Adjustable filter provides lowpass response

Posted on Oct 15, 2012

In an adjustable lowpass filter, each filter section uses commonly available components (Figure 1). This example uses filter-section cutoff frequencies for standard inductors and capacitors without the need for any extra components in series or parallel. Fixed inductors are Coilcraft 90 series axial-lead chokes with ±10% tolerance. Fixed capacitors are polyropylene units, available from any distributor, with ±2% tolerance for the 1200-pF devices and ±5% tolerance for the other values.

Adjustable filter provides lowpass response
Click here to download the full size of the above Circuit.
Adjustable filter provides lowpass response - image 1
Click here to download the full size of the above Image.

The adjustable lowpass filter is in a 3.625-in.-long×1.5-in.-wide×1.0625-in.-high Bud CU-123 die-cast aluminum box with input and output BNCs. Miniature toggle switches for the individual filter sections are accessible at the enclosure's exterior. Internal ground returns use solder lugs. The four filter sections have 50Ohm characteristic impedance and nominal 3-dB cutoff, from left to right in Figure 1, of 3.083, 6.586, 14.491, and 21.310 MHz. Table 1 shows the measured amplitude response for the box alone and for the four individual filter sections. The low-cost, adjustable lowpass filter delivers reasonable performance. As the frequency approaches 100 MHz, the transmission performance of the enclosure deteriorates with all filter sections switched to the off position. The interconnections between switched sections use available bus wire without any precautions to minimize parasitic circuit elements. Stray series inductance, estimated at approximately 55 nH, arises from the 3.5-in. physical length of the enclosure, plus 2 in. for the four switches.

Leave Comment

characters left:

New Circuits



Popular Circuits

RIAA Phono Equaliser
Fast Composite Amplifier Circuit
Operating Systems Latency Measurement and Analysis
15db uhf tv antenna booster circuit
tesla circuit
Non-Contact Power Monitor
ADUC845 12MIPS 8052 Flash MCU + 10-Ch 24-Bit ADC + 12-Bit DAC
Odd-ball PI circuit identification request and headroom question
GVI Digital Remote Control
450 800Hz signal generator circuit diagram