Op Amp Vlf Converter Sales

Tuning circuit using one fixed and one variable inductor.

CI 100 pF

C2 68 pF

In o

In o

Inductance Xenon Flash
« Out

13-7 Mutual reactance coupling: (Ay capacitance and (B) inductance.

In Fig. 13-7A, a small capacitance (33 to 120 pF) is used to couple the two LC resonant circuits (Li/CiA and L^Cib). In Fig. 13-7B, a common inductor {Lf) is used for the same purpose. Experiments show that a value of 150 to 700 §iH is needed for The ARRL Handbook for Radio Amateurs (all recent editions) provides details on selecting values for the components in these circuits.

A different approach to the design of receiver front ends is shown in Fig. 13-8. One of the problems in VLF receiver design is providing low-impedance link coupling into and out of the LC tank circuit. Large inductance coils are not very often available with low impedance transformer windings. For example, in the Digi-Key catalog, the largest coil with such a winding is a 220-jxH unit, which is intended for the AM broadcast band. The solution was the series inductor method (Fig. 13-6). In this case, however, a 56-mH variable inductor was connected in series with a xenon flash-tube trigger transformer. The version selected was intended to fire a 6000-V fast-rise-time pulse into a xenon flash tube, and it has a nominal inductance of 6.96 mH £20%. The measured inductance was 7.1 p.H, and the self-resonance frequency was 140 kHz. Without the trimmer capacitors, the tuning range was 30 to 60 kHz when a 2 x 380-pF variable capacitor was used. Adding a 600-pF trimmer across each sec-

Ca3240 Video Detecting Circuits

Tl, T2:: 7-|iH Trigger Transformer

C2 C3

Trimmer Trimmer

C2 C3

Trimmer Trimmer

Toko Rcl Ham Band Coil

CA) Using a flash-tube trigger transformer as a circuit element and (B) using custom wound coils.

Lion of the main tuning capacitor reduced the tuning range to 20 to 30 kHz. The approach in Fig. 13-8A uses a capacitor (C<t) for the inutttal reactance, and ill Fig. 13-8B, an inductor is used (L.\).

The trigger transformers are widely available from mail-order sources. How ever, I ordered several from an English source, Maplin Electronics (P.O. Box 3, Rayleigh, Essex SS6 8LR, England). They have variable capacitors, coil forms, and a number of other things of interest to amateur radio constructors. The U.S. credit cards accepted include VISA and American Express and the currency exchange is automatic Another alternative, although I've not tested i<, is to use pulse transformers. Unfortunately, these transformers typically have limited turns ratios (2:1:1).

A VLF receiver project

The circuit for a modified VLF receiver is shown in Fig. 13-9. it uses the same basic circuit as the Stokes design but with these modified tuning circuits. The 82-mH fixed inductors (LiA, ¿iAl andL:jA) are the Toko 181LY-82&J, which are available from Digi-Key (P.O. Box 677, Thief River Falls, MN 56701-0677, 1-800-344-4539) under catalog number TK-4424. The 56-mH variable inductors are Toko CLNS-TIM39Z, j

Vlf Sid Receiver
C3, C9, CI4: &-8Q pF trimmer capacitor (Sptague-Goodman GZC30000) Digi-key SG30IU

1 Schematic of the VLF receiver project.

available under Dig!-Key TK-1724, An additional degree of adjustment is provided by usii>g an 8- to 80-pF trimmer capacitor across each section of the 3 X 305-pF variable main tuning capacitor. These capacitors are Sprague-^Goodman GZC8000 units (Digi-Key SG3010).

The output circuit for this receiver reflects the fact that it is a SID monitor receiver, The detector is a voltage doubter (Di/Z^) made from germanium diodes. The original diodes specified were 1N34 but INfiOs work well also. If you cannot find these diodes (RaditiShack and Jtm-Pak sells them) then try using replacements from the "universal" service shop replacement tines, such as SK, NTE, and ECG. The NTE-I09s and ECG-l09s will work well. The output of the detector is heavily integrated by a 470-n.F electrolytic capacitor. The output as shown is designed to feed a current-input recorder or a microammeter. if a voltage output is desired then connect a resistor (3.3 to 10 kft) across capacitor CJH.

12 Vdc

12 Vdc

Toko Quad Coil
LtA, L2A, L3A: 82 mH (Toko 1S1LY-823J) DigJ-key TK4424 LtB, L2B( L3B: 56 mH (Tbko CLNS-T1039Z) Digi-key TKITS4

Figure 13-10A shows a printed circuit board for use with this circuit, and Fig. 13-10B is the components' placement "roadmap." The board is designed for these specific components (Fig. 13-9). Variations on the theme can be accommodated by using different value inductors from the same Toko series (see Digi-Key catalog; the Lrseries coils are Toko size 10RB and the L2-series coils are size 10PA). Also, if you don't want

Figure 13-10A shows a printed circuit board for use with this circuit, and Fig. 13-10B is the components' placement "roadmap." The board is designed for these specific components (Fig. 13-9). Variations on the theme can be accommodated by using different value inductors from the same Toko series (see Digi-Key catalog; the Lrseries coils are Toko size 10RB and the L2-series coils are size 10PA). Also, if you don't want

Vlf Receivers Circuit

13-10 PC board for the VLF receiver. (A) PCB pattern for VLF receiver; (B) parts layout.

to use two coils in each tuning circuit, then short out the holes for Li positions and use a coil in the L:> positions with the required inductance.

