Instruction manual

FMLP1 • 5
CIRCUIT DESCRIPTION
The FMLP1 is a lowpass filter. This means that certain frequency
elements can pass from the input to the output unimpeded while others are
almost stopped in their tracks! The name of this type of filter gives away its
response characteristics when a signal is applied to the input… lowpass filter…
low frequency signals pass through. Conversely, high frequency signals are
greatly reduced in amplitude virtually rendering them to insignificant levels.
The concept of a lowpass filter is quite simple, the design and
construction, however, require a good bit of engineering. All of the variables
that must be taken into account to construct a high-performance lowpass filter
are beyond the scope of this circuit description. The basic function of the actual
component elements used in this kit are what will be covered.
Take a look at the schematic diagram on page 11. A good quality filter
does not require a lot of components. The trick is to use the proper parts
chosen specifically for your application taking into consideration such things as:
-Cut-off frequency (f
c
), the frequency at which the signal amplitude drops by 3
dB, half the original power. The FMLP1 has an f
c
equal to about 120 MHz
giving plenty of headroom to pass the desired 88 to 108 MHz signal.
-Number of poles, the number of component elements or element groups
(parallel or series grouped reactive components i.e. ‘L1 & C3’ and ‘L3 & C8’)
determining things like the frequency roll-off characteristics and circuit cost.
-Input signal power, the amount of signal that can be applied to the circuit
without degrading its operation or causing damage. The FMLP1 is constructed
from 50 Volt parts setting its max input power at 50 Watts (thanks Mr. Ohm, R =
V / I; P = I x V; P = V
2
/ R therefore P = 50 volts
2
/ 50 ohms = 50 Watts). In
practice an over-rating of 20% yields dependable results at 40 Watts without
major concerns for cooling.
There are 13 basic electrical components consisting of 9 capacitors
and 3 inductors that work in conjunction to filter the applied signal. The
capacitor pairs ‘C1 & C2’, ‘C4 & C5’, ‘C6 & C7’, and ‘C9 & C10’ can each be
combined to form their respective parallel capacitive values. Using pairs in this
manner helps to reduce the amount of power each capacitor must dissipate
increasing operational efficiency. Remember, capacitors in parallel add, C1 +
C2 = 10 pF + 10 pF = 20 pF equivalent.
If these capacitors are combined into their respective capacitance, the
circuit can be reduced to 7 reactive elements. Note that ‘L1 & C3’ act together
as one element, the same goes for ‘L3 & C8’ as well. The basic structure of the
design consisting of shunt capacitors, except for C3 and C8, and series
inductors shows the classic lowpass filter layout.