Interference type versus signal detection type.
There are two types of interference from channelling standpoint,
co-channel and off-channel.
Co-channel interference (1st) takes place when one or more additional
Tx is operating on to a busy channel. Result is the reduction of
signal quality marked by the ratio = (signal) to (noise +
interference*). where (*) marks the variable.
Quasi-co-channel interference (2nd) takes place when a mirror image of
the wanted signal is reproduced by reflective object and find a way to
the same Rx. It is called multipath fading. Same effect as above.
Quasi-co-channel interference (3rd) takes place when metal to metal
junction is rolling, sliding or make/break in operation or under
vibration. It produce RF signal phase modulation. It share the same
appearance as above.
Two Tx co-channel in the extreme case produce a capture effect where
the unwanted co-ch-Tx is taking over the authority of the Rx when the
same Rx to Tx spacing is larger than 3~4X as far to the wanted Tx than
it is to the 2nd Tx. When the Rx is equal distance to the two, none of
the two Tx is in command. Some servo in a PPM system will bang from
limit to limit.
Off-channel interference (1st) takes place when one or more Tx is
operating next to a busy channel with no specific channel
relationship. Result is the reduction of signal quality marked by the
ratio = (signal x B*) to (noise). where (*) marks the variable. B is
the blocking factor, its effect is RF signal amplitude reduction.
Off-channel interference (2nd) is like (A) but with specific channel
relationship. (If desire channel is ch(c) and the off-channel are
ch(a) and ch(b) such that c=2a-b; or c=2b-a' a and b are integers) The
effect is taken as co-channel effect.
Perceived Rx interference takes place when we are not aware of a
normal servo operation may be subject to RF interference when the
servo signal cable acts like an antenna and its electronics acts like
a receiver under strong Tx signal at close proxmity. Servo twithcing
when Tx is ON may be the symptom. Such causes may be verified by an
experimental 5-cell battery, if symptom persists that proves that
electronics inside servo under test has poor RF immunity, or is
supporting co-channel the (3rd kind) interference. If problem goes
away, maintain the 5-cell battery system.
Interference priority in top down order:
co-channel (1st kind) ;deadly and wide coverage
off-channel Blocking ;deadly but 3meter coverage
co-channel (3rd kind) ;dangerous but avoidable
off-channel (2nd kind) ;dangerous when Tx are clustered
co-channel (2nd kind) ;concern for low and far planes.
Interference sources:
co-band RC equipments
co-field reflective objects
on-board metal junctions
servo under Tx radiation
co-band non RC equipments
off-band equipments.
Conclusion:
I can see two categories, Deadly and Not Deadly.
Deadly:
Any thing that is persistent and strong are those 1) stupid co-channel
RC Tx and 2) those big boys paging station. No known technology in RC
today can safe our planes from them. That include PCM, IPD etc...
Not Deadly:
Are event we can get away since those occurance are by chance (when
they exist) and do not take place every stretch of air we fly through.
Any radio with compliance to AMA guideline should be immune to that.
YK Chan
Seattle
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED]
