Measurement of the sensitivity of a crystal radio set when tuned to a weak fixed signal, as a function of the parameters of the detector diode; including output measurements on 15 diodes
By Ben H. Tongue
Summary: This Article shows how the 'weak signal' power loss, from the resultant operation in the square-law region of the detector, of a crystal radio set, varies as a function of the parameters of the diode. The measurements clearly shown how 'weak signal' detector loss is reduced when diodes having lower values of the product of saturation current and ideality factor are used. This results in obtaining greater volume from weak signals. Actual measurements compare closely to those predicted from equation #5, developed in Article #15A. It is assumed that the input and output impedances of the detector are reasonably well matched.
1. The Measurements:
The measurements of output power are made using a simpler and quicker method than that used in Article #11, since a CW instead of a modulated signal is used. This method involves measuring DC output voltage into a resistive load when the input of the detector is fed from a fixed source of available RF power.
A 3.2nW (-84.95 dBW) un-modulated source of available RF power of is applied to the diode for all measurements. This power level is about 12 dB below the LSLCP(i) of the average diode used in these tests. An AM broadcast signal of this power level will result in quite a weak sound in SP phones impedance matched to the output of a crystal radio set. The RF input power is applied through the AMCS described in Article 11. If the input to it is monitored by a DVM connected to the "T" connector, the AMCS should be considered to be an attenuator having an input resistance of 50 and an output resistance of 30.244 ohms. Its attenuation is 22.975 dB. The crystal radio set used in these measurements is described in Article #26. It was used because its performance has been well characterized and its input impedance can be changed over a wide range. The output resistor into which the output power is dissipated is R3. The primary windings of T1 and T2 are shorted when taking measurements.
The measurement procedure, for each diode, consists of applying an RF power source having an available power of 3.2 nW to the diode detector, adjusting the input impedance transformation (C7 and C8 in the crystal radio set described in Article #27) for maximum output voltage and recording that value.
The input conditioning device (AMCS), used to aid in measurement of input power is shown in Fig. 2 of Article #11. There, it was used in a procedure to measure input AM sideband and output audio power. Here it is used as a convenient way to provide an accurate source voltage having a known internal resistance. A CW signal generator tuned to, say, 1 MHz is connected to the AMCS as the source of RF power. If the generator has an AM modulation capability, that can be used with headphones as an aid in initially tuning the crystal radio set to the test signal. Table 2 and Fig. 2 show the results of the measurements.
* Is and n were measured using forward voltages of 39 and 55 mV (average current of between 3.8 to 5 times Is).
* Diode load is equal to Rx, its axis-crossing resistance
The red data points indicate actual measurements. The blue values are calculated from equation #5 in Article #15A. The blue line is a connection of the points calculated from equation #5. Note the close fit.
Discussion: Fig. 2 shows the close correlation of measured output power with measured diode n*Is, as predicted by equation #5 in Article #15A. This suggests that n*Is is a valid 'figure of merit' for a diode used to detect weak signals. Remember: The detector power loss figures shown in the last column of Table 2 would be even larger if the test signal of 3.2 nW were smaller. The assumption in all of this is that both the input and output ports of the crystal radio set are reasonably well impedance matched.
Note that the input and output resistances of a diode detector using diodes #10 and 11 are very high. Matched input source and output load resistances this high are hard to achieve in a low loss manner. A low loss high input resistive source is easier to achieve with a high Q loop driven crystal radio set that uses the loop as the tank than with one driven by an external antenna and ground that uses a separate high Q tank coil. This is because one of the sources of loaded tank resistive loss, the external antenna-ground system resistance, is eliminated. The radiation resistance of the loop is usually negligibly small compared to the loss in the loop when considered as a stand alone inductor. It is assumed in this discussion that the diode is connected to the top of the loop, the point of highest source impedance. Don't take this as a recommendation to go to a loop antenna for the best weak signal reception. A good outside antenna-ground system will outperform a loop by picking up more signal power. Conclusion: A diode with the lowest n*Is may be theoretically the best, but achieving impedance matching of input and output may not be possible. In practice, a compromise must be struck between a diode with the lowest n*Is and one having a lower axis-crossing resistance (Rxc). This means, in general, a higher Is. It can be achieved by paralleling several diodes or using a different diode type.
A good diode array to try in high performance crystal radio sets intended for weak signal reception is an Agilent HSMS-286L, with all three diodes connected in parallel. This diode array is packaged in a small SOT-363 SMD package but is easy to use even without a surfboard to aid in its connection. The three anode leads exit from one side of the package with the three cathode leads from the other. A quick connection solder blobbing all three anodes together and to a thin wire, and a similar connection to the cathodes is easy to do. Use a low temperature soldering iron and as little heat as possible to avoid injuring the diodes. This triple diode performs about the same as six Agilent 5082-2835 diodes in parallel, except that audio distortion will come in sooner on strong signals, because of its low reverse breakdown volt age. It performs best if used in a crystal radio set having RF source and audio load resistances of about 400k ohms, rather high values. An excellent diode for both weak and strong signal reception is the obsolete ITT FO-215 germanium diode, still available, think from Dave Schmarder at http://www.1n34a.com/catalog/index.htm . Crystal radio sets having RF source and audio load resistances of about 200k ohms may have better sensitivity with two of the HSMS-286L arrays in parallel. One section of the Infineon BAT62-08S triple diode should work the same as two HSMS-286L arrays in parallel. Agilent semiconductors are carried by Newark Electronics and Arrow Electronics, among others. Agilent or Infineon may sometimes send free samples to experimenters who ask for them.
