And Finally
How
do 'real world' lnbs stack up against the Eutelsat recommendations?
The
following table shows the specification of the Cambridge AE57 lnb, a popular,
attractively priced lnb.
| |
Manufacturer's Specified value.
|
Meets Eutelsat Requirement?
|
|
Model No.
|
AE 57
|
Not applicable
|
|
Size
|
115 X 55 mm
|
Not applicable
|
|
External Feed Dia.
|
40 mm
|
Not applicable
|
|
Weight
|
250g
|
Not applicable
|
|
Output Connector
|
SINGLE F-Type Female
|
Yes
|
|
Input Frequency 1
|
10.7 to 11.7 GHz
|
Yes
|
|
Input Frequency 2
|
11.7 to 12.75 GHz
|
Yes
|
|
Band selection RF1 to RF2
|
High band by 22KHz tone
|
Yes
|
|
Output Frequency 1
|
950 to 1950 MHz
|
Yes
|
|
Output Frequency 2
|
1100 to 2150 MHz
|
Yes
|
|
Input VSWR
|
2.5:1 typ
|
Not in spec, 1:1 is optimum,
but this is OK
|
|
Output VSWR
|
2.0:1 typ
|
Not in spec, 1:1 is optimum
|
|
Conversion gain
|
48dB min dB min
|
Yes - a little low
|
|
Gain Flatness
|
+/- 0.5dB over 26 MHz segment
|
Yes
|
|
Noise figure
|
0.6dB dB
|
Not in Eutelsat spec.
|
|
Local Osc. RF1
|
9.75 GHz
|
Yes
|
|
Local Osc. RF2
|
10.6 GHz
|
Yes
|
|
Loc. Osc. Stability setting
|
+/-2 MHz
|
Yes - even when combined with
below
|
|
Loc. Osc. Stability temp.
|
+/-2 MHz (-30 to 60°C)
|
Yes- even when combined with
above
|
|
Phase Noise
|
|
|
|
1Khz
|
-50dBc dBc
|
Yes (just)
|
|
10kHz
|
-75dBc dBc
|
Yes (just)
|
|
100kHz
|
-95dBc dBc
|
Yes (just)
|
|
Cross polar isolation
|
20 dB typ
|
Not in spec.
|
|
3rd Order Intercept
|
+15dBm typ
|
Meets the Prof.'s recommendation.
|
|
Current drawn
|
150mA max
|
Yes
|
|
Polarity
|
V/H
|
Yes
|
|
Supply voltage
|
11.5-14.0 v
|
Yes
|
|
Supply voltage
|
16.0-19.0 v
|
Yes
|
|
Digital LNB
|
Yes
|
N/A
|
Not
surprisingly, all the items in the Cambridge specification appear to meet the
Eutelsat requirements.
Hobbyists
looking for more difficult signals might profit from more gain, higher frequency
stability, and lower phase noise, but such things will cost them more money!
Lets
see what MTI gives us in their AP8-TW lnb for just a few Euros more
| |
Manufacturer's Specified
value.
|
Meets Eutelsat Requirement?
|
|
Input Frequency
Range
Low Band:
High Band:
|
10.7 GHz ~ 11.7 GHz
11.7 GHz ~ 12.75 GHz
|
Yes
|
|
Output Frequency
Range
|
Low Band: 950 ~ 1950
MHz
High Band: 1100 ~ 2150 MHz
|
Yes
|
|
Output Connector
Type
|
75 Ohm Female Connector
|
|
|
Output VSWR
|
2.0 : 1 (Max.) @ 20oC
|
Better than Cambridge
lnb.
|
|
Local Oscillator
Low Band:
High Band:
|
9.75 GHz
10.6 GHz
|
Yes
|
|
L.O. Frequency
Stability
|
±1 MHz(Max.)
±3 MHz(Max.) @ - 40oC ~ + 60oC
|
Yes, better than Cambridge
lnb.
|
|
L.O. Frequency
Phase Noise
(@ Room Temperature)
1
kHz
10
kHz
100
kHz
|
- 50 dBc/Hz (Max.)
- 75 dBc/Hz (Max.)
- 95 dBc/Hz (Max.)
|
Yes, possibly better
than Cambridge, as these are maximum values.
|
|
Conversion Performance
Conversion Gain
|
55 dB (Typ.)
60 dB (Max.)
|
Yes, better than Cambridge
lnb.
|
|
Gain Flatness
(across operating
band)
(across any 26MHz
segment)
|
5 dB p-p (Typ.)
±0.5 dB (Typ.)
|
Yes
|
|
1dB Gain Compression
|
5 dBm (Typ.)
|
Meets the Prof.'s
Criteria.
|
|
Noise Figure
Low Band:
High Band:
|
0.6 dB (Typ.)
0.6 dB (Typ.)
|
Not in Eutelsat spec.
same as Cambridge lnb.
|
|
Image Rejection
|
45 dB(Min.)
|
Not in Eutelsat spec.
|
|
Cross Polarisation
Isolation
|
25 dB (Typ.)
