Frontend - Input Band Pass Filters - 20M

20M Band Pass Filter

Series-Trap, Shunt-C filter topology is used. Just 3 sections are just enough to obtain sufficient image rejection. The filter design is based on a 0.1dB ripple Chebyshev response. The schematic is shown in the following drawing:

Because of design constraints, especially regarding IMD, the following theoretical and practical component values are chosen:

20M BPF, Fc=14150KHz, BW=1425KHz, Ql=9.9
Dipole Design Implementation Inductor
R (ohm) L (uH) C (pF) R (ohm) L (uH) C (pF) C (pF) ATC Core # turns wire (mm)
1 50 50
2 220 180 180
3 3.80 38.6 3.35 39 39 T94-10 24 0.65
4 470 470 470
5 3.80 38.4 3.35 39 39 T94-10 24 0.65
6 470 470 470
7 3.80 38.6 3.35 39 39 T94-10 24 0.65
8 220 180 180
10 50 50

Dipole 1 and 10 represent the filters input and output impedance in the simulation and are not physically present in the real circuit. The practical capacitors are made up from combinations of standard values. The first column shows the values I used with Polystyrene capacitors. The other column shows values when ATC100-B chip capacitors are used. The inductors are fine tuned by changing the distribution of turns around the toroid.

Dipole 2 and 8, that match the filter to 50 ohms, have been reduced by about 50pF in order to compensate for the parasitic capacitance of the complete motherboard system.

In simulation when the coils are modeled with a Q=150 the filter has the following key characteristics:

  • Insertion loss: 1.20dB
  • Image rejection at 32MHz: 101dB
  • IF rejection at 9MHz: 49dB
  • -3dB bandwidth: 1500KHz
  • Center frequency: 14200KHz
  • Loaded Q: 9.46

The 101dB image rejection at 32MHz is theoretical and might be realized in real life. The IF rejection of 49dB at 9MHz is not sufficient by itself. However the H-Mode mixer's symmetry (55dB) and the 9MHz notch filter (80dB) will add more than enough extra attenuation.


Because of the IMD requirement the following Micrometals toroid is tested: T94-10. Suflex 2.5% polystyrene capacitors are used.

The table below summarizes the filter measurement data. The 2-tones have 20KHz separation. IMD is measured at 0dBm and 5dBm input levels to detect non 3rd order law behavior. MDS is measured within 2.2KHz bandwidth:

20M BPF IMD, FSA3157 + T1-6T mixer, QT roofing filter
Core IIP3 2-tone level 0dBm IIP3 2-tone level 5dBm
(-dB) (-dB) (dBm) (dBm) (dBm) (dB) (dBm) (dBm) (dBm) (dB)
T94-10 1.40 132 49.8 50.8 48.8 120.5 52.1 52.1 48.8 120.5

The table requires some explanation. For each core the insertion loss of the resulting BPF and the MDS figure for the complete frontend with that BPF is given. Next, IMD results at 0dBm input tones and 5 dBm input tones are given. The columns marked 'BPF' show the IIP3 of the BPF only. The columns marked '-BPF' show the IIP3 of the frontend without the BPF in front and the columns marked '+BPF' show the IIP3 of the complete frontend including the BPF. Also the IMD3 dynamic range is computed and shown.

A number of additional remarks to the measurements:

  • All measurements are done with the complete BPF motherboard with 10 BPF filters, notch filter and the attenuator board mounted. So the IL and MDS figures include the small but measurable additional losses (about 0.2dB) introduced by the attenuator board (4 relays), the 9MHz notch and the 2 relays on the measured BPF board.

  • On 20M the additional loss in the IF notch filter can be neglected.

  • On 20M the T94-10 matches the expectations and the performance of the mixer!

  • The obtained inductor Q with the tested toroids seems to be around 150. Standard 0,65mm enameled wire is used.

20M BPF with T94-10

A picture of the assembled filter PCB is shown below. It measures 160x50mm, a half height eurocard. The assembly is meant to be plugged onto a motherboard with 9 other similar BPF's and a 9MHz notch filter board:

Measurement overview

The following table summarizes some measurements made to the T94-10 20M BPF:

20M BPF results, FSA3157 + T1-6T mixer, QT roofing filter
BPF Filter Center 14150 KHz
BPF -3dB Band Width 1425 KHz
BPF Loaded Q 9.9
BPF Insertion loss -1.40 dB
MDS level without BPF -134 dBm
MDS level with BPF -132 dBm
IP3 frontend without BPF @ 0dBm +50.8 dBm
IP3 of BPF only @ 0dBm +49.8 dBm
IP3 of frontend with BPF @ 0dBm +48.8 dBm
IMD3 dynamic range @ 0dBm 120.5 dB
Image rejection @ 32MHz -103 dB
BPF box IF rejection @ 9MHz @ 0dBm -110 dB

Single tone spurious responses

The following table shows the relevant single tone spurious responses of the H-Mode mixer frontend with the 20M BPF. Spurious signals are determined by the following equation: | n * Fo m * Fs | = Fif, where n = 0, 1, 2, 3, 4, 5 and m = 1, 2, 3, 4, 5. The number of possible spurs up to 5'th order is 55, but only those that fall within the -40dB pass band of the BPF are investigated. Higher order spurs and spurs outside the -40dB pass band are considered insignificant and have not been measured. The spur test signal is a very strong 0dBm (S9+73dB!) signal.

20M BPF + frontend single-tone spurious responses at 14.175.000Hz, @ 0dBm input level
Description Order: (n,m) F-spur (Hz) level (dBm)
IF (0,1) 9.000.000 < -127
Image (1,1) 32.175.000 -103
(1,2) 16.087.500 -122
(1,3) 10.725.000 < -127
(2,3) 12.450.000 < -127
(2,4) 13.837.500 < -115
(2,5) 11.070.000 < -127
(3,4) 15.131.250 < -123
(3,5) 12.105.000 < -127
(3,5) 15.705.000 < -123
(4,5) 16.740.000 < -125

The cyan colored measurements are limited due to the phase noise levels of the signal generator and / or LO. The given values are the best that could be done.


The next analyzer plot shows the pass band characteristics of the T94-10 20M filter. IL is around 1.40dB over the entire 20M band at a -3dB bandwidth of 1425KHz:

The following analyzer plot shows the wideband insertion loss up to the image frequency at 32MHz. At 9MHz the attenuation is about the predicted 49dB. The drastic dip of the notch filter is clearly visible. The stop band is close to the analyzers available 100dB range

The 9MHz IF rejection is measured at < -130dB (not measurable), with the 9MHz notch in place. The image rejection is measured at -103dB.

The following analyzer picture is a wideband plot up to 100MHz. Stop band reduces abruptly beyond 80MHz. Probable causes are self resonance of the big toroid inductors and / or resonance of the entire filter slot as a cavity resonator. With a pure enough LO signal this reduced stop band performance at those frequencies should not be of much concern.


Finally the impedance of the filter is measured with N2PK-VNA and Exiter software:

The measured curve almost overlaps the curve when simulating the theoretical filter and shows a good match to a 50 ohm system.

PCB artwork and schematic for the 3-pole filter board in PDF format.

160M BPF









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