LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

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glovisol
Posts: 491
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby glovisol » Wed Feb 13, 2019 5:05 pm

MEDIUM WAVE REJECTION WITH A CAUER C 05 15 40 PASSIVE LOW PASS FILTER

Network Analyser measurements

The two enclosed PC screens show the SWR performance of the LF Low Pass filter in the operating range.

- In the 560-1000 screen the AA-30 Network Analyser is connected to the antenna input and shows the SWR with the filter terminated or not terminated with a 1 KOhm resistor, simulating the load of the RSP balanced HI Z input.

- in the 1000-560 screen the network Analyser is connected to the 1 Kohm port while the antenna input is terminated with a 560 Ohm resistor. This screen also compares this reverse SWR with the forward SWR of the previous screen.

In the two screens the worsening of the SWR at low frequency is due: (a) to the proximity of the measurement limit of the AA-30 and (b) to the natural low frequency roloff of the ferrite isolation transfomer. These measurements underline the excellent performance of this filter.
Attachments
LOW PASS SWR 560-1000.jpg
LOW PASS SWR 560-1000.jpg (127.5 KiB) Viewed 1225 times
LOW PASS SWR 1000-560.jpg
LOW PASS SWR 1000-560.jpg (124.65 KiB) Viewed 1225 times

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glovisol
Posts: 491
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Location: Piedmont, Italy

Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby glovisol » Fri Apr 12, 2019 8:45 am

CLEAN FILTER PLOTS AND TEST SETUP

As mentioned here:

viewtopic.php?f=5&t=4130&start=30

when working at low frequency the Spectrum Analyser plots would show strong interference generated by the switch mode PSU feeding the Noise Generator. These plots were shown in the early posts of this thread. This problem has now been solved by replacing the PSU of the Noise Generator and clean low pass filter plots are uploaded below.
Attachments
Low Pass LF Filter 1.jpg
Low Pass LF Filter 1.jpg (144.32 KiB) Viewed 718 times
Low Pass LF Filter 2.jpg
Low Pass LF Filter 2.jpg (158.03 KiB) Viewed 718 times
Low Pass filter test April 19.JPG
Low Pass filter test April 19.JPG (271.63 KiB) Viewed 718 times

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glovisol
Posts: 491
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Location: Piedmont, Italy

Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby glovisol » Sat Apr 20, 2019 3:50 pm

MEDIUM WAVE REJECTION WITH A CAUER C 06 15 PASSIVE LOW PASS FILTER WITH 50 OHM TERMINATIONS

The filter described in the previous posts could only be used with RSP Processors equipped with the HI Z / 1 KOhm input. For LF work 0 to 500 KHz a sharp low pass filter is very useful in completely protecting the front end from strong MW Boadcast Band signals. This & following posts describe a Low Pass filter designed to work between 50 Ohm terminations and capable of working down to D.C. because it does not employ series capacitors and/or transformers.

The filter has a passband insertion loss of less than 2 dB (that makes it a natural accessory of the RSP-1A which features a LF sensitivity lower than that of its brothers so it is best not to add more attenuation in front) an attenuation of 35 dB @ 700 KHz and an ultimate attenuation of over 50 dB @ 900 KHz and up, as pictured below.

More detail in the next posts.
Attachments
LP Filter 50 Ohm 5.jpg
LP Filter 50 Ohm 5.jpg (219.48 KiB) Viewed 589 times
LP Filter 50 Ohm 6.jpg
LP Filter 50 Ohm 6.jpg (189.23 KiB) Viewed 589 times
LP Filter 50 Ohm 7.jpg
LP Filter 50 Ohm 7.jpg (216.84 KiB) Viewed 589 times

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glovisol
Posts: 491
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Location: Piedmont, Italy

Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby glovisol » Sun Apr 21, 2019 9:16 am

MEDIUM WAVE REJECTION WITH A CAUER C 06 10 PASSIVE LOW PASS FILTER WITH 50 OHM TERMINATIONS

Two versions of the filter have been calculated:

- From Cauer prototype C06 10 having a passband ripple of 0.5 dB, with a likely insertion loss of 2 dB max (provided element Q is in the order of magnitude of 150 or better) and an ultimate attenuation in excess of 40 dB. Infinite attenuation frequencies are: L2/C2 @ 884 KHz and L4/C4 @ 685 KHz.

- From Cauer prototype C06 50 having a passband ripple of 1 dB, with a likely insertion loss of 4 dB max (under Q hypotesis as above) and an ultimate attenuation in excess of 60 dB.

