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When you are interested in more technical information about crystal receivers read this article by:
Charles A Lauter. (email: Lautron@aol.com )
(Leider nur in englischer Sprache)
Background:
The following are standards and methods for testing Crystal Sets
for the purpose of comparing the performance of different Crystal
Sets, Crystal Set circuits, and Crystal Set components in an
objective manor. Standards for testing all other types of radios
have been in effect since about 1930. Crystal Sets have been in
use for almost 100 years but no performance measurement standards
exist that I can find. I have read many-many claims of
performance for various circuits, coil types, wire types and so
on but all without performance numbers that have been
measured. There are only two parameters or characteristics
of all Crystal Sets that are important. They are the sensitivity
or efficiency and the selectivity or bandwidth
characteristics. The ability of a Crystal Set to match its
antenna is very important and I am including it with the
efficiency or sensitivity characteristics. These tests cover Sets
designed to tune all or most of the AM broadcast band of
frequencies using a standard antenna. More about the
antenna below. I am not measuring characteristics such as
extended tuning range or the ability to work with very long
antennas or any other features not common to most Crystal
Sets. In all fairness, these tests were very
difficult or impractical to perform back in the 1920s or
earlier. Today these tests can be performed with what I
consider a modest investment in test equipment. These tests
will allow me to compare my collection of Crystal Sets with each
other and with other designs of circuits and components.
They will also permit comparison with other people's Sets and
designs. The reasons for the following test equipment and test
circuits will become apparent as the test procedures are
followed.
Test equipment:
The first item to consider for testing is the test equipment.
These tests require three pieces of test equipment; a RF Signal
Generator, a RF Voltmeter, and a Digital Multimeter. The Signal
Generator should have low output impedance. Most Generators by
HP, GR, and others have output impedance of 50 Ohms. When
terminated this impedance drops to 25 Ohms. Many so-called
Function Generators that have a Sine Wave Output will work just
as well for these tests. The impedance does not matter for the
sensitivity tests but can be factor in the selectivity tests. The
RF Signal generator does not need to have any modulation
capability at all. A less common piece of test equipment is the
RF Voltmeter. This meter must be able to measure RF voltages from
a few milliVolts to over one Volt. A high input impedance
of over 10,000 Ohms is also required. A well-calibrated
Oscilloscope will meet these requirements but will be less
accurate. I use a HP model 3400A RMS Voltmeter. The third piece
of test equipment is a battery powered 3-1/2 digit Digital
Voltmeter (DVM) or Digital Multimeter (DMM). Everyone that does
anything electrical or electronic should have one of these. The
DVM must be battery powered to make it RF isolated from
ground. Standard diode or diodes need to be used. Some sets
use two. I decided to use the popular and common 1N34 type
germanium diode.
Test circuits:
The next items are two simple test circuits. The first is the
circuit to connect the Signal Generator to the Antenna and Ground
terminals of the Crystal Set. This circuit is known as an
"Artificial Antenna", "Dummy Antenna", or
"Standard Input Circuit". Its purpose is to simulate a
typical "Long Wire" or "Marconi" antenna so
that the Crystal Set can be tuned in the normal manor. The values
chosen to be used are those recommended by the IRE in 1930 for
general radio testing. The circuit consists of a 25 Ohm resistor
in series with a 200 pF capacitor in series with a 20 uH coil. I
split the 25 Ohm resistor into a 15 Ohm resistor and a precision
10 Ohm resistor. The 10 Ohm resistor is connected between the low
or ground side of the signal generator output and the input
ground terminal of the crystal Set. This was done so that input
current to the Crystal Set can be easily measured. The input
current is then equal to voltage measured across the 10 Ohm
resistor divided by 10.
The second test circuit is the Crystal Set output load that
will be used in place of headphones. Headphones all have
three values of impedance; the first is simply the DC resistance,
the second is the AC impedance at audio frequencies, and the
third is the AC impedance at RF frequencies. In order to remove
the characteristics of different headphones from the tests I
simply replace the headphones with a precision 2,000 Ohm
resistor. In these tests I am only interested in measuring
the DC output voltage from the Crystal Set so a simple low-pass
filter that does not effect the output voltage is added. It is
simply a 100 kOhm resistor in series with the DVM and a .01 uF
capacitor in parallel with the DVM.
