Syllabus Sections:-
Filtering and remedial measures
6D1 43 Understand the use of high, low, band
pass and band stop (notch) filters of L, T and π
configuration, including coaxial stubs as notch filters
or traps, in improving the immunity of affected
devices.
Filters must be used in equipment at the earliest
part of the circuits and this is carried out in the design
of the equipment.
A low pass filter (LPF)
A low pass filter (LPF) is designed so that it
passes frequencies that are lower than that of the stop band
so in effect signals above the cut-off frequency (fc) are
reduced.
A high pass filter (HPF)
A high pass filter (HPF) is designed so that it
passes frequencies that are higher than that of the stop
band so in effect signals below the cut-off frequency (fc)
are reduced.
NOTE
: the LPF and HPF are sometimes called pi filters as they
resemble the Greek letter pi Π
A band pass filter (BPF)
A band pass filter (BPF) is a combination of low
pass and high pass filter that will passes a range of
frequencies in the pass band, any frequencies above or below
this range are reduced. Unlike the LPF and the HPF, the BPF
has two stop bands and two cut-off frequencies (fc) at the
meeting points of each of the stop and pass band.
with the use of filters remember that if one type
is not sufficient then you can also use another in what is
called a "cascade" of filters.
For the band stop filter see page 91 Fig 13.16 and there is text
to read.
Also
check out page 90 Fig 13.13 and Fig 13.14 for
different type of High pass and Low pass filters in the "L"
(inverted) configuration but using resistors in place of
capacitors.
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Notch
filters including coaxial stubs as notch filters or traps
A stub is connected to a feed line. (parallel to
it) It can be a length of co-ax, (connected inner to inner,
outer to outer when connecting to the feed line) or it can
be a length of open wire feeder. ("left wire" of the stub to
"left wire" of the feeder, right to right...)
Stubs are usually made to be an ELECTRICAL 1/4
wavelength at the frequency of interest. 1/2 and 3/4 wave
stubs can be used, but these are a) longer, and less
convenient, b) more lossy, c) less "Q" and hence have a
wider bandwidth (Although this may be desirable, and hence
is the reason such longer stubs are used).
What happens at the far end of the stub is
interesting. For odd 1/4 wavelength stubs (1/4, 3/4, etc) an
OPEN circuit at the end results in (AT THE FREQUENCY OF
INTEREST) a short circuit at the end connected on the feed
line. And vice versa...
For EVEN wavelengths (1/2, 1, 1/1/2 etc) what
happens at the end of the stub is what happens at the end
connected to the feed line. Again, ONLY AT THE FREQUENCY OF
INTEREST.
You can now see that it is possible to arrange for
a transmitter to have a short circuited 1/4 wave stub to be
connected on its feed line. As it is shorted at the far end,
then AT THE FREQUENCY OF INTEREST, the stub produces a very
high impedance across the feed line, and has little/no
effect. But at different frequencies, the stub can look like
a short circuit, and cause a bad VSWR, causing those
unwanted frequencies to be reflected back to the
transmitter, and dissipated in the output stage. (And not
sent to the antenna!).
The application for a receiver is that you have an
OPEN end on the stub. This causes signals coming from the
antenna (unwanted, strong, interfering signals, on a single
frequency) to see a bad VSWR, and return to the antenna, and
not be fed to the receiver. Typical application here is an
open circuit stub cut for two metres, to stop, say 2M
interference to an FM broadcast RX.
You have to be careful, because as stated above,
stubs also work when 3/4 wavelength long.
This means that you may find you use a 1/4 wave
stub on a 2M transmitter, hoping to stop the third harmonic
on 70cms from being radiated. But that stub is a 3/4 wave
stub on 70 cms :-(
The trick is to cut the stub short, and retune with
a trimmer capacitor across the end. The stub in the
transmitter feed line then does its job on 2M, allows that
through, but is no longer a 3/4 wavelength on 70cms.
So it does not allow that through.
Practical application of shortening and tuning is
to have a 2M 1/4 wave stub on a TV coax to keep 2M out of
the TV. This will also stop signals on 720MHz (5 times 144)
as it will be a 5/4 length stub (1 1/4 wave). If this
happens to be the TV channel of interest, you shorten the
stub, and retune with the trimmer to 2M, and then the
"overtone" is no longer on 720MHz. It will be somewhere, but
as long as it is not where the neighbour wants to watch a
particular channel, all well and good. Of course, if he is
trying to watch a channel on that frequency, you had better
have no 5th harmonic from the transmitter anyhow!
