frequency 1 MHz off

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crawfrdb

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Jun 5, 2006
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While blind scanning T5 last night, I noticed my receiver reported a transponder frequency 1 MHz off of what is listed on the various channel lists out there.

Should I keep the number derived in the blind scan or should I go in and edit the Tp freq by 1 MHz?

Thanks,
B
 
drhydro

drhydro

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Oct 19, 2004
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It really wont matter much, but you might get a very slight sq increase. Many times a lnb will be off a few MHz
 
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Mr Tony

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Nov 17, 2003
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LNB's register different frequencies. Some boxes are really touchy on that

As an example, Ohio News net (SBS6) is at 11742 but sometimes it blind scans at 11738 or 11740.
 
Vacosta

Vacosta

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Jun 27, 2005
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KCMO
I would leave as is. I have a Coship receiver that even if I input the freq from Lyngsat, it will default to 1 or 2 MHz up or down for some reason. Usually this default will give me a better quality reading. Don't know how it all works but I have another Samsung receiver that does not do this so I just input the same freq as the Coship, versus inputting what I saw in Lyngsat, it could make the difference between a signal or no signal.
 
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dish_in_the_sky

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May 18, 2006
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Frequency offset cause...

The LO in the LNBFs is usually +/- 1 to 3 MHz in frequency. The receiver assumes 5150, 9750, 10600 or 11250 MH LO.

The (transponder freq) - (LO freq) = IF freq. The receiver determines the IF frequency of best signal strength/lowest BER and ASSUMES the LO number to be one of those listed above (depending on the receiver's LO setting)

the ACTUAL LO freq can be that value +/- 1 to 3 MHz for most LNBFs. This will yield a +/- 1 to 3 MHz offset in your scanned TP frequency.

The LO freq can change with temperature, voltage or mechanical vibration.

In most cases the tolerance/drift is not a problem, but does increase the BER (reduces quality)

If you have a very weak signal and/or narrow transponder signal, plan ahead, let everything run for 20 minutes before trying to get it (so the LO is stabilized) and scan for it. You can also try to manually enter the TP freq at (nominal) (+ 1 MHz) (- 1 MHz) (+ 2 MHz) (- 2 MHz) etc until you hit it. More than +/- 4 MHz is not necessary. [I assume the receiver IF is +/- 1 MHz tolerance at most, it's probably less than that]

There are some commercial LNBs which have very accurate LOs, down to 5 kHz. They are higher noise, very expensive and require a polarization selector (or 2 LNBFs at 90 degrees to each other) plus a feedhorn for each, one such brand name is Norsat. (they don't care about low NF because they use huge dishes to guarantee a stron signal)

Unfortunately such stable LOs do not appear to be available in the low noise consumer LNBFs. Putting a good temperature control and tight ripple filter on the LNBF will help but these are tricky to build. I'm an experienced RF engineer but I'm not in a hurry to do this.

However, I'll try this trick with SatMex 5 (the 12080 transponder is zip for me) to see if I get anything.


I assume the blind scan locks onto the center freq of the TP.
 
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crawfrdb

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Jun 5, 2006
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dish_in_the_sky said:
If you have a very weak signal and/or narrow transponder signal, plan ahead, let everything run for 20 minutes before trying to get it (so the LO is stabilized) and scan for it. You can also try to manually enter the TP freq at (nominal) (+ 1 MHz) (- 1 MHz) (+ 2 MHz) (- 2 MHz) etc until you hit it. More than +/- 4 MHz is not necessary. [I assume the receiver IF is +/- 1 MHz tolerance at most, it's probably less than that]

This is very helpful given the signal outputs with my WF T90 antenna have often been right on the margin. Trying that may just lift it enough to get a clear picture instead of a broken one.

I also wonder how my 115° outdoor temperature might be effecting LNBF operation. Local friends receiving FTA note that FTAs are, at least mechanically, very tolerant to weather/temperature. But perhaps there may be an impact to RF performance.

Regards,
B
 
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dish_in_the_sky

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May 18, 2006
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(off topic) Focus spot (line in your case) and LNBF temperature

Keep the LNBF cool (aluminum foil around the BODY, NOT the front of it) to keep the noise floor down.

Try moving your LNBFs towards or away from the secondary reflector to see if the signal improves.

A toroidal dish cannot give the same strength per unit of dish diameter a single dish can, because the horizontal shape is not exactly a parabola. Vertically it is a parabola.

This is the compromise necessary to receive multiple satellites.
 
N5XZS

N5XZS

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Jan 23, 2005
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Albuquerque, NM, USA
For some reason we tend to get better signals in the wintertimes since the cold weather helps you get better gains from the LNA.

Maybe someday we might just build a refrigated LNA, for the summertime season to adjust for the 90*F or higher.:D

And also don't forget the Earth's Tilt seasonal pattern.:eek:
 
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dish_in_the_sky

SatelliteGuys Family
May 18, 2006
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Refrigerated LNBF

*L*:D Yes, this is a somewhat extreme suggestion.

Use a Peltier cooler with proper insulation, heatsinking, fans to keep the LNBF cold.

It's doable but not trivial. You'll need to run a pair of wires to the dish to run it.

You'll need up to 50W of power plus 2 fans and 2 heatsinks (CPU pin heatsink inside, bigger one outside,) insulated weatherproof enclosure and dessicant. Make sure not to overload the dish arm.

How much better??? Unsure, thermal noise is proportional to the square root of the absolute temperature of the LNBF, and LO short term stability/phase noise also play a part. I do suggest if someone does this, be sure to smoothly regulate the LNB case temperature so the LO is more stable.
 
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