Test frequencies:
The following are the test frequencies to be used. They are 400,
600, 800, 1000, 1200, 1400, and 1600 kHz. Many of the older
Crystal sets will not tune to frequencies above 1000 kHz but do
as many as the set can tune. This next aspect of the tests
may confuse some people. All tests to be conducted are made using
an UNMODULATED input signal. This increases the accuracy of the
tests and eliminates the need for a Signal Generator that can
produce an accurate percentage of modulation. (Most
Generators cannot)
All Crystal Sets produce a DC output voltage that is proportional
to the RF input level and this same proportionality applies to
modulated signals producing audio output signals. Proof of this
is too long to present here.
Test setup:
Set up the test equipment, the test circuits and the Crystal Set
as follows; Connect the output of the unmodulated Signal
generator to the 15 Ohm resistor in series with the 200 pF
capacitor in series the 20 uH coil. Connect the other end of the
coil to the Antenna terminal of the Crystal Set. Connect the low
or ground output of the Signal Generator to the 10 Ohm resistor.
Connect the other end of the 10 Ohm resistor to the Low or Ground
input terminal of the Crystal Set. Note; This Crystal Set
terminal is often not in common with (connected to) either of the
output terminals. Place the Crystal set on an insulated surface
such as a book for RF isolation purposes.
Connect the precision 2000 Ohm resistor across, or in parallel
with, the headphone terminals. Connect the 100 kOhm resistor to
one of the headphone terminals. Connect the other end of this
resistor to one input of the DVM. Connect the other headphone
terminal to the other input of the DVM. Connect a .01 uF
capacitor in parallel with the DVM. Place the DVM and all DVM
leads on a second RF insulated surface like another book.
Replace galena or other diode types with the 1N34
type. I use clip leads.
Test definitions or characteristics definitions:
The only item that I had to define so far were the values for the
Dummy antenna. Efficiency by general definition is the ratio of
the power output of any device to the power input to it or Pout /
Pin.
Selectivity is quite different, a number of definitions are
required. The most basic and popular definition of
bandwidth is the so-called half power or "-3db"
bandwidth. To obtain this number subtract the frequency below the
frequency of maximum output where the output decreases to 0.707
times the peak value (F below) from the frequency above the
maximum output frequency again where the output decreases to
0.707 times the peak value (F above). Or bandwidth =
(F above) minus (F below) In the actual tests
below I modify this definition slightly to simplify the
measurements.
Selectivity has a second and important characteristic. It
is related to Shape Factor. Filters are what make any
Radio selective. Unfortunately simple filters are far from
perfect. If a filter were perfect the bandwidth would be the same
for all values of attenuation. The term Shape Factor refers to a
bandwidth at a greater attenuation than
-3db. For our tests we will use the
bandwidth at -20db for comparing different Xtal
Sets. ( -20db = E max. x 0.1)
& ( -3db = E max. x 0.707)
I could also define the out of band selectivity in terms of
Filter Skirt Slope (db / octave) but I decided not to go there. I
am also not going to talk about circuit quality or
"Q" because there seems to be a lot of confusion
as to its meaning in overall Xtal Set performance.
Lets Test!!
SENSITIVITY OR EFFICIENCY TESTS:
Finally we are ready to make the Crystal Set Sensitivity or
Efficiency measurement as follows;
1. Set the Signal Generator to the first test frequency.
(unmodulated)
2. Set the Signal Generator to produce an output level of about
one volt rms.
3. Adjust the Xtal Set for maximum output as indicated by the
DVM.
4. Adjust the Signal Generator frequency for maximum output that
may be slightly different from the nominal test frequency due to
the inability of the Xtal Set to precisely tune if the Set uses
coil taps for tuning.
For extremely accurate tuning make the phase of the voltage
across the 10 Ohm resistor in phase with the Signal Generator
output using a two channel Oscilloscope.
5. Adjust the Signal Generator output level to give a 1.000 volt
DC reading on the DVM.
6. Measure the RF Voltage at the input of the Dummy
antenna. Call this value "E1".
7. Measure the RF Voltage across the 10 Ohm resistor. (Between
the Signal generator common and the Xtal Set ground
terminal) Call this value "E2".
