Frontend - Input Band Pass Filters - 6M

6M Band Pass Filter

Series-Trap, Shunt-C filter topology is used. 5 sections are used to obtain as much image rejection as possible. The filter design is based on a 0.1dB ripple Chebyshev response. The schematic is shown in the following drawing:

IL is a problem with this filter as more inductors are needed and high Q is difficult to achieve with iron powder on this band. So a compromise between IL and image rejection is needed.

Rather small inductors have been used in order to achieve rather big in/out matching capacitors. This allows for compensation of these capacitors in the mother board (50pF need to be subtracted at least!).

Only up to size T50 toroids can be used. Bigger is not possible physically as 5 toroids are needed for the image rejection requirement. And even 5 toroids don't give 100dB image rejection...

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

6M BPF, Fc=51500KHz, BW=4390KHz, Ql=11.8
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 90.9 22 22
3 0.400 32.9 0.350 33 33 T50-10 9 0.50
4 224 220 220
5 0.400 28.8 0.350 27 27 T50-10 9 0.50
6 302 270 + 33 300
7 0.400 27.9 0.350 27 27 T50-10 9 0.50
8 302 270 + 33 300
9 0.400 28.8 0.350 27 27 T50-10 9 0.50
10 224 220 220
11 0.400 32.9 0.350 33 33 T50-10 9 0.50
12 90.9 22 22
14 50 50

Dipole 1 and 14 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 12, 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=120 the filter has the following key characteristics:

  • Insertion loss: 2.30dB
  • Image rejection at 68MHz: 81dB
  • IF rejection at 9MHz: 96dB
  • -3dB bandwidth: 7000KHz
  • Center frequency: 52000KHz
  • Loaded Q: 7.43

The 81dB image rejection at 68MHz is theoretical and should be realizable in real life. The IF rejection of 96dB 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 and the compromise between IL and image rejection, only T50-10 Micrometals toroids have been tested. Suflex 2.5% polystyrene capacitors have been 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:

6M 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)
T50-10 3.36 126,0 44.6 41.1 44.6 113.7

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 6M the additional loss in the IF notch filter can be neglected.

  • The tested toroids give fair results on 6M. The toroids match the performance of the mixer!

  • On 6M the Q produced with the T50-10 is fair, but IL is mounting with 5 toroids. 6M is clearly a difficult band in a 9MHz IF down conversion receiver.

  • The obtained inductor Q with the black toroids seems to be around 100. Silver plated 0,50mm enameled wire is used.

6M BPF with T50-10

The tested toroids perform rather well. I have done an experiment with air-coils, but the problem of 5 coils coupling slightly with each other did undo the advantage of a much better Q. 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 T50-10 6M BPF:

6M BPF results, FSA3157 + T1-6T mixer, QT roofing filter
BPF Filter Center 51500 KHz
BPF -3dB Band Width 4390 KHz
BPF Loaded Q 11.7
BPF Insertion loss -3.36 dB
MDS level without BPF -133.0 dBm
MDS level with BPF -126.0 dBm
IP3 frontend without BPF @ 0dBm +41.0 dBm
IP3 of BPF only @ 0dBm +44.6 dBm
IP3 of frontend with BPF @ 0dBm +44.6 dBm
IMD3 dynamic range @ 0dBm 113.7 dB
Image rejection @ 68MHz -72 dB
BPF box IF rejection @ 9MHz @ 0dBm -102 dB

Single tone spurious responses

The following table shows the relevant single tone spurious responses of the H-Mode mixer frontend with the 6M 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.

6M BPF + frontend single-tone spurious responses at 52.100.000Hz, @ 0dBm input level
Description Order: (n,m) F-spur (Hz) level (dBm)
IF (0,1) 9.000.000 < -127
Image (1,1) 70.100.000 -72
(2,2) 56.600.000 < -122
(3,3) 58.100.000 < -122
(3,4) 48.075.000 < -122
(4,4) 58.850.000 < -122
(4,5) 47.080.000 < -122
(4,5) 50.680.000 < -112

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 T50-10 6M filter. IL is around 3.36dB at best with -3dB bandwidth of 3056KHz. The value of the IL indicates that the Q's of the inductors are close to about 100:

The following analyzer plot shows the wideband insertion loss up to 100MHz. This filter is behaving rather different from its simulation especially in the stop band areas! There appears to be a parasitic pole at 40MHz!

The 9MHz IF rejection is measured at < -130dB (not measurable), with the 9MHz notch in place. The image rejection is measured at only -72dB. With a low-side LO the image rejection looks actually a bit better: below -80dB.


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

The measured curve comes out too high around 70 ohms, but still shows a fairly good match to a 50 ohm system. It looks like I have been able to find an adjustment without much ripple but with a mismatch!

PCB artwork and schematic for the 5-pole filter board in PDF format.

160M BPF









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