The final receiver is shown in Fig. 13-11. A front-panel view is in Fig. 13-11 A, and an internal view is shown in Fig. 13-1 IB. The tuning capacitor was a three -

The Gyrator Iii Vlf Receiver

13-11 Completed receiver.

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section model purchased from Antique Electronic Supply, Tempe, A2- Although I first thought it was a 3 X 365-pP unit, it measured at 550 pF (which is better for VLF anyway). A 2* vernier dial (Ocean State Electronics, P.O. Box 1458, Westerly, RJ 02891, 1-800-866-6626) was used to drive the variable capacitor (notice: Ocean State also sells multisection variable capacitors). The final receiver tuned from 16.5 to 31 kHz.

A later VLF receiver design by Stokes used the gyrator concept to simulate the inductance. Recall that the inductance is the hardest problem in the design of VLF receivers. A gyrator is an operational amplifier circuit (j4] and 4 a in Fig, 13-11) that acts as if it were an inductor. The value of the inductance is set by the values of the resistors and of capacitor C:j. Op amp A.? is a noninverting follower with a gain of i 00 and provides half of the RF gain and most of the dc gain for the circuit.

After being amplified in As, the signal is split into two paths. The original Stokes design went directly to a precision rectifier/integrator, but I needed an RF output as well. When J built the original design, 1 found that the RF output waveform was distorted by the action of the precision rectifier. In order to overcome that problem, 1 changed the design to that of Fig. 13-11. Amplifier A?> buffers the input of the precision rectifier (A«) to isolate it from the RF output circuit. The RF output could be taken directly from the output of A& but 1 opted instead to use a gain-of-2 noninverting follower (A4) to buffer the RF output and provide some additional gain.

This receiver is a siiigle-channet model and will tune the 17- to 30-kHz range preferred by SID hunters. Because it is single-channel, 1 used 15-turn PC-mounted trimmer potentiometers for and #7. The receiver was built into a SFSCOM SB-7 RF box.

The operational amplifiers used in this circuit must have a gain-bandwidth product high enough to support amplification at the frequencies involved (741-family op amps wont cut it). The original Stokes design used four op amps, so he used the 4136 device. The 4136 is a quad op amp. Other alternatives include the 5534, CA-3140, or CA-3160 (all single op amps) or the CA-3240 (dtiai op amp).

The dc power supply is extremely important, in this circuit. The circuit will op* erate from any potential over ±9 to ±12 Vdc. It is critical that the dc power supply lines be well-bypassed. In Fig. 13-12, only a single op amp is shown in the power coit-nections inset but it serves as a reference for all stages. Each package (whether quad, dual, or single op amp) will have V- and V+ terminals. Each of these terminals must be bypassed to ground through 0.1-jlF capacitors. These capacitors should be placed as close to the body of the op amp as possible (otherwise oscillation might occur). The V— and V+ lines should be bypassed with electrolytic capacitors (be mindful of polarity!) in the 470- to 2000-^F range.

A multiband variant is shown in Fig. 13-13. This receiver is identical to the version of Fig. 13-12, except that the band-set capacitor (C3) is replaced by a bank of four switched capacitors.

The inductance simulated by the gyrator is calculated from:

The inductance required for any given frequency (in hertz) is calculated from:

Gyrator Vlf Receiver Vlf Converter Circuit

HVW4

ne asi

Vlf Receiver Circuit
13*13 Multibaiwi VLF receiver.

l+A3

Output

100 kQ

R7 100 kil

Gain

R8 100 kil

Vlf Converter

RIO 10011

The use of a VLF converter is shown in Fig. 13-14A, and an actual converter circuit is shown in Fig. 13-14B. The converter is placed in the signal path between the antenna and the antenna input of the receiver The converter circuit in Fig. 13-14B is based on the NE-602 chip. The oscillator section operates at 4000 kHz (i.e., 4 MHz), and the input is tunable over whatever range needed in the range 10 to 500 kHz. Tuning is accomplished by resonating the secondary winding of 71, with capacitor C^. The difference frequency will be either 3500 to 4000 kHz (difference IF), or 4000 to 4500 (sum IF) f so the VLF signals will appear in either range (the sense of the tuning will be low to high if the sum IF is selected).

Antenna

Shortwave

Ant

Receiver

Vlf Converter Circuit

1J-14 (A) Converter connection to SW receiver and (B) typical VLF converter circuit

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