Bandpass Ladder Filters.
Quartz Crystals
In addition to low- and high-pass filters, ladder filters can also
be used to construct higher-order bandpass and bandstop (i.e.,
“notch”) filters. Circuit diagrams for a four element (i.e., second order) bandpass and bandstop filters are shown below.
For bandstop filters, simply interchange the two sections
These filters can be synthesized using the same filter tables we
used for low- and high-pass ladder filters.
However, there are some important differences. Here is the
procedure for a bandpass filter:
1. The filter tables are used to compute the series inductances
and shunt capacitances as we did with the low-pass filter.
2. When un-normalizing, use the 3-dB bandwidth ?? rather
than ?c to find the L’s and C’s.
3. Finally, compute the series C’s and parallel L’s using the
resonant frequency condition ?0 =1 under root LC .
We will now consider the same example shown in the text on
pp. 104-105. (In Prob. 8 you will build and test the series LC
network portion of the RF Filter.)
Example – Here we will design a second order Butterworth
bandpass filter (Fig. 5.7a) using L1 and C1 as one section. We
require that f0 = 7 MHz and R = 50 ?.
With L1 and C1 specified, we have one half of the filter already:
We will now consider the same example shown in the text on
pp. 104-105. (In Prob. 8 you will build and test the series LC
network portion of the RF Filter.)
Now, from Table 5.1, a1 =under root 2 . Recall that
Consequently, ?? = a1 R /15×10?6red or ?f = ??/2?= 750 kHz. This ?f is the 3-dB bandwidth of the filter. (At 7 MHz, this ?f is somewhat large, meaning this is a relatively low- Q filter.)
Next, using the filter table again, we will determine the shunt C2
From
The complete filter design is shown below
While this example was seemingly just an exercise, believe it or
not, the actual RF Filter in the NorCal 40A is a two-element
Butterworth bandpass filter! L1 and C1 are the series L and C,
but where is the parallel L and C?
This second section is provided by the primary winding of T2
(? 66 nH). The transformer also transforms the impedance of C2 to the primary side (you’ll see this later in Lecture 15).
Consequently, 20 p C ?? nF or so. That’s just what’s needed for
this second order Butterworth bandpass filter! You’ll see more with this aspect of the RF Filter in Prob. 16 when you construct and install T2. Quartz Crystals
The maximum Q (minimum bandwidth) of LC ladder filters is
usually limited by the inductor Q (i.e., the inductor losses).
Other types of resonant elements must be used if Q’s higher than
a few hundred are desired. Such Q’s are useful from audio to
microwave frequencies.
Quartz crystals are one such element. These are made from
silicon dioxide, which is cheap. The Q’s of such quartz crystal
resonators range from 25,000 to 150,000!
Another advantage is that a small temperature coefficient can be
obtained for quartz crystals. This is useful so that the resonant
frequency drift with temperature is minimized as the transceiver
warms up, or in other situations. Quartz crystals make good electrical resonators because of the
piezoelectric effect. This effect is a combination of a mechanical
vibration and bound electric charges. When a quartz crystal is squeezed, a voltage is produced:
Electric starters for gas furnaces, water heaters and grills use
such a piezoelectric effect. This piezoelectric effect also works the other way. A voltage applied across a quartz crystal causes a small expansion of the crystal
A microscopic view of the atoms in the quartz lattice helps us
understand this piezoelectric effect
These quartz crystals can be modeled as RLC filters in electrical
circuits:
The equivalent electrical circuit for the quartz crystal is
You will operate your quartz crystals in series resonance in the
NorCal 40A. Why is there variation with frequency? Because the mechanical vibrations of the lattice will not be as favorable for llfrequencies of voltage excitation. At some frequencies, the lattice vibrations are maximum.