8. Now some calculations;
a. Compute the input current: Iin =
E2/10 (Amps)
b. Compute the power into the Dummy antenna; Pin = E1 x
Iin (Watts)
c. Compute the power loss in the Dummy antenna: Pda = (Iin x Iin)
x 25
d. Compute the power going into the Xtal Set: Px = Pin -
Pda
e. The power output from the Xtal Set = Pout = Eo x Eo / 2000 =
0.0005 Watts
f. Now for the Xtal Set Efficiency = Pout / Pin = .0005 /
Pin <<<<<<<
Note that this value does not include any mismatch with the Dummy
antenna.
g. Compute the Xtal Set input Resistance. (Assuming that Xtal Set
is tuned)
Rx = Ex / Iin , where Ex = E1 - (Iin x
25)
The resistance of the dummy antenna is 25 Ohms. The ideal
resistance of the Xtal Set is also 25 Ohms. This means that the
best possible efficiency of a Xtal Set and its antenna and ground
is 50%. This would mean a Xtal Set that is 100%
efficient. Compute the power into the Dummy antenna
if Rx was 25 Ohms; P best = (E1 x E1) /
(25 + 25).
h. Now compute the total efficiency of the Xtal Set with
the dummy antenna. Total efficiency = Pout / P
best
<<<<<<<<<<<<
Now lets look at some numbers from an actual Xtal Set. The
set is "The Beaver Baby Grand vest pocket Radio
Receiving Set". This is a little set that was made in
1922 and is shown on the cover of the book "Crystal
Clear - Volume 2" by Maurice L. Sievers.
Test frequency used = 600.4 kHz. Measured
values; E1 = 1.26 Vrms, E2 = .015 Vrms Computed
values; Iin = .0015 Amps, Pin = .00189 W, Pda =
.0000563 W, Px = .00183 W, Pout = .0005 W, Xtal
Set efficiency = Pout / Px = 27.27%
<<<<<<
Now for the rest of the story; Rx = 815 Ohms, P best
= .0317 W, and the total efficiency Pout / P
best is now only 1.57%. As you can see, the efficiency of
the set alone is not too bad, but when it is used with a standard
antenna it is very poor.
This set was apparently designed as a low cost Xtal Set to
receive a one and only nearby Radio Station. This Set is an
example of no matter what type of coil or coil wire is used, the
performance would not change enough to matter.
Now lets look at a better Xtal Set. The set is from world war one
and its U.S. Army designation is "Type SCR-54-A". This
set is also known as a BC-14A. The Set is shown in both editions
of Crystal Clear by Maurice L. Sievers and was described by Alan
Douglas in the September 1978 edition of Radio Age.
Test frequency used = 600 kHz. Measured values; E1 =
.605 VRMS, E2 = .048 VRMS.
Now the computed values; Iin = .0048 A, Pin = .0029 W, Pda
= .000576W, Px = .00233W, Pout = .0005W, The Xtal Set
efficiency = Pout / Px = 21.48% <<<<< Now lets add
the dummy antenna. Rx = 101 Ohms, Pbest = .0073W then the
total efficiency is 6.8%. It is Interesting that there is
more energy or power loss in the set but the total efficiency is
better by about 4.3 times. This Set has more coils and
capacitors which is why the losses are greater but the sets
ability to better match the antenna is the reason that the
overall efficiency or sensitivity is better. The set is much more
selective as will be shown below.
SELECTIVITY or BANDWIDTH:
These tests are a little more complex than the sensitivity tests
because selectivity is more complex to define.
Make the sensitivity measurement above and before making any
adjustments to
the Xtal Set or to the Signal Generator output level make the
following measurements. I recommend using a frequency counter to
measure the following frequencies.
1. Decrease the Signal Generator frequency to below the first
test frequency and until the Xtal set output decreases to 0.707
volts DC (-3db). Readjust the output level from the Signal
Generator until the value of Voltage "E1" is the same
as that used above. Again adjust the frequency for 0.707 VDC on
the DVM and again adjust the Generator above value of
"E1". Keeping the value of "E1"
constant removes the output resistance of the Signal Generator
and keeps it from effecting the bandwidth to be measured. The
higher the resistance or impedance of the Signal Generator the
more interaction there will be to obtain the final value
frequency at which the Xtal Set output is 0.707 Volts DC. This is
because as the Set is tuned to other than the selectivity test
frequency the impedance looking into the dummy antenna (the
Signal Generator load) changes. >>> Keep the value
"E1" constant for this and all of the following tests.
