A Simple 80M Receiver

Introduction

This circuit originally published in a book named Amateurfunk-Superhets (Ham Radio - Superhets) by Gerhard E. Gerzelka. This was a small pocket book that was part of the RPD Electronic Pocket Book series, published in 1979.

I used the Google translator to convert from German to English. It does a pretty good job, but the converted wording often sounds a little funny and uses odd words. Where necessary, I have fixed some of the wording. For the most part, I paraphrased and added my own interpretation.

I redrew all of the drawings. I don't like schematics with big empty boxes that have pin numbers. I like more detail. So I redrew all the schematics. The original schematics were readable but pretty small. And, there was too much in each one. So I split the drawings into smaller, logical sections.

Schematic Notes:

Each schematic (Fig 14, Fig 17, and Fig 18) for this receiver has been split into two logical sections, A and B. This is because the original schematics are too small and cramped. Fig. 19, the power supply, is simple enough and doesn't need to be split.
- Off page arrows are signals that go to or from the A and B sections of a board schematic. The text is the signal name and should match across the A and B schematics.
- Board Connectors symbols are for wires that are going to and from different boards. The number is for referencing a particular wire. But note that, the direction of the arrows indicate signal (input/output) direction. Refer to the interconnect diagram, Fig. 21.
The parts that are in dashed boxes are Front and Rear Panel controls, external to the circuit boards. However they are included with the board schematics for clarity.
Power connections are shown on the B section of a schematic only. Note that the input capacitors on the power lines, are different for each board.
Reference Designators - The schematics in the book only lists reference designators for certain parts, like diodes, inductors, trimmers, and external controls/switches. With few exceptions, basic resistors and capacitors have no reference designators. So, I added my own reference designators. I kept the original reference designators for diodes and inductors, but created new designators for all resistors and capacitors, and external controls/switches. Designators R1 and C1 start on the far left of Fig. 14 and progress left to right, top to bottom. The designators then resume on Fig. 17 and across to Fig. 18. The Power Supply, Fig. 19, has it's own set of designators starting with R1, C1, T1, etc..
The inductor values and physical makeup my be difficult to determine from the schematics and the parts list. However, I attempt to explain them at the bottom of the page, in section 2.7 Notes on the overall circuit.

2. A Simple 80M Receiver

2.1 The Circuit Diagram

Good receiver performance can be achieved on the lower amateur bands with simple circuits. Fig. 13 shows the circuit diagram of a simple 80-m receiver for CW, SSB and AM, i.e. all of the usual operating modes in HF amateur radio.

The received signal, in the 3.5-4.0 MHz range, passes through a two-circuit band pass filter to the mixer, where it is converted to 460 KHz IF. The converter VFO oscillates in half of the receiving frequency. A four-circuit and a two-circuit L/C filter, which are coupled to one another via an IF amplifier, ensure selective selection. A conventional diode holder is used for demodulation, which is supplied by a BFO with the auxiliary carrier during CW and SSB operation. The audio signal passes through a two-stage amplifier for reproduction via headphones or small speakers. Power is supplied from the lighting network.

IF and first AF amplifiers are connected to an automatic amplification control (AVC) which is controlled by the Nf and equipped with a separate amplifier.

This receiver can also be recreated by newcomers without difficulty. It is well suited for gaining initial construction and operating experience.

2.2 Input Filter, Mixer and VFO

The Mixer circuit uses two sections of U1, the CA3046 (RCA). See fig. 14A-Input Filter and Mixer and 14B.-VFO.

L2 and L5 are the inductors for the Input Filter. The Varactor Diodes D1/D2 and D3/D4 tunes circuits. Windings L3 and L4 inductively couple L2 and L5. Capacitive coupling would not be constant with capacitive frequency tuning.

C1/L1 act as a IF trap circuit. Matching positions are covered in all of the coil cores.

