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1、Highly sensitive and selective single-tuned four-band crystal radio set using a new contra wound dual-value inductor, and having a sharp selectivity setting; along with a way to measure the unloaded Q of an L/C resonatorBy Ben H. TongueThe Crystal Radio Set Design, in a (large) Nutshell: The design

2、approach is to divide the AM band into several sub-bands in an attempt to keep the selectivity relatively constant and the insertion power loss low across the whole band. The first step is to divide the BC band into two halves: band A (520-943 kHz) and band B (943-1710 kHz). Two-step shunt inductive

3、 tuning is employed to switch between bands. A tank inductance of 250 uH is used in band A and 62.5 in and B. Band A is further subdivided into two bands: sub-bands 1 (520-700 kHz) and 2 (700-943 kHz). The band B is also subdivided into two bands: sub-band 3 (943-1270 kHz) and 4 (1270-1710 kHz). In

4、the normal selectivity mode, two different resonant RF resistance levels, measured at the top of the tuned circuit (point A in Fig. 5), are used at the center of the sub-bands. This impedance level is about 125k ohms at the center of sub-bands 1 and 3. It is 250k at the center of sub-bands 2 and 4 (

5、excluding the resistive losses of the components used). These resistance values are made up of a parallel combination of the transformed RF antenna-ground system resistance and the input RF resistance of the diode. These two resistances should be equal to each other to achieve minimum insertion powe

6、r loss, at the design bandwidth. This means that the transformed antenna-ground system and diode RF resistances are each about 250k in sub-bands 1 and 3 and 500k in sub-bands 2 and 4 at point A. The two different transformed RF antenna-ground system resistance values are achieved by proper adjustmen

7、t of a variable capacitor in series with the antenna (C7 in Fig. 5). The higher diode RF tank loading resistance value for sub-bands 2 and 4 are achieved by tapping the diode onto the tank at a point that is 70% of the turns up from ground. The tank is not tapped for sub-bands 1 and 3 and connection

8、 is to the top of the tank. In the sharp selectivity mode the diode is tapped half of the turns down on the tank from the point used for normal selectivity. The weak-signal RF input and audio output resistances of a diode detector are approximately the same and equal to 0.026*n/Is ohms (Is means dio

9、de saturation current, see Article #0-Part 4). The strong-signal audio output resistance of a diode detector is approximately equal to 2 times the RF resistance of its source. Compromise audio impedance transformation ratios are used to optimize performance on both weak and strong signals. The desig

10、n is scalable. Less expensive parts that may have somewhat greater losses may be used with some penalty in sensitivity and selectivity, especially at the at the high end of the BC band and at the Sharp Selectivity setting. See the Parts List for a listing of some more easily available and lower cost

11、 parts than the ones used in the original design.Fig. 1 - Single-Tuned Four-Band Crystal Radio Set, Version C. These are actually pictures of Version B asdescribed in Article #22, converted to version C, as described in this Article, but modified with theaddition of the amplifier in Article #25. Thi

12、s Article does not include the amplifier.Design objectives: A relatively constant -3 dB bandwidth of 5 to 6 kHz across the full range of 520 to 1710 kHz at normal selectivity, with a relatively constant RF power loss in the RF tuned circuits of less than 3 dB. A switched adjustment to achieve about

13、3 times sharper selectivity than normal. Optimal performance with external antenna-ground systems having a fairly wide range of impedance. To provide a simple-to-use switching setup for comparing a test diode with a standard one. To provide a volume control with a range of 45 dB in 15 dB steps that

14、has the minimal possible effect on tuning, This was incorporated in the design because the two local 50 kW blowtorch stations WABC and WOR (about 10 miles away) deliver a very uncomfortably loud output from SP headphones from my attic antenna. A means of volume reduction that did not reduce selectiv

15、ity was needed. This method of volume reduction actually increases selectivity by isolating antenna-ground resistance from the tank circuit. Introduction fo a new (to me) method for constructing low inductance high Q coils.1. TheoryThe frequency response shape of the circuit shown in Fig. 2 is that

16、of a simple single tuned circuit and can be thought of as representative of the nominal response of a single tuned crystal radio set. Consider these facts:1 If Lt and Ct have no loss (infinite Q), zero insertion power loss occurs at resonance when Rs equals Rl. This is called an impedance matched co

17、ndition. The power source (Vs, Rs), looking towards the tank, sees a resistance value equal to itself (Rl). Also, the load (Rl), looking towards the input sees a resistance (Rs), equal to itself. In the practical case there is a finite loss in Lt and CT This loss can be represented by an additional

18、resistance Rt (not shown), shunted across the tuned circuit. The input resistance seen by (Vs, Rs) is now the parallel combo of Rt and Rl and it is less than Rs The perfect impedance match seen by (Vs, Rs) when the tank Q (Qt) was infinite is now destroyed. The impedance matched condition can be res

19、tored by placing an impedance transformation device between the source, (Vs, Rs) and the tank.2 In Fig. 2, if tuning could be done with Lt alone, leaving CT fixed, the bandwidth would be constant. The problem here is that high Q variable inductors that can be varied over an approximately 11:1 range,

