On this page measurements are described on my
detector unit 1 .
In this detector unit is as diode used the HSMS-282K (2 diodes parallel), and
the load resistance by
transformer unit 1 is 1.6 MΩ.
For the choice of diode and load resistance I assumed that maximum sensitivity
would occur if load resistance and diode resistance were about equal to the
impedance of the unloaded LC circuit ( this assumption is however not correct).
Via some experiments I will now determine if the sensitivity and selectivity can
be improved by the use of another detector diode and/or another load resistor.
Also frequency shift of the detector circuit is measured as a function of the
voltage across the circuit.
Test setup used for the measurements on this
page.
The oscilloscope measures via themeasuring amplifier
the voltage across the LC circuit.
The voltmeter measures the detected DC voltage.
The load resistor is formed by resistor R parallel with the voltmeter
with 10 MΩ input resistance.
The value of capacitor C is: 100 nF.
The RF signal is supplied by a DDS signal generator, the signal is via a
coupling coil coupled to the LC circuit.
All measurements are done at a frequency of about 1500
kHz.
At 1500 kHz, the impedance of the unloaded detector unit is: 1.97 MΩ.
There are 4 (combinations of) diodes tested.
HSMS-282K Schottky diode ( 2 diodes in one case
parallel)
HSMS-282K Schottky diode ( 1 diode)
5082-2835 Schottky diode ( 2 diodes parallel)
5082-2835 Schottky diode ( 1 diode)
Detected voltage at constant input
voltage.
In this measurement, the detected DC voltage is measured, so the voltage at
the output of the diode.
The diode is in all cases loaded with a load resistor of 1.52 MΩ,
this is a combination of a 1.8 MΩ resistor parallel to the voltmeter of 10 MΩ.
The value of 1.52 MΩ is however a little bit lower then the impedance of
transformer unit 1.
The input voltage (RF signal across the LC circuit) is constant (20 mV, 50 mV,
100 mV etc.) when measuring the different diodes.
Detected voltage (mV dc) as function of the voltage
across the LC circuit (peak-peak value across the LC circuit).
Below 200 mVtt input voltage these diodes work in the square law detection
region.
When doubling the input voltage, the output voltage increases about 4
times.
Above 2V input voltage, we more come into the linear detection
region
When using 2 diodes 5082-2835 parallel, the highest DC
voltage is measured.
The (single) diode 5082-2835 gives the least output voltage.
From this, one might conclude that the use of 2 diodes 5082-2835 parallel gives
the most sensitive receiver.
And the use of a single 5082-2835 the least sensitive receiver.
This conclusion is however not correct, as we shall see later in this article.
Q factor at a constant input voltage.
In this measurement the Q factor is measured of the (by diode and load
resistor) loaded circuit.
The diode is in all cases loaded with a load resistor of 1.52 MΩ.
De input voltage is constant (50 mV, 100 mV etc.) for the different diodes.
Voltage across LC circuit
HSMS-282K
2 diodes parallel
HSMS-282K
1 diode
5082-2835
2 diodes parallel
5082-2835
1 diode
20 mV
*
*
*
*
50 mV
442
600
395
601
100 mV
429
536
376
578
200 mV
385
469
358
518
500 mV
306
333
289
341
1 V
273
300
259
284
2 V
264
283
247
278
5 V
247
250
236
251
Table 2
Q factor as function of the voltage across the LC
circuit.
The diode is loaded with 1.52
MΩ.
* There is not measured at 20 mV input voltage, this voltage is too low for an
accurate Q measurement.
The use of 2 diodes 5082-2835 gives the lowest Q in this measurement.
A low Q means the LC circuit is (relative) heavy loaded by the diode.
A low Q also means a lower selectivity of the receiver.
Frequency change of the LC circuit as
function of the input voltage.
Every diode has a certain capacitance (capacitor value in pF).
This capacitance is depending on reverse voltage across the diode, the higher
the reverse voltage, the lower the capacitance.
At zero volt the capacitance is the highest.
A diode connected to a LC circuit shall rectify the RF signal, the rectified
voltage forms a reverse voltage across the diode.
The higher the voltage across the LC circuit, the higher the reverse voltage
across the diode, the lower the capacitance, and the higher the resonance
frequency.
