mm]
Temp/quantity: [°C; units]

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