20 dB (Min.)
|
Not in Eutelsat spec.
possibly better than Cambridge lnb.
|
|
DC Current Consumption
|
110 mA (Typ.)/150
mA (Max.)
for AP8-TW
|
Yes
|
|
Operation Voltage
Vertical:
Horizontal:
|
11.5 ~ 14 Vdc
16 ~ 19 Vdc
|
Yes
|
|
Band Switching
Low
Band:
High
Band:
|
0 Hz
22 ± 4 kHz
|
Yes
|
|
Operating Temperature
Range
|
- 40oC
~ + 60oC
|
Not applicable
|
|
Storage Temperature
Range
|
- 55oC
~ + 80oC
|
Not applicable
|
|
Spurious Response1700
MHz
|
- 57 dBm (Max.)
|
No (almost good enough)
Not specified for
Cambridge lnb.
|
Spending
a little more money does apparently get us a slightly better lnb, there's more
gain, the frequency stability is a bit better, but the noise performance
is about the same. Perhaps this would be a better lnb for searching out
weaker MCPC channels on a weaker satellite (such as Sirius in the UK).
What
is interesting here is the spurious signal response measured at 1700 MHz. This
says nothing about what the spurious signal level at other frequencies might
be, but it is claimed to be a maximum (worst case) figure, so perhaps the one
you buy will be significantly better than this. The Cambridge lnb makes
no mention of the spurious responses in its spec so it seems reasonable to assume
that it can be no better than the MTI lnb, and very likely worse in this respect.
What
happens if we spend a lot more money?
Here
are the specs for the Invacom TWH-031 lnb - it's not cheap!
| |
Manufacturer's Specified
value.
|
Meets Eutelsat Requirement?
|
|
Input Frequency
Low Band
High Band
|
10.7 - 11.7 GHz
11.7 - 12.75 GHz
|
Yes
|
|
Output Frequency
Low Band
High Band
|
950 - 1950 MHz
1100 - 2150 MHz
|
Yes
|
|
Noise Figure
|
0.3 dB typ
|
Not in Eutelsat spec.
|
|
Gain
|
50 - 60 dB
|
Yes
|
|
Gain Ripple
in 26 MHz bandwidth
Low Band
High Band
|
<+/-0.5 dB
<5 dB typ
<5 dB typ
|
Yes
|
|
Local Oscillator
Frequency
Low
High
|
9.75 GHz
10.6 GHz
|
Yes
|
|
Local Oscillator
Phase Noise (typ)
1kHz
10kHz
100kHz
|
-65 dBc/Hz
-95 dBc/Hz
-110 dBc/Hz
|
Yes, much better than
Eutelsat spec.
|
|
Local Oscillator
stability
(including Setting,
aging and temperature drift)
|
+/-1 MHz typ
+/-2 MHz max
|
Yes, better than MTI
or Cambridge.
|
|
Current Consumption
|
190mA typ
|
Yes
|
|
Image Rejection
|
>40 dB
|
Not in Eutelsat spec.
Worse than MTI.
|
|
Cross Polar Isolation
|
>20 dB
|
Not in Eutelsat spec,
same as MTI
|
|
High to Low Band
Isolation
|
>25 dB
|
Not in Eutelsat spec
- could be better.
|
|
Two Tone 3rd
Order intercept point (output)
|
>15 dBm
|
Meets the Prof.'s
criterion.
|
|
Output Connector
|
2x female F-Type
|
Yes
|
|
Impedance
|
75 Ohm
|
Yes
|
|
Return Loss
|
>10 dB
|
Yes
|
|
Operating Temperature
Range
|
-40°C to +70°C
|
Not in Eutelsat spec.
|
|
Storage Temp Range
|
-40°C to +70°C
|
Not in Eutelsat spec.
|
|
Band Polarization
Selection
Vertical Polarization
Horizontal Polarization
|
11.5V to 14V
15.5V to 19V
|
Yes
|
|
High Band Selection
(22kHz tone) Frequency ( square wave with controlled rise/fall transition
time) Level
Transition time
Duty Cycle
Load Impedance
at 22kHz
|
18 kHz - 26 kHz
0.4 Vpp - 0.8 Vpp
5µs -15µs
40% - 60%
>70 Ohm
|
Yes
|
|
Low Band Selection
|
No tone
|
Yes
|
|
In Band Spurious
(primarily 1700MHz)
|
<-65 dBm
|
Yes
|
|
Out of Band Spurious
(primarily 850MHz)
|
<-45 dBm
|
Not in Eutelsat spec.
|
|
Output Gain Difference
(between the outputs
in 26MHz Bandwidth)
|
<6 dB
|
Not in Eutelsat spec.
|
Spending
the extra money appears to buy something, but you do not get more gain than
the MTI lnb gives you, and the noise factor, though lower, will not make much,
if any, difference to the weak signal capture abilities of your receiver. The
local oscillator frequency stability is better, and the phase noise is much
lower, so if you are interested in winkling out those SCPC feeds, then this
should give you a much better chance.
It
is a lot of money, however, and if your interests are with MCPC broadcasts
from the weaker satellites, then buying a bigger dish would certainly seem to
be the better solution.
Summary
Expensive
lnbs do have something to offer the enthusiast, but for most people they aren't
necessary.
The
golden rules are:
1. Read the specification sheet, and know what it's telling you - and what
it might be trying to hide.
2. If there isn't a spec sheet available, be very careful. There are plenty
of lnbs about that do have a meaningful specification sheet, which will give
you some idea of the performance you can expect.
3. Noise figures are only a part of the story, and, according to Eutelsat,
they aren't even relevant!
4. A bigger dish is a better solution for most people who have reception
difficulties.
The Professor
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