Voltage across LC
circuit
HSMS-282K
2 diodes parallel
HSMS282K
1 diode
5082-2835
2 diodes parallel
5082-2835
1 diode
20 mV
0.0
0.0
0.0
0.0
50 mV
0.0
0.0
+ 0.2
+ 0.1
100 mV
0.0
0.0
+ 0.3
+ 0.2
200 mV
+ 0.1
0.0
+ 0.5
+ 0.3
500 mV
+ 0.9
0.6
+ 1.5
+ 1.0
1 V
+ 2.4
1.0
+ 3.1
+ 1.4
2 V
+ 4.2
2.0
+ 4.8
+ 2.5
5 V
+ 6.2
2.8
+ 6.8
+ 2.9
Table 3.
This table gives the measured
frequency shift (in kHz) of the LC circuit, as function of the voltage
(peak-peak value) across the LC circuit.
The resonance frequency of the circuit is 1500 kHz.
The load resistor is 1.52 MΩ in this
measurement.
Detected voltage and power at a constant
magnetic field (large signal).
In the previous measurements, the voltage across the LC circuit was constant,
when switching from one diode to another.
This corresponds however not with the situation in practice.
In a real crystal receiver, the voltage across the LC circuit shall depend on
both used diode and load resistor.
If the LC circuit of a receiver is loaded more heavy by diode or load resistor,
the voltage across the circuit shall reduce, and also circuit Q shall reduce.
In the next measurement (table 4) the LC circuit is placed in a constant
magnetic field.
We get such a constant magnetic field by placing the coupling coil at a fixed
distance from the LC circuit, and keep the amplitude of the signal generator
constant.
The constant field simulates a received station of constant strength.
The load resistor is varied between 0.91 and 10 MΩ.
The voltage across the LC circuit is now depending on both load resistor and
used diode, but the level of several volts across the circuit compares to
reception of quite a strong local station.
Table 4
Voltage across the LC circuit (AC) and detected voltage (DC) as function of load
resistor and used diode.
Load resistor
HSMS-282K
2 diodes parallel
HSMS-282K
1 diode
5082-2835
2 diodes parallel
5082-2835
1 diode
0.91
MΩ
DC= 0.546 V AC= 1.48 V
DC= 0.561 V AC= 1.58 V
DC= 0.562 V AC= 1.54 V
DC= 0.564
V AC= 1.55 V
1.07
MΩ
DC= 0.625 V AC= 1.65 V
DC= 0.644 V AC= 1.72 V
DC= 0.643 V AC= 1.70 V
DC= 0.647 V AC= 1.70 V
1.30
MΩ
DC= 0.721 V AC= 1.87 V
DC= 0.745 V AC= 1.95 V
DC= 0.742 V AC= 1.90 V
DC= 0.747 V AC= 1.94 V
1.52
MΩ
DC= 0.790 V AC= 2.00 V
DC= 0.819 V AC= 2.19 V
DC= 0.815 V AC= 2.07 V
DC= 0.821 V AC= 2.08 V
1,80
MΩ
DC= 0.867 V AC= 2.18 V
DC= 0.900 V AC= 2.27 V
DC= 0.981 V AC= 2.20 V
DC= 0.900 V AC= 2.26 V
2.13
MΩ
DC= 0.966 V AC= 2.38 V
DC= 1.003 V AC= 2.48 V
DC= 0.992 V AC= 2.40 V
DC= 1.005 V AC= 2.49 V
2.48
MΩ
DC= 1.055 V AC= 2.58 V
DC= 1.094 V AC= 2.66 V
DC= 1.079 V AC= 2.60 V
DC= 1.097 V AC= 2.68 V
3.19
MΩ
DC= 1.191 V AC= 2.85 V
DC= 1.243 V AC= 2.96 V
DC= 1.222 V AC= 2.90 V
DC= 1.243 V AC= 2.97 V
3.97
MΩ
DC= 1.318 V AC= 3.10 V
DC= 1.376 V AC= 3.21 V
DC= 1.350 V AC= 3.17 V
DC= 1.370 V AC= 3.21 V
4.38
MΩ
DC= 1,362 V AC= 3.19 V
DC= 1.428 V AC= 3.33 V
DC= 1.395 V AC= 3.26 V
DC= 1.424 V AC= 3.34 V
5.00
MΩ
DC= 1.424 V AC= 3.30 V
DC= 1.498 V AC= 3.49 V
DC= 1.467 V AC= 3.40 V
DC= 1.495 V AC= 3.49 V
6.67
MΩ
DC= 1.566 V AC= 3.60 V
DC= 1.657 V AC= 3.80 V
DC= 1.614 V AC= 3.70 V
DC= 1.657 V AC= 3.80 V
10.0 MΩ
DC= 1.743 V AC= 3.98 V
DC= 1.852 V AC= 4.20 V
DC= 1.775 V AC= 4.10 V
DC= 1.843 V AC= 4.20 V
The given voltage across the LC circuit (AC) is the peak-peak value.