20、 as would be needed to tune from 520 to 1710 kHz do not exist. On the other hand, tuning by varying CT by 11:1 will cover the range, but have two disadvantages. (1) The -3dB bandwidth will vary by 1:11 from 520 to 1740 kHz. (2) In the practical case, if the bandwidth is set to 6 kHz at the low end o

21、f the BC band, and an attempt is made to narrow the bandwidth at 1710 kHz by placing a capacitor in series with the antenna, the insertion power loss will become great.3 The compromise used here is a coil design that can be switched between two inductance values differing by 4:1. The high inductance

22、 setting is used for the low half of the BC band and the low inductance for the high half. Capacitive tuning is used to tune across each half. The new technique used here, of using a combination of two inductors, enables the Q of the low value inductance (used in the high half of the band) to to be

23、much higher than would be the case if a single coil of the same diameter and wire size, but with fewer turns, were used. This technique uses two coils, closely coupled, and on the same axis. They are connected in series to obtain the large inductance and in parallel for realization of the small one.

24、 The small inductance has a value 1/4 that of the large one and about the same Q at 1 MHz. (if coil distributed capacity is disregarded). The innovation, so far as I know, is to use the full length of wire used in the high inductance coil, occupy the same cubic volume, but get 1/4 the inductance and

25、 keep the same Q as the high inductance coil (at the same frequency). See Table 4.4 The high and low bands are each further subdivided giving a total of four sub-bands (1, 2, 3 and 4). If this were not done, we would be faced with a bandwidth variation of about 1:3.3 in each band. The geometrically

26、subdivided bands are: sub-band 1 (520-700 kHz), 2 (700-943 kHz), 3 (943-1270 kHz) and 4 (1270-1710 kHz). The bandwidth should vary about 1:1.8 across each of these sub-bands. The bandwidths at the center of each of the four sub-bands are made approximately equal to each other by raising the loading

27、resistance of the antenna-ground system and the diode on the tank by a factor of two in sub-bands 2 and 4, compared to the value used in sub-bands 1 and 3.2. Design Approach for the Center of each of the four sub-bands.Fig. 3a shows the simplified Standard Dummy Antenna circuit, described in Termans

28、 Radio Engineers Handbook for simulating a typical open-wire outdoor antenna-ground system in the AM band. R1=25 ohms, C1=200 pF and L1=20 uH. See Article #20 for info on how to measure the resistance and capacitance of an antenna-ground system. These values are used in the design of the crystal rad

29、io set. R1 represents the antenna-ground system resistance, C1 the capacitance of the horizontal wire and lead-in to ground and L1 represents the series inductance of the antenna-ground system.The values of R1, C1 and L1 in Fig. 3a are considered to be independent of frequency. To the extent that th

30、ey may vary with frequency, C7 and C8 in Fig. 4 can be adjusted to compensate. The current-source equivalent circuit of the antenna-ground circuit is shown in Fig. 3b. To a first degree of approximation, C2 in Fig. 3b is independent of frequency. R2 will vary approximately inversely with frequency.

31、We will ignore the effect of L2, since its value is large, except when approaching the first resonance of the antenna-ground system. The design approach is to place a variable capacitor C3 in series with the antenna circuit (Fig. 3a) to enable impedance transformation of the antenna-ground circuit t

32、o an equivalent parallel RC (Fig. 3b), the R component of which can be adjusted by changing the value of the C3 to follow a desired relationship vs frequency. One of the objectives of the design is to enable as constant a bandwidth as possible vs. frequency. This requires the aforementioned equivale

33、nt parallel component (R2) to vary proportionally with the square of the frequency if capacitive tuning is used in each sub-band (loaded Q must be proportional to frequency for a constant bandwidth). The shunt variable capacitor to ground, shown across the tank coil, is used to tune the tank to reso

34、nance. This design attempts to accomplish this in the center of each sub-band. Performance is close at the band edges.3. The single tuned crystal radio setThe topology of the single tuned circuit is changed from band to band as shown in Fig. 4 below.Resonant RF resistance values at the top of C8 (Fi

35、g. 4) from antenna loading are designed to be: 250k ohms at the center of sub-bands 1 and 3 and 500k ohms for sub-bands 2 and 4. Since the diode is tapped at the 0.7 voltage point for bands 2 and 4, it sees a source resistance at resonance of: 125k for sub-bands 1 and 3 and of 250k ohms for sub-band

36、s 2 and 4. These figures apply for the theoretical case of zero loss in the tuned circuit components (infinite Q). In a shunt capacitively tuned crystal radio set, loaded with a constant resistive load, the bandwidth will vary as the square of the frequency. To understand why, consider this: When th

37、e resonant frequency of a tuned circuit loaded by fixed parallel resistance is increased (from reducing the total circuit tuning capacitance), the shunt reactance rises proportionally, giving rise to a proportionally lower circuit Q. But, a proportionally higher Q is needed if the bandwidth is to be

38、 kept constant. Therefore, the square relation.In the practical case, we are faced with two problems. (1) How should we deal with the fact we work with finite Q components? (2) At high signal levels (above the LSLCP), the RF load presented by the diode to the tuned circuit is about 1/2 the audio load resistance, and at low signal levels (below the LSLCP) the RF load presented by the diode is about 0.026*n/Is ohms. Compromises are called for. .此处忽略！