July 1957 Radio & TV News
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television
News articles I have already posted. All copyrights are hereby acknowledged.
This article on crystal filters will probably be more useful to
people responsible for maintenance on old RF systems than for new
designs. The technology has come a long way since 1957. Crystal
filters were heralded as godsends as airwaves became more crowded
and simple LC filters could not provide the required Q to prevent
cross-channel interference. Of course the problem is many times
worse today, but components are better now than then with low-cost
integrated circuit front ends that handle a lot of the selectivity
issues and SAW filters with better performance than many crystals.
Super Selectivity with Crystals
by Richard F. Burns, W9NVC
c.w., phone, and single-sideband reception for your receiver with
an effective crystal filter.
Because of excessive crowding in the amateur bands, an i.f. stage
filter in the communications receiver is considered a necessity
by most amateur radio operators today and it is for this reason
that radio manufacturers include a filter as standard equipment
with their higher-priced communications receivers. In their striving
for extreme selectivity, manufacturers have resorted to crystal
filters, mechanical filters, complicated coil and capacitor filters,
and dual conversion in conjunction with a low second i.f. The crystal
filter, in one form or another, is still the favorite among hams,
however, because of its simplicity, stability, and relative low
cost. The filter to be described rivals the best narrow-band mechanical
filters available today insofar as selectivity characteristics are
concerned; has no greater passband insertion loss than a mechanical
filter; and can be constructed for less than the cost of an additional
i.f. stage, if the FT-241-A crystals now available on the surplus
market are utilized.
The filter circuit itself is a modification of the pilot frequency
selecting filters used extensively by the British Post Office Department.
It can be constructed from standard i.f. stage parts and presents
few constructional difficulties, as can be seen from the photographs.
Alignment of the filter may present some problems; but, if the instructions
given in the text are followed, success is assured.
The filter consists of two "half-lattice" filter sections connected
"back-to-back" in a configuration designed to make it possible to
insert the filter into the i.f. section of any receiver having the
appropriate i.f, frequency with a minimum of impedance matching
difficulty. In addition to impedance matching ease, the "back-to-back"
configuration has several advantages over the conventional crystal
filter circuit which employs one crystal in conjunction with a phasing
capacitor in a single section "half-lattice": Twice the discrimination
possible with the usual commercial crystal filter may be obtained;
and, since the attenuation outside the maximum rejection frequencies
is so much greater than that obtainable with the conventional filter,
the external phasing control may be eliminated - the maximum rejection
frequencies being permanently fixed by trimmer adjustment during
the alignment procedure. Much of the inconvenience experienced with
phasing out undesired signals with the conventional filter is thus
avoided, since the filter is always in correct operating condition
and may be merely switched in and out of the circuit like a mechanical
Theory of Operation
In operation, each "half-lattice" section of the filter will have
a maximum attenuation point ("rejection slot") at the frequency
where the reactance of the crystal arm is equal to that of the capacitor
arm. It will attenuate in the region where the reactances of the
crystal and capacitor arms are most nearly equal and will have a
passband where the reactances of the arms are of opposite sign.
This is illustrated in Fig. 1. In practice, due to resistance in
the circuit, stray coupling, impedance mismatch, and other factors,
the attenuation vs frequency curve of one section of the filter
will have the form of the dashed curve shown in Fig. 2.
Fig. 1. Reactance and attenuation curves.
Fig. 2. Solid curve is i.f. stage response without filter.
Dashed curve shows response with the single-section "half-lattice"
Fig. 3. Crystal filter circuit diagram.
Fig. 4. Measured response curve of filter.
Fig. 5. Set-up used to check the filter.
Fig. 6. Filter in a.v.c.-controlled stage.
As to the bandwidth possibilities of this type of filter, it
should be noted that the difference between the parallel resonant
frequency and the series resonant frequency of the crystal and its
shunt capacitance - (fb-fa) in Fig. 1 - is
one limiting factor. With a given value of crystal shunt capacitance,
the smaller this difference, the smaller will be the possible maximum
bandwidth of the filter. Thus, we may make the filter as narrow
as we like by increasing the crystal shunt capacitance, but, with
the FT-241-A crystals, we are limited to a maximum passband width
of approximately one kilocycle at the 3 db points, assuming a symmetrical
attenuation vs. frequency curve similar to that of Fig. 4. A wider
passband will result if both attenuation peaks are chosen to fall
on one side of the passband.
It may be noted that with the "back-to-back" configuration it
is possible to place one rejection slot on each side of the center-band
frequency, as indicated in Fig. 4, or unsymmetrically with both
on one side of the center frequency. If the filter is to be used
for c.w. exclusively, as was the one built here, the symmetrical
arrangement is to be preferred. For phone, the unsymmetrical arrangement
is preferable because of the greater passband width possible. The
filter, even with the unsymmetrical arrangement, however, may be
somewhat too sharp for pleasant phone listening for many operators.
The transformers, T1 and T2 in Fig. 3,
which were used in this filter were Miller push-pull, 455 kc. output
i.f. transformers, one of which was modified by the removal of one
input capacitor lead so as to achieve the series-tuned circuit shown
in the diagram. The center-tap lead of each secondary coil was brought
out through one of the holes in the top of the transformer can.
These two operations were performed after removing the fastening
nut at the top of the can and withdrawing the entire i.f. coil and
capacitor assembly, Extreme care should be exercised in handling
the coils as the Litz wire used in the coils is very fragile - breakage
of several strands of the wire will result in a change of the d.c.
resistance of the coil and will decrease the possible attenuation
at the maximum attenuation frequencies, In one of the transformers
used here there was a difference of 2 ohms in the d.c. coil resistance
as measured from the center tap to each coil end so resistive balancing
was utilized. This balancing consisted in the addition of a 2-ohm,
non-inductive resistor to the arm connected to the low resistance
winding. Resistive balancing, while not absolutely necessary, enables
both resistive and reactive balance to be obtained at the maximum
rejection frequencies and results in improved filter discrimination
characteristics. The balancing resistor is not shown in the circuit
diagram but, if one is needed, it should be inserted in the low
resistance arm in series with the ungrounded crystal or capacitor
electrode, as the case may be.