Although component values for both versions are given below, only the 0.5 dB filter version has been built and tested. The higher ripple version is sure to work with no problem, but it will have a higher out of band attenuation at the price of higher insertion loss.

The most important parameter that determines filter performance is the Quality Factor at cutoff frequency (500 KHz) of the coils used. It was planned to use powder iron toroids and Micrometals material 3 (gray) was chosen, with a mu=35, good temperature stability and rated for the 0.05 to 0.5 MHz frequency range. Core T44-3 (o.d. 11 mm, Al= 180) was tested with 28 turns of 0.355 mm wire. Inductance was measured to be 14.5 uH (spot on as calculated on the Al value). The Q Meter measurement gave a Q=100 @ 500 KHz. This Q value being too low, it was decided to test a 18 uH air core coil. The coil was to be wound on a 15.2 mm dia. with 0.355 enamelled copper wire. Calculation with Coil32 software yelded 46 turns for a Q=99. The actually built test coil had 44 turns and measured Q=200-20%=160. Therefore it was decided to go for air coils and to use the multi-coil technology already described here:

viewtopic.php?f=5&t=3973&start=50

The complete filter is shown below. Construction details next
Attachments
LP Filter 50 Ohm 9.jpg
Filter pass-band VSWR with 50 Ohm load
LP Filter 50 Ohm 9.jpg (83.4 KiB) Viewed 558 times
Cauer N=6 MW elimination Low Pass Filter.jpg
Filter schematic and data for the two options
Cauer N=6 MW elimination Low Pass Filter.jpg (115.3 KiB) Viewed 558 times
Assembled filter.JPG
Filter assembled inside utility aluminium die cast box
Assembled filter.JPG (118.5 KiB) Viewed 558 times

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sdrom33
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Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby sdrom33 » Sun Apr 21, 2019 9:44 am

Hi glovisol, the filter looks good, but in the response pics I see glitches that look like signals to me. Shielding does not look so good even if you use a metal box

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glovisol
Posts: 491
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby glovisol » Sun Apr 21, 2019 3:41 pm

Sdrom 33, the measurements shown in previous post were done as soon as the filter was assembled and outside the shielding box, as illustrated in the pic below. One of the advantages of the multi-coil technique is that you can check the filter and be sure it works as per design BEFORE you shut it up in the box. Thanks for your observations and checks, mistakes always happen.....
Attachments
Naked filter measurement.JPG
Naked filter measurement.JPG (146.98 KiB) Viewed 530 times

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glovisol
Posts: 491
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Location: Piedmont, Italy

Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby glovisol » Mon Apr 22, 2019 7:18 am

MEDIUM WAVE REJECTION WITH A CAUER C 06 10 PASSIVE LOW PASS FILTER WITH 50 OHM TERMINATIONS

The filter is built on a rigid PVC conduit tube with 15.2 mm external diameter. First 1 mm dia.holes are drilled into the pipe for pins 1 to 6 plus the two ground pins at the end: these support the ground bar. Spacings are as follows:
Pins 1-2: 25 mm.
Pins 2-3: 5 mm.
Pins 3-4: 25 mm.
Pins 4-5: 5 mm.
Pins 5-6: 30 mm.

Pin 1 is pushed in, enamelled wire is soldered to it and L2 is hand wound. At the end the wire is temporarily taped, pin 2 is pushed in and the end of L2 is soldered to the pin. Then L4 is started and so on. At the end the two ground pins are soldered in and joined with the solid copper 1 mm dia. ground bar. Finally pins lsted above are strapped & soldered together.

Pictures below show it all.
Attachments
Filter build 1.JPG
Filter build 1.JPG (162.33 KiB) Viewed 489 times
Filter build 2.JPG
Filter build 2.JPG (150.63 KiB) Viewed 489 times
Filter build 3.JPG
Filter build 3.JPG (196.42 KiB) Viewed 489 times

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glovisol
Posts: 491
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION

Postby glovisol » Tue Apr 23, 2019 9:00 am

MEDIUM WAVE REJECTION WITH A CAUER C 06 10 PASSIVE LOW PASS FILTER WITH 50 OHM TERMINATIONS

Uploaded below information on filter VSWR with output terminated into a 50 Ohm load and practical filter performance in attenuating the MW broacast band stations.
Attachments
MW with no Filter.jpg
MW with no Filter.jpg (188.88 KiB) Viewed 430 times
MW with Filter.jpg
MW with Filter.jpg (194.97 KiB) Viewed 430 times
LP Filter 50 Ohm 8.jpg
LP Filter 50 Ohm 8.jpg (82.13 KiB) Viewed 430 times

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