The voltage across the circuit is proportional to the loaded Q of the LC
circuit.
So, the higher the AC voltage, the higher the Q, and the more selective the
receiver is.
We see the voltage across the circuit and the detected voltage are not very
depending on the used diode, all diodes give about the same result.
But AC voltage is very depending on the load resistor (at equal diode).
This is a typical symptom at high signal levels, where the diode is working in
the linear detection region.
In the following table, the detected voltages from table 4 are calculated to
power in the load resistor.
Table 5
Power in the load resistor (in nW = nano-Watt) as function of the load resistor
and used diode.
Load resistor
HSMS-282K
2 diodes parallel
HSMS-282K
1 diode
5082-2835
2 diodes parallel
5082-2835
1 diode
0.91
MΩ
328
346
347
349
1.07
MΩ
365
388
386
391
1.30
MΩ
400
427
424
429
1.52
MΩ
411
441
437
443
1.80
MΩ
418
450
441
450
2.13
MΩ
438
472
462
474
2.48
MΩ
448
483
469
485
3.19
MΩ
445
484
468
484
3.97
MΩ
438
477
459
473
4.38
MΩ
423
466
444
463
5.00
MΩ
406
449
430
447
6.67
MΩ
368
412
391
412
10.0 MΩ
304
343
315
340
Graph 1
Power in the load resistor (in nW) as function of the load resistor and used
diode.
The results from table 5 are shown in this graph.
Conclusion:
The highest detected power occurs at a load resistor of about 2.5 to 3 MΩ.
As diode, a single HSMS-282K and a single 5082-2835 give the best results.
Connecting 2 diodes parallel gives a little bit less output power, but the
differences between are small.
Once again: this is the situation when receiving strong signals.
Detected voltage and power at a constant magnetic field (small
signal).
The same measurement from table 4 is done once again, but now with a much
smaller signal level, so with a lower amplitude of the signal generator.
This signal level compares to reception of a very weak station.
Table 6
Voltage across the LC circuit (AC) and detected voltage (DC) as function of the
load resistor and used diode.
Load resistor
HSMS-282K
2 diodes parallel
HSMS-282K
1 diode
5082-2835
2 diodes parallel
5082-2835
1 diode
0.91
MΩ
DC= 6.2 mV AC= 98 mV
DC= 6.5 mV AC= 119 mV
DC= 7.2 mV AC= 111 mV
DC= 6.6
mV AC= 138 mV
1.07
MΩ
DC= 7.1 mV AC= 99 mV
DC= 7.5 mV AC= 120 mV
DC= 8.2 mV AC= 112 mV
DC= 7.8 mV AC= 139 mV
1.30
MΩ
DC= 8.1 mV AC= 99 mV
DC= 8.8 mV AC= 121 mV
DC= 9.4 mV AC= 113 mV
DC= 8.9 mV AC= 140 mV
1.52
MΩ
DC= 9.0 mV AC= 100 mV
DC= 9.7 mV AC= 122 mV
DC= 10.3 mV AC= 115 mV
DC= 9.9 mV AC= 141 mV
1.80
MΩ
DC= 9.8 mV AC= 102 mV
DC= 10.9 mV AC= 124 mV
DC= 11.3 mV AC= 116 mV
DC= 11.0 mV AC= 142 mV
2.13
MΩ
DC= 10.9 mV AC= 104 mV
DC= 12.3 mV AC= 126 mV
DC= 12.6 mV AC= 117 mV
DC= 12.3 mV AC= 143 mV
2.48
MΩ
DC= 11.9 mV AC= 105 mV
DC= 13.5 mV AC= 128 mV
DC= 13.8 mV AC= 118 mV
DC= 13.7 mV AC= 144 mV
3.19
MΩ
DC= 13.4 mV AC= 106 mV
DC= 15.6 mV AC= 130 mV
