HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

General discussions
glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby glovisol » Mon Jan 21, 2019 6:42 pm

Hi Sdrom33,

1) My setup is a noise generator feeding the filter under test. The output from the filter goes to the input of an RSP-1A whch is driven by the RSP SPECTRUM ANALYSER, look here below for information:

viewforum.php?f=12

This software you can download from the SDRpaly site. My setup is described in detail here:

viewtopic.php?f=5&t=3943

If you have an RSP-1A you can do the same easily & quickly.

2) I am busy designing and testing, because I have realised that the topological configurations shown before (if you change filter type to shunt first) can be drastically simplified, eliminating transformers, by using auxiliary windings on the first and last coil of the filter. For example the filter shown in my previous post is measured at 50 OHm input/output by using a 50 OHm winding on input and the output coil. Will show data soon, with detailed descriptions.

Cheers,

glovisol

Reason: No reason

ON5HB
Posts: 126
Joined: Sat Dec 29, 2018 1:07 pm
Location: Heppen - Belgium
Contact:

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby ON5HB » Mon Jan 21, 2019 7:24 pm

Hi Glovisol,

I don't get it, why are you doing this?

There are 2 build in filters that can and will do this for you.

Have a look here: https://www.sdrplay.com/wp-content/uplo ... m-RSP2.pdf

What you see is that Hi-Z has no filters or anything, where the 50 Ohm inputs do have them all.

Reason: No reason
Greetings,

Bas - ON5HB

glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby glovisol » Mon Jan 21, 2019 8:08 pm

Hi Bas,

There are several reasons to design, develop and build a high pass filter for the RSP Processors, I shall try to explain.

1) There are RSP units which do not possess the MW notch filter: the RSP1, for instance. Using an antenna filter will remove the possibility of overload from strong MW & LW signals.

2) The MW filter in the RSP-1A / RSP2 / RSPduo is a NOTCH filter suppressing the MW band alone, but what about the frequencies between 0 and 560 KHz? You will remember that I posted, in the thread about LW noise & reception, a PC screen showing extremely strong signals in the LW band. In fact, as I described there, the most powerful radio stations in the world operate in LW & MW. Using a High Pass filter, such as one based on my published specification, will free the front end of ALL TYPES of RSP Processors from overload, leaving them free from spurious and distortions.

3) But why start from scratch? Because, as far as I know, such a filter cannot be bought, nobody markets it. But, more than this, it is the pleasure of conceiving and building something useful with your own brain and hands, which is a fascinating endeavor. The more so if your goal is difficult and involves different fields of knowledge, which you must all consider at the same time.

4) Finally there is the pleasure of sharing experience and knowledge: many Forum Dvellers will be versed in these techniques and will devote their attention to something else; others, perhaps, will obtain from this thread the impulse to delve more deeply in this fascinating filter technology. Is it not knowledge, perhaps, one of the scopes of our Forum?

Thanks for asking. '73,

glovisol

Reason: No reason

ON5HB
Posts: 126
Joined: Sat Dec 29, 2018 1:07 pm
Location: Heppen - Belgium
Contact:

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby ON5HB » Mon Jan 21, 2019 8:28 pm

Hi Glovisol,

I understand about the MW-notch filter not being present in older units.

However the RSP1A is far more advanced then any of them, is has more filters.
Like a 0-2 MHz LPF and bandpass 2-12 MHz.

Now I see your problem, you are using devices that don't have these filters whereas the RSP1A does possesses them.

I get it now, it got me puzzled as the RSP1A's that I use have no problem filtering out those big guns on VLF-MW as well as FM or DAB stations.

Thanks for clearing that up, I was under the impression that the RSP2's had those too, but it only has a 0-12MHz LPF....it makes sense now.

As for marketing, RTL-SDR.com does make this filter:
https://www.rtl-sdr.com/rtl-sdr-com-bro ... -now-sale/

I have one of those from the time I was using RTL-dongles, and it does work.

Sorry I caused confusion.

Reason: No reason
Greetings,

Bas - ON5HB

glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby glovisol » Tue Jan 22, 2019 8:48 am

Hi Bas,

Thanks for your comments and for sharing your experience. Just to set the record straight, the advantge of an external filter is that is is external, so that it eliminates problems even before they get into the receiver circuitry.

All the built-in RSP filters are either notch, low pass or band pass, so there is really no provision in any RSP receiver for efficient elimination of the 0 to 560 KHz frequency spectrum. The more so when using the HI Z balanced input, the best between 2 and 29 MHz. This is important, considering that local industrial noise is often concentrated in this frequency spectrum.

If you look at my proposed filter topologies, which will be modified & simplified while keeping their functionality, you immediately realise that the proposed filters transform a coaxial input into a balanced input, thereby bringing to single input receivers (the RSP-1A for example) the advantages of the balanced HI Z termination, especially useful when using long wire antennas, but sometimes irreplaceable when coping with local noise.