<<< Note the frequency. Call this frequency
"F low".
2. Subtract "F low" from the Sensitivity test frequency
used above. Multiply this difference frequency by
two. This value is the basic (half power) "-3db"
Bandwidth " of the Xtal Set for comparison
purposes. Normally one would measure the frequency
above the peak or resonant frequency at which the output also
drops to 0.707 Volts and then subtract F low from this frequency
to obtain the bandwidth. This measurement was done in this manor
because in some cases if you try to increase the frequency to the
point where the output voltage drops to 0.707 Volts a number of
resonant frequencies of the Xtal Set will prevent obtaining a
good measurement. (usually due to self resonance of the Xtal Set
coil or coils) The error due to this method is very small.
3. For the next test, reduce the Signal generator frequency until
the Xtal Set output voltage decreases to 0.1 VDC. (-20db). Call
this frequency "F min"
4. Subtract "F min" from the Sensitivity test frequency
above. Multiply this difference frequency by two. This is
the "-20db" Bandwidth.
Now lets again look at the little "Beaver Baby Grand "
Xtal Set for Selectivity.
F test = 600.4 kHz, F low = 543.4 kHz, F test -
F low = 57 kHz, 57 kHz x 2 = 114 kHz. This is the Basic or
"-3db" Bandwidth. Now "F min" = 434
kHz, F test - F min = 166.4 kHz, 166.4 kHz x 2 =
332.8 kHz = the "-20db" Bandwidth.
As you can see this is not a very selective Xtal set. For the
intended use, this was probably not a problem in 1922. In
today's world, the Set would be receiving several stations at the
same time in almost any city in the US.
Now lets look at the selectivity of the Type SCR-54-A. F
test=600 kHz, F low= 589.3 kHz, F min = 556 kHz. This means
that the half power or -3db bandwidth is 21.4 kHz and the -20db
bandwidth is 88 kHz. With this Set we have the ability to
possibility select a station we want to listen to.
BANDWIDTH AT REDUCED SENSITIVITY:
More complex Xtal sets have some means of improving
Selectivity at the expense of Sensitivity. There are a
number of ways of accomplishing this. In most cases reducing the
coupling between the antenna circuit and a second tuned
circuit is the method used. This is the purpose of
the "Loose or Slide Coupler" that is used in many of
the better Xtal Sets. In some Sets changing a coil tap and
re-tuning a capacitor will accomplish the change in coupling. It
may require some experimenting to find the best adjustment method
and control settings for any given Xtal Set. The
procedure is as follows;
1. Set up the Signal Generator and the Xtal set the same as in
the sensitivity measurement above.
2. Increase the Signal Generator output until the DVM reads 1.414
volts DC. This will now be the new value for the input voltage
"E1". Maintain this value for the rest of these tests.
3. Decrease the effective Xtal set coupling until the DVM reads
1.000 volts DC. This may effect the Xtal set tuning. If so retune
the Xtal set to the Signal generator frequency and repeat steps
two and three. Remember to keep E1 constant.
4. Again measure the bandwidth using the previous method above.
Now go back and measure the sensitivity again.
5. This process can be repeated over and over for better and
better selectivity.
This bandwidth will be less (narrower) than when the Xtal set was
adjusted for maximum output.
Xtal Sets with more tuned circuits and variable coupling will
have in better selectivity.
Now lets see what this reduced coupling did for the Type
SCR-54-A. The bandwidth at -3db is now only 9 kHz and the -20 db
bandwidth is now 73.6 kHz. Lets see what this change did to
sensitivity. Crystal Set efficiency decreased from 21.5% to
12 35% and total efficiency decreased from 6.83% to 3.67%. These
efficiency numbers were expected because they are about a -3db
decrease which is the same as the decrease in step 3 above.
The set of three measurements above is then repeated at each of
the test frequencies listed above.
All of the above measuring may sound like a lot of work but once
the equipment is in place the tests do not take long to
perform. I use a computer spreadsheet so that after I
enter the measured values, all of the calculations are performed
automatically. These measurements will definitely determine
which are the best Xtal sets, circuits, or
components.
Happy Testing