Note: The original book had a small error. The bottom ends of R5 and R6 were not tied to -5.6V

L6 is the VFO oscillator inductor. The frequency is adjusted by means of the capacitance diodes D5 ... D7. Adjustment positions for the VFO corner frequencies are L6 and C10.

The tuning diodes receive their control voltage via potentiometer R1, which must have a linear resistance characteristic for largely linear frequency scale calibration.

2.3 The CA3046 Integrated Circuit

The IC internal circuit, in Fig. 15, shows five transistors, that can be used for frequencies up to 120 MHz. There are no internal passive components. T1 and T2 are a matched differential pair and are suitable for bridge and push-pull circuits. The IC is delivered in a Dual In-Line 14 Pin (DIL-14) plastic package. For all circuits described in this book, the CA3086 (RCA) can be used instead of the CA3046, without any circuit changes.

Note: The collector of each transistor of the CA3046 is isolated from the substrate by an integral diode. The substrate (Terminal 13) must be connected to the most negative point in the external circuit to maintain isolation between transistors and to provide for normal transistor action.

Fig. 16 shows how you can create an interesting amplifier circuit from three of the IC transistors: The signal voltage is at the base of U1B, which works in a collector circuit: the output voltage drops at U1B, which serves as an electronic emitter resistor for U1A (left); U1A (right) takes over this voltage at the emitter and amplifies it at the collector, so the transistor works in the base circuit. with the help of a certain direct voltage at the base of U1B, the amplification level of the arrangement can be set, if you connect an AVC voltage here, so you get an automatically controlled amplifier, and with a mixed voltage on the U1B base, the circuit works as a mixer.

2.4 IF amplifier, Demodulator, and AVC control stage

Fig. 17A and 17B the shows the IF Amplifier, Demodulator, and AVC Control circuit which uses a second CA3046, U-2.

In Fig 17A you can see the two selective IF filters. On the left side is a Four-Section filter (L8, C22, L9, C24, L10, C26, L11, C18) and on the right side is a Two-Section filter (L1, C32, L13, C34). Each section of the filters are capacitively coupled. The cores in the coil forms are used to to aligne filter sections. The selectivity should be about 5 KHz/6 db.

Diode D8 is used for IF Demodulation, which is biased by means of D9, at about -1.0 Volts, so that even very small IF voltages can be processed. The Beat Frequency Oscillator, for the demodulation of CW and SSB signals, is supplied via the circuit board connection 10 (BFO). The audio (AF) is then coupled to the output via a Low-Pass Filter, which removes residual noise.

Two of the IC transistors work as regulating amplifiers for the AVC circuit, which is controlled by the AF. The regulation is adjusted by means of the R4 for undelayed use, because the receiver sensitivity is determined by the unregulated mixer stage. The control time constant can be selected using switch S1: fast with approx. 150 ms and slow with approx. 2.5 s decay time, the rise time is <10 ms in both cases; Time-determining terms are C6/R7 for fast and additionally C7/R6 for slow.

The S-meter has an almost linear S-value calibration, which the diode D12A takes care of. The zero position of the instrument is calibrated with VR6, the full scale with VR5.

2.5 BFO and AF Amplifiers

Fig. 18 shows this circuit part equipped with the U3 of type CA3046.

The BFO circular coil is L14, L15 is the coupling winding for the feedback voltage. The frequency is bit adjustable to +/- 3.5 KHz around the mean value 460 KHz, in addition the tuning diodes D13/14 and the potentiometer R11 with linear characteristic. Adjustment positions for the BFO corner frequencies are L14 and C8, the BFO voltage of 0.1 ... 0.3 V_eff at the circuit board connection 10 is to be leveled by means of R9 (VR8).

The two-stage AF amplifier, Fig. 18B, is pretty straight forward. Its first stage works according to the example shown in Fig. 16 and is connected to the AVC regulator. The volume control, R10, is located between the two AF amplifier stages. Transformer T1 can drive a small speaker or a pair of low impedance head phones.