Finally I have looked at the HP filter by RTL-SDR.com, but unfortunately its cutoff is between 2.2 and 2.5 MHz, thereby eliminating operation in the 160 m band.

Kind regards,

glovisol

Reason: No reason

ON5HB
Posts: 126
Joined: Sat Dec 29, 2018 1:07 pm
Location: Heppen - Belgium
Contact:

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby ON5HB » Tue Jan 22, 2019 10:30 am

Hi Glovisol,

True, 160m is a bit of a problem, I solved that by using an upconverter and turned all the filters on.

But Hi-Z has no filters at all, it goes straight into the ADC, not good if you have big signals near you.

Reason: No reason
Greetings,

Bas - ON5HB

glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby glovisol » Wed Jan 23, 2019 12:55 pm

GENERAL DISCUSSION ON PRACTICAL FILTER REALISATION PROBLEMS

During this month of January I have built and tested several prototypes. The outcome of of the results is as follows.

1) Practical realization of filters with in/out 50 Ohm impedances is difficult, because capacitor values near or above 1000 pF are needed and temperature stable high value precision capacitors are difficult to find. At this cutoff frequency (2.200 KHz) it is advisable to go for higher internal impedances (such as 500 Ohm) which require more easily available components.

2) My choice of Series First topology (see previous posts) in order to minimise the number of coils and hence the insertion losses, was not ideal, because then one would be obliged to use transformers (additional components) for impedance trasformation, so losses would go up again, apart from making the filter more complex.

NEW HP filter topologies .jpg
NEW HP filter topologies .jpg (74.51 KiB) Viewed 2134 times

3) To conclude, the best topology, after all, is the Shunt First as shown above.With this configuration the filter has a shunt inductor at the beginning and one at the end. If we change these inductors into transformers by adding an additional link (winding) we obtain a big simplification, hence smaller size, and lower losses in practice. With this topology we have the advantage we can use the impedance level as a parameter for filter component optimisation, because the turns ratio between main winding and link permits to accommodate any impedance shift. This configuration makes the use of toroids mandatory, at least for the two end inductors, in order to have strong coupling between the windings.

4) Another advantage is that any filter, whatever the level of the internal design impedance, can be easily measured with standard 50 Ohm instrumentation by just adding a link to the two external coils. Furthermore the links can even provide double ground isolation if desired, resulting in exceptional ground balance, useful for local noise rejection.

Following the above criteria, I calculated and built the experimental proptotype whose excellent performance was illustrated in the previous posts. In the next post complete data for this prototype will be uploaded.

Reason: No reason

glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby glovisol » Wed Jan 23, 2019 2:30 pm

The N=7 Chebyshev High Pass Prototype

Uploaded below detailed data of the High Pass Chebyshev N=7 prototype. At this frequency the technical characteristics of available components are as important as the design, in fact the best design will fail if implemented with low quality components.

The other problem with this High Pass filter is that it must reject very strong signals, so that shielding is essential and effective shielding requires low filter volume and hence low size components. In this respect the air cored coils wound on Kodak film roll containers have a very good Q factor, but are very large. They are good for prototype measurement, but should be replaced by smaller toroid cores, if possible. This is what I have done substituting the two internal coils with T80-2 toroidal cores.

The next step will be the replacement of the T94-26 external toroid cores with T94-2 cores having a much higher Q factor at 2 MHz.

See errata corrige for the schematic: "Experimental prototype illustration" in one of the next posts. Added 26/01/19
Attachments
Chebyshev N=7 Experimental prototype.jpg
Experimental prototype illustration
Chebyshev N=7 Experimental prototype.jpg (141.72 KiB) Viewed 2137 times
Prototype scripted.JPG
Experimental prototype schematic & data
Prototype scripted.JPG (169.01 KiB) Viewed 2140 times

Reason: No reason

arcosine
Posts: 30
Joined: Wed Jan 02, 2019 6:24 pm
Location: 41.678472, -88.051762

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby arcosine » Wed Jan 23, 2019 3:59 pm

I think material 1 (mu 20) or 15 (mu 25) would be a better choice for powdered cores for MF. Ferrites have much higher mu.

Good work!

Reason: No reason
Tony
AA9CC

glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION

Postby glovisol » Wed Jan 23, 2019 4:32 pm

Tony,

Thanks for the suggestion. I hope to get back my HP "Q" meter this week, I have sent it for repairs. Then I should be able to assess Q of various coils & materials.

The three materials should all be sutable at 2 MHz. The higher mu ones, that you advise, use less turns and hence should have less copper losses. However it is not easy to get toroids and one must get what is available. I shall try to get materials 1 & 15 as well and do a Q comparison.


'73

glovisol
Attachments
Iron Powder materials.jpg
The three materials compared
Iron Powder materials.jpg (71.59 KiB) Viewed 2125 times

Reason: No reason


Return to “General”

Who is online

Users browsing this forum: No registered users and 9 guests