Low pass filters / Attenuation of DBS signals.

[gillham]

So let me start by saying that RF is not my specialty.

I've been looking at some data sheets recently for low pass filters. Particularly a DC to 825Mhz filter that has an interesting insertion loss graph.

From the specs:

Mhz     Insert Loss (dB)
850     0.99
950     1.19
1100    1.58
1300    3.89
1400    11.97
1475    22.09
1800    39.04

What I'm wondering about is what does it take to "quiet down" the DBS birds? Looking at satbeams and similar data the power difference for my area between, for example, 119° and 123° is roughly 4.5dBW.

Based on the data above it appears this particular filter with a standard ku LNBF (LO 10750) would provide 1.19 to 3.89 dB of signal attenuation on the first ~17 transponders of a FSS satellite. (I/F 950mhz - 1300mhz) At the same time, it would provide at least 22.09 dB of attenuation, again on a standard ku LNBF, beginning with the first DBS transponder with an I/F of 1474Mhz.

So my question is, would this type of filtering work when combined with a basic tone style signal meter to effectively filter out any DBS satellite being picked up on a standard ku LNBF? This would make a simple analog meter much more useful for dish pointing. The filter I was looking at (VLFX-825) is not impedance matched or the correct connector type, but I'm really just thinking conceptually here.

 

[armyvet]

...guessing that would work like an APA for wildblue yeah?

 

[gillham]

...guessing that would work like an APA for wildblue yeah?

Exactly. I wish I could find some specs on those APA attenuators. They might be exactly what I need.

Anyone have any specs off the APA attenuators? They mention specific spot beams, but I'm hoping to get something for the whole lower end of the linear spectrum. At least the North American standard ku spectrum.

 

[Smith P.]

Using spectrum analyzer will give full picture not just relative signal level. It would best approach for tuning sat dishes.

 

[rv1pop]

I think you are-- (or I am) missing something. A "LOW PASS" filter pases lower frequencies and blocks higher ones. With this chart, you would get the older wireless phone signals. The 4 to 5 year old ones would be iffy and all signals above that would be missing. All satellite signals would be 40 db or more down - and you lose signal if you go 5 db down. If this is OK then I am missing what you want to do.

 

[gillham]

Using spectrum analyzer will give full picture not just relative signal level. It would best approach for tuning sat dishes.

Any idea where I can get one for $50 US?

 

[gillham]

I think you are-- (or I am) missing something. A "LOW PASS" filter pases lower frequencies and blocks higher ones. With this chart, you would get the older wireless phone signals. The 4 to 5 year old ones would be iffy and all signals above that would be missing. All satellite signals would be 40 db or more down - and you lose signal if you go 5 db down. If this is OK then I am missing what you want to do.

I might not have been clear that I want to put the low pass filter on the RG6 to attenuate the Intermediate Frequency side of the LNBF. Since with the 10750 LO freq the I/F for 11700 is 950, I would want minimal attenuation on that band.

So signal at 12.224Ghz (DBS TP1) collected on a standard ku LNBF will have a rough center intermediate frequency of 1474mhz. The filter I am looking at will attenuate that I/F at 22dB while passing 950mhz with only 1.19dB of attenuation.

I was just curious if anyone thinks approximately 21dB difference in attenuation, on the RG6, would be sufficient to reduce the DBS signals to below FSS signals.

The DBS TP1 attenuation would be about the same as if there was 300-325 feet between the LNBF and the meter. The FSS TP1 attenuation would be comparable to loss over only 20 feet.

I guess I need to conduct an experiment.

 

[Anole]

I was just curious if anyone thinks approximately 21dB difference in attenuation, on the RG6, would be sufficient to reduce the DBS signals to below FSS signals.

I guess I need to conduct an experiment.

Oh, I got it when you proposed the idea.
Can't think of a good argument against.
And, you were really only suggesting this filter ahead of your METER, not your receiver.