Low frequency amplifiers 1 and IF amplifiers produce an AVC control depth of around 95 db in total.

Power Supply

2.6 The power supply

The schematic of the power supply is shown in Fig. 19. It consists of a dual voltage supply, +/-5.6 VDC, and a single voltage supply, +24 VDC. The power supply in the book used a single 220 VAC transformer with two secondary windings, 18 VAC and 48 VAC. However, I modified the schematic to use two 120 VAC transformers. With two transformers the power supply can be wired for 120 VAC or 220 VAC. As shown in the schematic, the two primaries are wired in series for 220 VAC and parallel for 120 VAC. The power input should be fused for 100ma.

For the bridge rectifiers I specified 1N4003 diodes. These greatly exceed the requirements of the circuit. However any of the small, 1 Amp, through hole or surface mount bridge rectifiers can be used. Even in small quantities, the small bridge rectifiers can be had for less than $1.00. A good choice to help minimize parts.

The dual voltage supply has a center tapped, 18 VAC, secondary. The output of the bridge rectifier (DB1) should then output about 25.2 VDC. With the transformer center tap grounded, a +/- 12.6 VDC is created. These voltages are then applied to their respective filters and Zener regulators, which create +/- 5.6 VDC.

The single voltage supply has a 45 VAC secondary. The output of the bridge rectifier (DB2) should then output about 63 VDC. This voltage is then applied its associated filter and Zener regulator, which creates +24 VDC. A stable voltage is particularly important here in the interests of high frequency accuracy.

There are three output pins for the +5.6 VDC and three for -5.6 VDC. This provides two power lines for each receiver circuit board. Note that these voltages are also routed to the AVC switch (-5.6 VDC), the BFO switch (+5.6 VDC), and the Volume control (+5.6 VDC). The +24 VDC output only has one output pin because it is simply routed to two chassis mounted controlls, VR1-Main Tuning and VR7-BFO Tuning.

When installing the power supply unit in the receiver housing, one must take into account that the low-frequency amplifier and the lines of the tuning voltage are sensitive to hum; shielding measures may therefore be necessary.

2.7 Notes on the overall circuit

Figure 20 is a listing for the inductors, straight out of the book. Initially, I found it to be a little difficult to understand. I just wasn't use to the terminology. But here is what I was able to decipher.

References like "10x0.03" mean "10 strands of 0.03 mm diameter wire". This would be equivalent to a SWG #36 - #38 wire. I believe the intent is to use "Litz" wire. While Litz wire may make a difference at 460 KHz, the IF frequency, it will have little effect at 3.5-4.0 MHz. Using a solid enameled would probably work just fine and may provide a better Q.
The "Vogt Coil Kit D41-2165 and D41-2519" are no longer available. I can't find any reference on the internet. The best that I can figure out is that, these are small (1/4" to 3/8" diameter) coil forms with adjustable ferrite cores. The core is adjustable allowing the user to adjust the inductance value.

So I did some searching, and performed some calculations, to come up with the required inductors. I looked for pre-wound inductors that were within the range required. I found that Coil Craft had all the necessary parts. To select a inductor, an approximate inductance for each inductor was calculated, based on the other associated circuit components. I have added my inductor selections in Figure 20. Overall, the inductors that I have selected will cost around $30, in small quantities.