The transformers are mounted skew-wise with a 455 kc. crystal
on one side of each transformer as is shown in the photographs.
Such a mounting procedure helps to minimize the shielding problem.
Since one electrode of the crystal is at ground potential, the crystal
may be plugged into its socket so as to have the grounded electrode
outside with the other electrode shielded by the transformer can;
thus, the crystal is essentially self-shielding. If the filter is
to be installed inside the receiver near a potential noise source,
the crystals should be shielded more completely; but, since no such
difficulty was experienced here, extra shielding was not employed.
Capacitors C3 and C4 are securely fastened
to the chassis as far apart as possible. By mounting these capacitors
back-to-back with the back sides grounded, no extra shielding will
be needed - the grounded coax braid and chassis base cover offering
adequate isolation. Quarter-inch holes are drilled into the ends
of the chassis base cover so as to enable the capacitors to be adjusted
after the entire filter is assembled and the cover secured. Since
stray capacity may function as a part of the circuit capacity, the
chassis and base cover should be in place and securely fastened
The double-pole, double-throw switch used in the filter is a
double section of a two position band switch taken from an old broadcast
receiver. The contacts farthest apart were chosen as the input and
output contacts so as to minimize feed through. Some apprehension
was experienced regarding the feedthrough problem, but all of the
connecting leads were made with coax having the external braid grounded
to the chassis and the input-to-output capacitance was so low with
the switch in the "crystal-in" position that further shielding
was found unnecessary.
The electrical stability of the filter will depend upon the rigidity
and stability of the components, hence, all parts should be well
anchored and protected from vibration. As can be seen in the photograph,
the coax input and output leads have been securely fastened to the
chassis with metal holders to prevent their moving around after
alignment of the filter.
The crystals used were of the FT-241-A surplus variety, but any
set of crystals whose series-resonant frequencies are equal and
close to the receiver i.f. frequency may be used. It isn't necessary
that the crystals be of the same cut, but a difference in crystal
shunt capacitance will affect the position of the attenuation peaks;
hence, only the crystals which are to be ultimately used in the
filter should be used during the alignment process.
After the filter has been constructed, it is advisable to make
a few simple continuity tests with an ohmmeter just to be certain
that none of the transformer leads has been broken and that no high
resistance solder joints have been made. A high resistance connection
may ruin the performance of the filter.
The ideal filter alignment set-up is indicated in Fig. 5. A commercial
signal generator, a grid dipper, or the b.f.o. in the receiver may
be used as the signal generator indicated in the diagram. The b.f.o.
would, of course, have to be variable and rewired so as to inject
a signal into the grid of one of the i.f. stages preceding the filter,
if the receiver were to be used as the amplifier indicated in the
test set-up diagram. It is naturally preferable to align the filter
in the receiver for which it is intended, since the tuning of the
input and output circuits will depend upon the circuits to which
they are connected. An SX-43 receiver was used as both the amplifier
and voltmeter in the lab here - the S-meter being used as the v.t.v.m.
indicated in the diagram.
The filter is aligned in three steps: First, the input section
to the point marked A (see Fig. 3) is aligned. Second, the output
section of the filter, from A through T, is aligned, and; third,
the filter is connected to the receiver and both filter and receiver
In the first step, the lead connecting the two transformer center
taps is cut at the point marked A in Fig. 3. A signal at the crystal
frequency is fed into the filter by means of the input coax and
the primary and secondary of T, are peaked-measurement being made
with the crystal in place by means of the test set-up of Fig. 5;
output from A going through coax to the amplifier as indicated in
the diagram. The phasing capacitor, C3, is varied so as to place
the rejection slot on one side of the i.f. frequency. The closer
the rejection slot is to the i.f. frequency, the narrower will be
the bandwidth of the filter and the less will be the attenuation
outside the rejection frequencies. Half-a-kilocycle from the i.f.
frequency is a satisfactory setting.
In the second step, the filter is turned around and the identical
procedure as was used for the input section is followed to align
the second section of the filter.
The third step consists in re-connecting the two transformer
center taps and adjusting the transformer capacitors once more after
the filter is installed in the receiver. If the receiver has no
a.v.c. applied to the i.f. stage into which it is desired to insert
the filter, the filter may be simply inserted in series with the
grid of the i.f. tube, re-adjusting the transformers to resonate
at the i.f. frequency. If the stage has a.v.c. voltage applied to
the grid circuit, the circuit of Fig. 6 is recommended - the a.v.c.
voltage being fed to the grid through the r.f. choke. In any event,
some provision must be made for the grid return.
If the filter is correctly aligned and the crystals are of the
correct frequency, an attenuation vs frequency curve similar to
that of Fig. 4 should result. If the FT-241-A crystals are used
in the filter, do not take it for granted that the frequencies will
be exactly as stated in the specifications. In checking a dozen
crystals for frequency variation, over two kilocycle spread was
observed in one channel number. It is advisable to purchase half-a-dozen
crystals so as to obviate the necessity for edge grinding the crystals
In operation, the desired signal is tuned in without the filter
- the b.f.o. being adjusted so as to give a good note with the signal
in the center of the i.f. passband - and the filter is switched
in or out at will. The r.f. gain control is run at as low a level
as possible, to prevent ringing, and the audio gain is used to adjust
Posted February 7, 2014