The apparent error in output power for diode #1 has been checked many times. The figure appears to be accurate. I don't know the reason for the anomaly, except that the diode probably has an increased value for n*Is at the 21 Na rectified current, compared to the value of Is*n from the measurements in Table 1 (made at a higher current). It is known that there are extra causes for conduction in a diode beyond those modeled by the Shockley equation. Other measurements show that this diode also does not follow the Shockley diode equation at high currents (see Article #16). Diodes A, B and C are randomly selected 1N56 germanium units. They also show poorer performance than would be expected from Schottky diodes of the same Is and n. Note that germanium diodes diodes #1, A, B and C provide less output than would be expected from Equation #5. The output from germanium diode #5 is close to that expected from Equation #5 as is the output from all the Schottky diodes.
Two charts are presented in Article #16 showing measurements of Is and n for 10 different diodes. The Schottky diodes seem to have fairly constant values of Is and n as a function of current. The silicon p-n junction and germanium measurements show how Is and n can vary, in other diode types, as a function of current.
Appendix: The objective of these measurements is to measure the performance of various diodes when used as detectors; at a signal level well below their LSLCP so that their weak signal performance can be compared. The measurements described in this Article were made with an 'available RF power' of -84.95 dBW (3.2 nW) applied to the diode. Here is how that value was chosen:
Initial measurements were made using a Tektronix model T922 scope having a maximum sensitivity of 2 mV/cm. The scope was connected to P1 in Fig. 1; the horizontal sweep rate was set to display about 3 cycles of RF. The RF voltage that could be read on the scope, with reasonable precision, was considered to be about 2 mV minimum peak-to-peak, providing a vertical display of 1 cm. The voltage at P1 drives a 25 ohm resistor, the resistance of which is transformed in the crystal radio set up to a value that matches the input resistance of the diode. The correct input impedance match is attained by interactively adjusting C7 and C8 on the crystal radio set to maximize the rectified DC output voltage. 2 mV pp RF voltage equates to 2/(sqrt8) mV RMS. Since available power=Pa=(Erms^2)/(4*source resistance), Pa=5 nW. If one allows for about 2 dB loss between the input to the crystal radio set and that to the diode, the available power that actually reaches the diode becomes about 3.2 nW. To obtain better measurement precision a Fluke model 8920A true RMS RF digital voltmeter was connected to the "T" connector in Fig. 1 and used in the final measurements. Since there were internal noise issues with the Fluke, the 20 dB attenuator (SW3) in the AMCS was switched in to enable increasing the signal to the DVM by 20 dB to overcome the noise. The resistor values in the "inverted L" pad in the AMCS (45.0 and 5.55 ohms), along with the 25 ohm resistor, provide a source resistance of about 30 ohms and an attenuation of 22.98 dB, exclusive of any attenuation introduced by SW1, SW2 or SW3.
At first an HP model 3312A function generator was used as the RF source. Final measurements quoted were made using an HP model 33120A synthesized signal generator.
Measurements of the diodes having the lower values of Is were made at 892 kHz (Band A, sub-band 1 of the crystal radio set). Insufficient impedance transformation range was available to match diodes having the higher values of Is, so those were measured at 1205 kHz (Band B, sub-band 3). These frequencies were chosen so as to eliminate signal pickup from local stations.
The actual insertion power loss in the crystal radio set caused by losses in its L and C components was accounted for in each diode measurement by feeding an RF signal of -84.95+20.98+20+X dBW into the AMCS. (The raw internal power loss in the AMCS is 22.98 dB, and the 20 dB attenuator (SW3) was activated). X represents the L/C losses from the tank inductor and C7 and C8 in the crystal radio set used for the tests. Its value was determined from a computer simulation of the crystal radio set, using a source resistance of 30 ohms and a load resistance equal to the Rxc of the diode to be tested, and noting the insertion power loss as X. The value of X varied from 0.378 dB for diode #1 up to 1.711 dB for diode #11. The simulated crystal radio set consisted of two impedance-matching/tuning capacitors (C7 and C8 in Article #26) with a tank inductor having a Q value extrapolated from the values given in Table 4 of Article #26. The simulation program was 'SuperStar', by Eagleware. Those uncomfortable with the concept of 'Available Power' may find Part 3 in Article #0 helpful. Note that the diodes having the lowest n*Is value (and the lowest detector loss) result in the greatest loss from the tuning components in the crystal radio set. This means that to gain the greatest benefit from using a diode having a low n*Is, the parallel resonant loss resistance of the tank circuit must be made as high as possible(unfortunately reducing selectivity).
Note re diode performance when receiving strong signals: A high diode reverse-breakdown voltage rating is important in this case to prevent tank resistive loading from diode reverse conduction caused by the high reverse voltage present during the non-forward-conduction half-cycle. When this happens, volume is reduced. Diodes that have a high reverse breakdown voltage rating usually have a high value for the product of their saturation current and ideality factor and are best for obtaining maximum volume on strong stations. The diodes that are best for weak signal reception usually have a low n*Is product. Many of those who use the HP 5082-2835 report inferior volume on strong stations. I believe the cause is explained above. One solution to this problem is to provide switching means for two diodes as is done in the crystal set described in Article #26. Another possible approach might be to to use several HP 5082-2800 high-reverse-breakdown-voltage diodes in parallel. Several diodes would probably be needed because of the low saturation current of one '2800. One could also use only one '2800 and supply a little forward bias voltage to make it simulate several in parallel. These approaches have not been experimentally investigated.
#27 Published: 08/14/2003; Revised: 05/23/2007