Inductor L1 resonates with C1 (560 pF) to form a 460 KHz trap. This make the required inductance, when tuned, to be about 213.8 µH. A good choice, from the Coil Craft selection guide would be the SLOT TEN-2-15, which has a nominal inductance of 216 µH and is adjustable via the ferrite core.
L2/L3 and L4/L5, used in the RF Input Filter, each have a 10-40 pF Ceramic trimmer, a fixed 30 pF capacitor, and two BB105 Variable Capacitance Diodes. If the trimmer and the BB105s are adjusted to their midrange, the approximate capacitance would be 73 pF. At 3.75 MHz, the mid tuning range, a capacitance of 73 pF would require an inductance of ~25 µH. A good choice might be the SLOT TEN-2-9, which has a nominal inductance of 25 µH. L2 will need to be modified to add a tap, 2 turns from the ground end, and a 2 turn loop, L3/L4, for coupling to L2/L5. L5 will need to be modified to add a tap, 5 turns from the ground end.
L6 is part of the HF VFO tuning, which should span a range of 3.040 - 3.540 MHz. In parallel with this inductor is a 10-40 pF Ceramic trimmer, a fixed 30 pF capacitor, and three BB105 Variable Capacitance Diodes. If the trimmer and the BB105s are adjusted to their midrange, the approximate capacitance would be 82 pF. At 3.290 MHz, the mid tuning range, a capacitance of 82 pF would require an inductance of ~28.5 µH. Again, A good choice might be the SLOT TEN-2-9, which has a nominal inductance of 25 µH.
L7 is simply a 100 µH RF Choke that should be available almost anywhere.
L8, L9, L10, and L11 form the IF Amplifier Input Filter and L12 and L13 form the IF Amplifier Output Filter. Since they all run at 460 KHz and have the same parallel capacitance (360 pF) the inductance should be the same. A quick calculation indicates that a 332.5 µH inductor should be used, for all. A good choice might be the SLOT TEN-2-16, which has a nominal inductance of 312 µH. L8 and L12 will require a tap at 90 turns from the cold end. L11 and L13 will require a tap at 14 turns from the cold end. These taps will take a bit of skill.
L14 and L15 are for the BFO and will run at 460 KHz, +/-3.5 KHz. In parallel with this inductor is a 10-40 pF Ceramic trimmer, a fixed 330 pF capacitor, and two BB105 Variable Capacitance Diodes. If the trimmer and the BB105s are adjusted to their midrange, the approximate capacitance would be 373 pF. At 460 KHz, the mid tuning range, a capacitance of 373 pF would require an inductance of ~320.9 µH. A good choice might be the SLOT TEN-2-16, which has a nominal inductance of 312 µH and a high inductance of 426 µH. L15 would then be a 23 turn link, for feedback.

Figure 20
Component	Description
L1	20 windings, 10x0.03 mm CuLS or 7x0.04 mm CuNy on Vogt coil kit D41-2165 Coil Craft - SLOT TEN-2-15
L2	34 windings, 15..20x0.05 mm CuLS. Tap 2 windings from the cold end. Vogt coil kit D41-2519. Coil Craft - SLOT TEN-2-9
L3	2 windings, 15..20x0.05 mm CuLS, over L2 winding
L4	like L3, but over L5
L5	like L2, but tapped at 5 wind. v. cold end. Coil Craft - SLOT TEN-2-9
L6	Same as L2, but without tap. Coil Craft - SLOT TEN-2-9
L7	Ferrite choke 100uH, commercially available
L8	115 windings, 12x0.03 mm CuLS, tap at 90 windings from the cold end, on Vogt coil kit D41-2519. Coil Craft - SLOT TEN-2-16
L9	like L8, but without tap. Coil Craft - SLOT TEN-2-16
L10	like L9. Coil Craft - SLOT TEN-2-16
L11	like L8, but tapped at 14 windings from the cold end. Coil Craft - SLOT TEN-2-16
L12	like L8. Coil Craft - SLOT TEN-2-16
L13	like L11. Coil Craft - SLOT TEN-2-16
L14	like L9. Coil Craft - SLOT TEN-2-16
L15	23 windings, 12x0.03 mm CuLS, wrap over L14

Fig. 21 shows the line routing between the partial circuits. The connections 7 and 10 must either be kept short (<= 3 cm) or laid in a shielded manner. All other lines are not critical if the note on hum sensitivity in 2.6 is observed.