A while back, I used a $10 meter to find and align on White Springs.
I wondered at the time, if I was really tuning up on the DBS bird at that location?.
With your filter in line, will there be sufficient FSS signal for the meter?
They're not really built for the job.
So, the sky might look very quiet.

I look forward to your experimental data. - :up

 

[Smith P.]

Any idea where I can get one for $50 US?

Yes. I made it by myself - BLSA.

 

[gillham]

Oh, I got it when you proposed the idea.
Can't think of a good argument against.
And, you were really only suggesting this filter ahead of your METER, not your receiver.

Absolutely, no way I want to deprive my receiver of any signal.

With your filter in line, will there be sufficient FSS signal for the meter?
They're not really built for the job.
So, the sky might look very quiet.

I think so since I would be right next to the dish, with maybe 6 feet of RG6 between the attenuated meter and the LNBF. It should be less loss than the 50 feet of RG6 running inside to the receiver. At least based on average loss per 100' I can find online.

I look forward to your experimental data. - :up

Me too. I need to find a place to acquire one. I can find a datasheet, but none of my favorite vendors list the part. I might need to just build it myself since the parts are cheaper, but I don't really have the best equipment to prove the filter is working as designed prior to trying to prove it will do what I think it will.

I'll definitely report back anything I can figure out over the next few weeks.

 

[mark_calgary]

Based on your comments here it wouldn't take much to make RF your specialty. You certainly have the practical/commonsense knowledge that many "RF specialists" I've met do not have.

The only thing I would add is that the specifications are typical and you may see a bit more or less attenuation than stated - but not hugely different.

I see no reason why this won't work for the application you have described. Even the extra loss from the impedance mismatch may not be noticeable if you choose to use a 50ohm filter (I assume it is). Given the quality of the consumer satellite equipment we use, there are worse things than that in the system.

Hardest part might be trying to get something to adapt between an SMA connector and your 75 ohm cable.

I hope you try it.

 

[Anole]

another thought

A meter equipped with your proposed filter would work fine on the FSS satellites we seek.
Assuming it had enough gain to hear them.

But, it would still be fine for the DBS birds the meter is marketed for, too.
About the only place it'd fall short, is if someone were using it to line up using any bandstacked LNB, because both FSS and DBS would be partially attenuated.

And thinking back to the example in the previous post, of finding 129/White Springs . . . .
I wonder if the reason some folks report success and some report failure with those meters may have to do with whether there is a strong DBS bird at the location they are tuning up on?

 

[gillham]

I see no reason why this won't work for the application you have described. Even the extra loss from the impedance mismatch may not be noticeable if you choose to use a 50ohm filter (I assume it is). Given the quality of the consumer satellite equipment we use, there are worse things than that in the system.

Thanks for the encouragement. The filter I'm looking at, VFLX-825 is the model number, has an MSRP of $39.95 and I haven't found a place to order it online yet. So I decided to order a couple of filter ICs from Digikey with similar specs and solder something up first. If I can't get a $1 part working, or it can't handle 500ma@18v then I'll have to look harder for the VFLX-825.

Also I don't know if the filter ICs will work backwards or not, but that is why it is an experiment. Hopefully my Digikey order ships quickly and I'll be able to experiment next weekend.

These are the parts I'm looking at:
http://www.avx.com/docs/catalogs/lp0603.pdf
http://www.johansontechnology.com/products/rfc/lpf/JTI_Low-Pass-0869LP14A090_3-03.pdf
http://media.digikey.com/pdf/Data%20Sheets/Johanson%20Technology/0892LP07A136(E).pdf

 

[gillham]

But, it would still be fine for the DBS birds the meter is marketed for, too.
About the only place it'd fall short, is if someone were using it to line up using any bandstacked LNB, because both FSS and DBS would be partially attenuated.

Exactly, so it should still "Just Work" for the low transponders on either FSS, DBS or vertical of a bandstacked. Basically any I/F below the stop frequency on the filter.

