Bandpass and I.F. Filter Information





Web Page Introduction

On this page; a popcorn highpass filter, some inductors and their Q , double tuned bandpass filters and simple Cohn Crystal ladder IF filters are presented.
This page last updated --> Sept 22, 1999.



A Popcorn High Pass Filter

Like many people, I have a general coverage receiver for casual listening in the shop. Occasional broadcast band QRM from the lower portion of the broadcast band has been a problem with this receiver and the simple, no-tune, standard value capacitor filter shown below has been helpful. The 3.3 uH inductors were build using 26 turns #26 AWG wire wound on T50-2 cores. Junk box ceramic capacitors were used to build this 5th order high pass filter. The filter should be used with a 50 ohm impedance at both ends. The 160 meter band is relatively uneffected. The attenuation is not very high in the upper part of the BC band and this filter is therefore not suitable for all BCI applications. A graph of the filter response is shown below the main schematic. The solid line gain plot on the left hand side shows the relatively poor attenuation in the upper portion of the broadcast band.





Inductors and Their Unloaded Q
Here is data collected on sample inductors with their data as follows: frequency, winding instructions, inductance value and unloaded Q. The unloaded Q is used to calculate the parallel capacitance for each end of the filter. If you have any values to add from chart or measured, please email us.

10 MHz: 2.60 uH, 25 turns #22 AWG on a T50-6, Q = 260 (from chart)
10 MHz: 2.54 uH, 25 turns #22 AWG on a T50-6, Q = 281 (measured by W7EL)
14 MHz: 1.54 uH, 19 turns #20 AWG on a T50-6, Q = 248 (from chart)
14 MHz: 2.08 uH, 20 turns #22 AWG on a T68-6, Q = 255 (measured)
19 MHz; 0.86 uH, 14 turns #18 AWG on a T50-6, Q = 235 (extrapolated to be 220 at 21 MHz from chart)


Example Bandpass Filters
Here are a couple of H.F. DTC filters. This filter topology requires that both ports be terminated in 50 ohms in order to achieve the designed bandpass response and it's associated stop band attenuation.

20 Meter Band:

Below is a second order Butterworth bandpass filter designed for the 20 meter band. The center frequency is 14.2 Mhz and the bandwidth is 0.575 MHz. L1 and L2 are 0.5 uH and are wound on T50-6 toroids with 11 turns of #24 AWG wire. The unloaded Q of the inductors is around 200 ohms. The 150 pF parallel caps are rounded to fit a standard value. C tune should include a variable cap in parallel with a fixed value capacitor to tune each resonator. A suggested value might be a 180 pF in parallel with a 7-40 pF trimmer cap on each side of the 7.2 pF coupling capacitor.
Below the main filter is an equivalent filter using transformers to match the filter to a 50 ohm load. Note C tune is now 250 pF and again a fixed value cap should be used in parallel with a variable trimmer cap to tune each resonator. A 2 turn secondary is wound over the grounded end of the main coil. The 7.2 pF coupling cap can be achieved by placing a 12 pF capacitor in series with an 18 pF capacitor.



30 Meter Band:

Below is a second order Butterworth bandpass filter designed for the 30 meter band. The center frequency is 10.125 Mhz and the bandwidth is 0.40 MHz. L1 and L2 are 2.1 uH and are wound on T50-6 toroids with 23 turns of #24 AWG wire. The unloaded Q of the inductors is around 225 ohms. A portion of each C tune should be variable to allow tuning of the filters resonators. Below the main design is a filter using transformers to match the filter to its load with a 3 secondary to make the impedance transformation. The 3.3 pF coupling cap may be achieved by placing in series a 5 pF with a 10 pF or a 3.9 with a 4.7 pF capacitor.
A filter was built and the resonators tuned at the center frequency of 10.125 MHz. The coupling caps were NP0 ceramics (5 pF in series with a 10 pF). The variable cap was a DigiKey bottom-adjusted (SG20016-ND) 5-20 pF ceramic trimmer cap. In parallel with the each trimmer cap was a fixed 82 pF silver-mica capacitor. After tuning, the coils were disconnected and the total resonating capacitance was measured at 94 pF. This illustrates the fact that you need to be able to tune well under or in some cases over the designed C Tune frequency. Many builders use variable caps as high as 60 - 100 pF for the C Tune trimmer to allow for deviation. The bottom-adjusted type trimmer cap was selected, as the leads are easily bent for use with Ugly construction and tuned from the top.





Example Cohn IF Filters

Building I.F. filters out of low-cost microprocessor crystals was popularized for the amateur homebrew community by Wes Hayward, W7ZOI. Below is a schematic and the component values for a number of simple Cohn filters which I have used. The values for Y, C and R term are all the same for a given filter. R term represents the input and output impedance of the filter and impedance matching is usually performed with transformers and/or resistors. You may practically use from 3 to 6 crystals, however only 3 are shown for brevity. The insertion loss for a given filter is not shown as this will vary with factors such as the crystal Q. Please read the article on building crystal ladder filters by W7ZOI mentioned on the QRPHB web page entitled Selected QRP Reading List. The crystal frequencies are standard value units right out of the Mouser and Digi-Key catalogs.

These examples are meant to show a few ready-made filters and if you have any additions to contribute, please send the values by email.

CW Filter : IF = Y = 1.8432 MHz , 6dB bandwidth = 800 Hz, C = 33 pF , R term = 1500 ( matches NE602 without transformer).

CW Filter : IF = Y = 3.579548 MHz ( colorburst xtal ), 3dB bandwidth = 403Hz , C = 200 pF, R term = 150.

CW Filter : IF = Y = 4.000 MHz , 6dB bandwidth = 400 Hz , C = 300 pF , R term = 200.

CW Filter : IF = Y = 4.032 MHz , 6dB bandwidth = 650 Hz , C = 120 pF , R term = 300.

CW Filter : IF = Y = 4.9134304 MHz , 6dB bandwidth = 600 Hz , C = 130 pF , R term = 240.

CW Filter : IF = Y = 9.600 MHz , 6dB bandwidth = 600 Hz , C = 120 pF , R term = 227.

CW Filter : IF = Y = 12.000 MHz , 6dB bandwidth = 500 Hz , C = 150 pF , R term = 227.


SSB Filter : IF = Y = 8.000 MHz , 6dB bandwidth = 2.2 KHz , C = 100 pF , R term = 300.

SSB Filter : IF = Y = 12.000 MHz , 6dB bandwidth = 2.1 KHz , C = 160 pF , R term = 128.





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