DC= 15.6 mV AC= 120 mV
DC= 15.8 mV AC= 146 mV
3.97
MΩ
DC= 14.8 mV AC= 108 mV
DC= 17.5 mV AC= 132 mV
DC= 17.4 mV AC= 122 mV
DC= 17.9 mV AC= 147 mV
4.38
MΩ
DC= 15.3 mV AC= 109 mV
DC= 18.2 mV AC= 134 mV
DC= 17.9 mV AC= 124 mV
DC= 18.8 mV AC= 148 mV
5.00
MΩ
DC= 16.0 mV AC= 109 mV
DC= 19.3 mV AC= 136 mV
DC= 18.9 mV AC= 126 mV
DC= 20.1 mV AC= 150 mV
6.67
MΩ
DC= 17.7 mV AC= 110 mV
DC= 21.9 mV AC= 138 mV
DC= 21.2 mV AC= 128 mV
DC= 22.9 mV AC= 152 mV
10.0 MΩ
DC= 19.8 mV AC= 110 mV
DC= 25.3 mV AC= 140 mV
DC= 24.0 mV AC= 130 mV
DC= 27.2 mV AC= 156 mV
We see the voltage across the circuit and the detected voltage are rather
depending on the used diode.
But the AC voltage as function of the load resistor (at equal diode) is just
relative constant.
In other words: the circuit Q (which is proportional to the AC voltage) is
mostly depending on used diode and not so much on load resistor.
This is a typical symptom at low signal levels, where the diode is working in
the
Also now, the detected voltages are calculated to power in the load resistor.
Table 7
Power in the load resistor (in pW = pico-Watt) as function of the load resistor
and used diode.
Load resistor
HSMS-282K
2 diodes parallel
HSMS-282K
1 diode
5082-2835
2 diodes parallel
5082-2835
1 diode
0.91
MΩ
42.2
46.4
56.9
47.9
1.07
MΩ
47.1
52.6
62.8
56.9
1.30
MΩ
50.5
59.6
68.0
60.9
1.52
MΩ
53.3
61.9
69.8
64.5
1.80
MΩ
53.4
66.0
70.9
67.2
2.13
MΩ
55.8
71.0
74.5
71.0
2.48
MΩ
57.1
73.4
76.8
75.7
3.19
MΩ
56.3
76.3
76.3
78.3
3.97
MΩ
55.5
77.1
76.3
80.7
4.38
MΩ
53.4
75.6
73.2
80.7
5.00
MΩ
51.2
74.5
71.4
80.8
6.67
MΩ
47.0
71.9
67.4
78.6
10.0 MΩ
39.2
64.0
57.6
74.0
Graph 2
Power in the load resistor (in pW) as function of the load resistor and used
diode.
The results of table 7 are shown in this graph.
Conclusions:
Of the tested diodes, the 5082-2835 (one diode) gives the most output power
at low signal levels.
This diode gives the maximum output power at a load resistor of about 5 MΩ.
The other diodes give less output power, and have their power peak at lower load
resistance.
Improvements on detector unit
1
The maximum sensitivity of
detector unit 1
does not occur when all impedances (of LC circuit, diode and load resistor) are
equal.
The sensitivity and selectivity of the detector
unit can be improved, by using a single 5082-2835 diode instead of a dual
HSMS-282K.
And besides that increasing the load resistor from 1.6 MΩ to 5 MΩ.
In that case the output power at low signal levels increases with a factor 1.52
(80.8 pW / 53.3 pW see table 7) or + 1.81 dB.
At high signal levels, the output power increases with a factor 1.09
( 447 nW / 411 nW see table 5) or + 0.36 dB
The Q factor (selectivity) at low signal levels increases with a factor 1.50
(150 mVac / 100 mVac see table 6).
The Q factor at high signal levels increases with a factor 1.745 (3.49 Vac /
2.00 Vac see table 4).