And thinking back to the example in the previous post, of finding 129/White Springs . . . .
I wonder if the reason some folks report success and some report failure with those meters may have to do with whether there is a strong DBS bird at the location they are tuning up on?

Right, I think the locations sharing FSS/DBS are easy to find with a cheap meter since you're actually listening to the DBS on the meter and that gets you close enough to see the FSS on the receiver.

 

[gillham]

Some actual test data! (finally)

Thanks to a recently acquired BLSA, I've been able to more precisely test a "LPF-1075" low pass filter I bought off eBay. It doesn't have the exact specs I would like, but it was the closest I could find readily and makes a good baseline.

I'm using an Invacom Quad QPH-031 LNBF, 90CM Fortec Star and I have a few receivers slaved, but the only variable was inserting the filter or not.

I've attached three images. All are using the linear side of the LNBF. All are scanning 11700 - 12700. (950-1950 IF) This is vertical polarity.

The first is a normal scan of 101W with AMC4 on the left (950-1450) and DirecTV on the right (1450-1950). The second is showing the exact same scan except with the filter inline. The third is my attempt to combine them both to one image.

Roughly around 11860 (1110 if) the filtered signal begins to be severely attenuated. By 12200/1450mhz the signal is *gone*, just like I wanted. I tested this setup with a cheap meter installed and found that the signal was attenuated enough that the meter barely reacted to a linear signal. Looking at the specs, I need something that gives me another 250-280mhz before severe attenuation kicks in. Now that I have an easy way to test, I can hunt for some parts!

I tested the meter & filter on the circular side of the LNBF as well and it was getting plenty of signal through from DirecTV 101.

I need to test this with just a short piece of cable between the LNBF and filter/meter setup, instead of the long run I have, but right now the attenuation is too much for these cheap meters on linear with this filter. Note that my DVB-S PCI card was still able to lock low transponders.

Anyway, I wanted to update this old thread now that I have some test data not just hypothesis.

 

[Anole]

I'm using an Invacom Quad QPH-031 LNBF, 90CM Fortec Star ...

I tested the meter & filter on the circular side of the LNBF as well and it was getting plenty of signal through from DirecTV 101.

Glad to hear more on this subject. - :up

I was confused for a few minutes, reading what I clipped above.
Then, I realized the circular side with its different LO -should- get signal, so that's fine.
But, what I was scratching my head about, was whether the Invacom Standard output had significant response into the DBS end of the band?
It might be instructive to use a lesser Standard (or even Universal) LNB with a (possibly) worse bandpass filter.

You who might have input on the Invacom?
SatelliteAV (Brian) might.
But if it's convenient, you might try a lower quality LNB.

 

[cracklincrotch]

Where are people finding BLSAs these days? Fleabay?

 

[gillham]

Where are people finding BLSAs these days? Fleabay?

Smith, P. is selling them in the classifieds here. I thought it was a decent deal since it was assembled and tested and cheaper than the one I saw on eBay. Less headache trying to get my first one going. Now I am prepared to build my own spare.

 

[gillham]

But, what I was scratching my head about, was whether the Invacom Standard output had significant response into the DBS end of the band?
It might be instructive to use a lesser Standard (or even Universal) LNB with a (possibly) worse bandpass filter.

But if it's convenient, you might try a lower quality LNB.

My Coolsat 6000 can't lock circular on the linear LNBF. Quality is right below threshold. I didn't mess with it much, just a couple minutes while getting these readings.

I have a "LAVA" linear that I can wave around the sky a bit, hopefully this weekend. More to report later.

 

[Smith P.]

Where are people finding BLSAs these days? Fleabay?

You can still get it here while it lasts or will sell them soon on eBay without SG member's discount.
http://www.satelliteguys.us/satelli...4-fs-blsa-finished-pci-spectrum-analyzer.html