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Crystal Set Testing

 

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)

Crystal Set Testing

 

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

 

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