local oscillator info

[ynnedibanez]

i have been trying to dig up some decent info on how the local oscillator in a lnb works, and also what the local oscillator frequencies for different lnbs are.
so far, what ive read says that the local oscillator frequency in a lnb mixes with the frequency received from the satellite, filters them both out, and sends the difference out to the receiver.
so according to that, if you are looking at a transponder that is at 12060mhz and your lo freq is 10750mhz the signal sent down the wire is 1310mhz
12060-10750=1310
that seems simple enough, but here is a curve ball,
i have a superdish stacked lnb.
it apparently has 2 local oscillators, selected by voltage on the cable.
i have read a lot of info on them, but everyone seems to disagree on what the actual frequencies of the two local oscillators are in these lnbs.
i have read 10000mhz, 10600mhz, 10750mhz, 11250mhz, 13850mhz, and 14350mhz!!!!
try to replace the 10750mhz lo freq. in the above equation with a few of these frequencies and it just doesnt make sense.
so does anyone know how this really works, and what the local oscillator frequencies are in this lnb?
thanks,
Denny

 

[Anole]

There are actually two Local Oscillators, but they are not selected by voltage on the cable.
The correct voltage is 18 volts, and doesn't change.
This gives you the ability to have long cable runs without so much voltage loss as to cause problems.
Your receiver will get both vertical and horizontal signals, and they are located in the 950-2150mhz range (as I recall).

Here are some links to good discussions by Iceberg in the past:
Iceberg on Bandstacked Dish Network LNB frequency considerations:
http://www.satelliteguys.us/free-air-fta-discussion/117442-surplus-super-dish.html
- which LNB to use on SuperDish:
http://www.satelliteguys.us/free-air-fta-discussion/159383-amc-21-superdish.html

 

[Mr Tony]

on SD bandstacked LNB's the vertical side (the "normally" vertical) is 10750 so the frequencies line up right. The high band I have no idea what it is. The bandstacked ones I had (Eagle Aspen) had 13850 for LNB LO and I just blind scanned vertical

Most receivers dont care what LNB LO you put in as it scans the IF frequency. So if you try 13850 it should work fine for the whole band

 

[ynnedibanez]

thanks, but how does the lo actually work?
im curious about the math, where does the 24600 come from?

 

[Anole]

the math:

Okay, subject to confirmation, let's see if I got this right.

In a normal LNB, the LO is 10750.
Being lower than the incoming signal, this is called low-side injection.
The incoming range is 11700 - 12200, MINUS the LO Of 10750 equal 950 - 1450mhz.
These are the vertical transponders in a stacked LNB.

rcvFreq....LO.......receiver tunes
11700 - 10750 = _950
12200 - 10750 = 1450

But, having the two local oscillators too close in frequency, leads to all sorts of bad things.
So, having the second oscillator set for high-side injection, you'd get this:
The incoming range is 11700 - 12200, MINUS the LO Of 13850 equal 2150 - 1650mhz.

rcvFreq....LO.......receiver tunes
11700 - 13850 = -2150
12200 - 13850 = -1650
This is where the horizontal transponders appear on your cable between LNB and receiver.

Ignore the negative sign on the frequency. It's meaningless.

So, the Vertical transponders occupy the range 950-1450 on your cable, and Horizontal is located in the 1650-2150mhz range, and backwards.
Now, any simple splitter can feed multiple receivers, and each receiver can select vertical or horizontal, without interfering with any other receiver.

 

[Mr Tony]

you are right oh green one

very well laid out

 

[ynnedibanez]

very good info!! thanks
i pointed this thing at 101w,
set my lo freq. to 10750 and did a blind scan on my pansat 2700a
it scanned RTS in at 12119 and lyngsat has it posted at 12120,
so i assume that the vertical lo is currently oscillating at 10751mhz
i then set the lo to 13850 and did another blind scan
Esperanza scanned in at 15715, so 15715-13850=1865
lyngsat has it listed at 11983, so 11983+1865=13848
so i assume that the horizontal lo is currently oscillating at 13848mhz
i assume that the temperature being 31 deg f outside might be affecting this.
i have read that the frequency of the local oscillator in a lnb is controlled by a small piece of ceramic in the lnb.
i would guess that expansion and contraction of this piece of ceramic by temperature outside would cause the lo frequency to shift slightly.
what do you guys think?

 

[Mr Tony]

if the frequency is off by 1 thats fine. Blind scanning will do that

When I blind scan a sat I've seen the frequency be off by + - 3 off the "published" frequency.

Cold actually gives more stability. I notice that here in MN when it was -31 in January

 

[Pepper]

Your frequency is off by only 1, that's great. Some of those DishPro LNBFs are notorious for frequency drift. I have a couple that are off by about 8.

 

[ynnedibanez]

well, with a little more research and math with the figures i have gotten so far,
for horizontal,
tp14 at 11983mhz with my horizontal local oscillator oscillating at 13848mhz makes my intermediate frequency 1865mhz
my pansat maxes out at 2150mhz which means that 12268mhz is my horizontal max
the low side on the pansat is 950mhz
so 2150-950 gives me a range of 1200mhz on the pansat
max freq. -receiver range for the lowest horizontal frequency i can receive with this setup.
12268-1200=11068 horizontal min
for vertical,
tp21 at 12120mhz with my vertical local oscillator oscillating at 10751 means my IF is at 1369mhz so with 2150 as my max IF input, that leaves me with 781mhz further that i can go up, so my vertical max is 12901mhz
and 12901-1200=11701 for my vertical min
i have noticed that with this setup, i was able to scan in tp 27v at 11655mhz
so it seems like my receiver is actually going lower than 950mhz because this transponder seems to be coming in at 904mhz
11655-10751=904
is all this correct?