A stub is intended to work on one set of
frequencies. Hence if it is working at the frequency of
interest, it will "kill" or allow. If you are not in this
ballpark, the stub will cause the feed line to see either an
inductance, or capacitance, and you will get a bad VSWR. So
no, you cannot use stubs on a feed line unless they are cut
for the frequency involved. This means that you need to have
a "T" piece in the coax, and connect the appropriate stub.
In the case of a receiver, one stub per band, which will
exclude the nearby transmitter on another band, BUT remember
the nearby transmitter will be producing unwanted harmonics,
which will be where you are trying to receive, so the stub
will allow these through :-( Also, if you have a stub on the
receiver, at 7MHz, and the nearby TX is 21MHz, the stub
won't help, as it will pass odd multiples of its design
frequency.
You can use more than one stub on a feeder when
they are all cut for the same band. You make each an
electrical quarter wavelength long and you space them along
the feeder an electrical quarter wavelength. This
arrangement works on one frequency, as a single stub does,
but the attenuation of unwanted products is much higher :-)
You MAY be able to use a 144MHz stub on an HF feed,
but it will cause some VSWR. Remember, it will be short in
length compared to the HF frequency, so effectively, you are
connecting a capacitor across the HF feed line. A much
better arrangement is to have a low pass filter for the HF
RX. If a transceiver, you should have a low pass filter
anyhow!
Another useful application for an open wire
feeder/stub is to make a short circuited version to only
allow the frequency of interest to pass. The far end from
the feeder is shorted. And can be connected to earth. Useful
lightning protection. Will work on 7MHz and 21MHz but that
you can't do this with coax.
Stubs have other uses, mainly on open wire feeder.
In this case, you can actually design the stub to tune the
feed line, as it will add capacity/inductance. The trick
there is to know how long it should be, and on what part of
the feed line it should be connected!
6D1 43 continued Recall
the use of ferrite beads or rings in internal and external
filtering.
Sometime
the filtering has to be carried out inside the equipment. Here
the "tiny" ferrite bead (FB) is actually used on the "BASE"
leg of a transistor or adjacent PCB track is broken and a bead
and wire soldered in as a replacement.
See also page 94
Fig 13.22 which explains in more detail.
Note the FB is placed on
the BASE leg of the Transistor
The ferrite bead is acting as an RF choke just like
it did when using the ferrite ring on interconnecting cables
externally, it is used internally in equipment over the BASE
legs of the transistor.
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6D2 43
Understand the construction and use of a typical mains
filter.
For this
see
page 92 Fig 13.19 which shows the circuit diagram of a mains
filter.
The
simplest mains filter to fit is the "ferrite ring" where
several turns of the mains cable is wound onto a ring or
several rings stacked up - with the assembly bring placed as
close to the chassis of the equipment as possible.
The ends
of the cable should be kept apart and the turns on the ferrite
only occupy about 2/3 of the diameter.
See also
page 92 Fig 13.20
This
filter is know as a ferrite ring choke.
Whilst
here it is coaxial cable wound on the ring the principle is
the same.
The
diagram above is the equivalent circuit of the ferrite ring
filter - note the earth core of the cable is not shown as it
is only the live and neutral cores that would carry any
interference into or away from the equipment.
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6D2 43
continued Identify a
typical circuit of a braid-breaking filter and a combined
high-pass/braid-breaking filter. Understand their use.
The section deals with filters used on antenna
feeds and NOT mains filtering.
Ferrite Ring
The braid
breaking filter can also be the simple ferrite ring as shown
above.
The
circuit is similar to that above.
The
ferrite ring may be the usual one you can buy at rallies but
sometimes the ferrite is not "correct" for the frequency of
the interference and a different "mix" of ferrite is required.
More
information on this is in page 93 Fig 13.21
High
pass/ braid breaking filter
The
combined high pass/ braid breaking filter is more complex but
is basically a high pass filter with a resistor across one of
the capacitors to allow a continuous link for the braid
through the resistor.
Each of
the filters works to stop the unwanted signals and pass the
wanted signals. They are placed as close to the chassis of the
equipment as possible.
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6D2 43
continued Understand why
a ferrite ring will attenuate common mode currents without
affecting the differential mode wanted signal.
Many
problems with stereos can be traced to common-mode propagation
currents on long speaker leads and other interconnecting
cables.
A
common-mode current is an unbalanced current flowing in one
cable which is not balanced by a similar current flowing in
the return cable when making the circuit complete and is
caused by unwanted external signals being applied.
The
differential mode or signal current is the wanted current.
When a cable is wound on a ferrite ring it creates
a series inductance in the lead which increases the
inductance of the leads and is seen as a high impedance to
RF but allows the differential mode or wanted signal to pass
unhindered.
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