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Re: [Discuss-gnuradio] DBS Dish Feeds
David I. Emery
Re: [Discuss-gnuradio] DBS Dish Feeds
Tue, 6 Dec 2005 23:59:18 -0500
On Tue, Dec 06, 2005 at 04:34:34PM -0800, Matt Ettus wrote:
> DBS small satellite dish systems often have LNB feedhorns capable of
> multiple polarizations and/or multiple bands. This is often controlled
> by changing the voltage which powers the device. However, some have a
> more complex control system involving tones in the 20kHz range if my
> memory serves correctly. Is there anyone who has a reference on this
> control system, or who knows how it works? I want to use these for
> radio astronomy with the USRP and need to know how to control them.
This is something called DiSEqC control and is documented in
various ETSI specs available on the web (you have to register an account
with ETSI to get copies) There are several generations of this
protocol in use, all of which use superimposed 22 khz tones on the 13 or
18 volt power supply to the LNB or LNBF.
A quick check shows that EN 61319-1 is one of the controlling
specs with several implementation levels specified. I think I remember
there are a couple of others involved as well.
Basically the simplest versions use the presence or absence of
low level 22 khz tone on the LNB power on the 75 ohm coax to select the
lower or upper of two LO frequencies in "universal" European LNBs which
cover 10.65 to 12.7 ghz and require two selectable LO frequencies to get
all that bandwidth to fit into the standard 950-2150 satellite TV IF
range. Usually the presence of 22 khz selects the higher LO frequency.
The most standard (Level 1) configuration is for the
polarization (either RHC or LHC or Vertical or Horizontal) to be
selected by choice of 13 volts or 18 volts up the cable and the upper or
lower LO frequency to be selected by the 22 khz.
But as home satellite systems in Europe where this protocol
originated have become more complex with multiple clusters of DBS
satellites at various longitudes and thus potentially multiple feeds on
multiple dishes in a typical installation this protocol has been
extended greatly by allowing the 22 khz tone to be amplitude shift keyed
(ASK) with digital signaling that can be used to control both cascades
of rf switches and more significantly small motorized antenna
positioners powered by the LNB 13 or 18 volts up the cable. This
allows small (usually under 1 meter) Ku band dishes to be completely
steerable (in the plane of the Clarke orbit) with only one single coax
and no other control wires running back to the receiver which obviously
has considerable advantages cost and convenience wise.
The latest versions of the DiSEqC spec anticipate use of small
microcomputers (PIC class) to interpret the 22 khz ASk signaling on the
cable and thus can provide quite complex control of antenna mounted
intelligent devices (including the ability to position to point at
particular longitudes or satellites as in the USALS version of DiSEqC).
This all replaces a group of interfaces that grew up with the US
domestic C band TVRO market that has traditionally used three wires for
PWM signaling for polarity control using a modified RC airplane servo
motor to rotate a probe in the feed, and +-36 volt DC power to drive a
motorized jack-screw to move the dish back and forth (with several amps
of current in larger installations). Position feedback has usually been
via optical interrupters or reed switches driven by rotating magnet
wheels - both of which generate contact closures as the jack-screw shaft
turns which get counted to determine postion.
Obviously this US TVRO interface requires multiple wires (ribbon
cables with all the usual wires and a couple of coaxes for C and Ku band
IF were the standard) which is more expensive than just one 75 ohm coax,
but of course with 36 volts at several amps there is much more energy
available to move a dish than with perhaps half an amp at 13 volts.
But maybe more relevant, the US DBS industry (currently DISH and
DirecTV) adopted the 22 khz and 13/18 volt signaling scheme to control
selection of multiple LNBs and polarizations in US DBS systems operating
at 12.2 to 12.7 ghz. Universally US DBS systems use the 13 and 18 volt
signaling on the cable from the set top box to the dish to select RHC or
LHC polarization, and current DBS systems with multiple feeds on on one
dish (local channels or HDTV or both are on satellites at other
longitudes because of limited bandwidth in 12.2-12.7) now use 22 khz
tone to select which LNB to get signal from. AFAIK, use of ASK of the
22 khz tone to perform more complex functions in US DBS antenna systems
is only just starting (I think in some DISH network installations),
most just use 22 khz on and 22 khz off as the only choices, though there
has been some limited use of 11 khz to select RF inputs as well.
Multiswitches for the US DBS industry are a standard cheap
product, and provide the ability to select one of four (or more) RF
basebands by the 13/18V and 22 khz signaling. All US DBS systems use
LNBFs so polarity control is universally via the 13/18V and most DBS
LNBFs have two output ports allowing two receivers to share one LNBF or
a multiswitch to have both LHC and RHC inputs for each LNBF on the dish.
On a related topic, in addition to the PWM modified RC airplane
servo polarity control, some older LNBFs including some commercial cable
gear have used 12 volt and 0 volts to control polarization and good bit
of commercial satellite head end gear had provision for outputting 12
volts or 0 volts on polarity changes to switch RF relays or control
older LNBFs that used this protocol. But more or less the completely
universal standard in the last 5-10 years has been the 13/18 volt
polarity shift with 22 khz selection of LO frequency being also pretty
standard in contempory gear.
I have yet to have time to study the dbsrx to see if you
provided any mechanism for injecting external LNB power (the tuner used
surely must) or 22 khz signaling - but as I have some projects involving
using the USRP to actually look at real satellite signals I surely shall
have to eventually handle this somehow.
Obviously a fully satellite compatible dbsrx would include
provisions for selectable 13/18/24 volt power up the cable and for
software controlled 22 khz tone injection on the power with a bit
in a register somewhere to turn the tone on and off. I believe the
signalling rate is low enough so control of this by bit banging in
host code is quite practical, so support for this can be in the
code and not hardware.
I might add that if I was trying to do radio astronomy using
commercial satellite LNBs or LNBFs I'd probably want to use higher end
commercial grade (made for VSAT, TV stations, cable head ends and the
like) units which have much better flatness, gain stability, out of band
signal rejection and LO frequency drift and accuracy than the cheap crap
sold for consumer use. Norsat and NJR are the big brands here, with
Calamp also making some decent stuff.
Dave Emery N1PRE, address@hidden DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."