Introduction to the Electroluminescent Receiver Kit
A Superluminescent Experience

It seems that as Hams get older, the boards get smaller. I thought a receiver made from an NE602 portable phone chip was small until I saw English kits to "learn about Surface Mount Technique."

The curves have crossed and it's time for an upturn. So I designed a competition grade receiver for the Ham bands, or SWL bands, on two huge single sided silk-screened boards (8 1/2" by 8 1/2").

The interconnections are infrared and there are 21 functional LEDs that add to the circuit, not just to tell you when it's on. And this radio has all the audio the average ham can use.

The Electroluminescent Receiver Kit uses very low noise devices, modern circuits that are documented in current literature, with boards designed to help the kit builder have a fun and educational experience.

The Electroluminescent Receiver Kit

The receiver uses low noise dual gate MOSFETs in which all the drain/source current travels through an LED. Therefore, mistakes in the MOSFET circuit show in the brightness of the LED. No oscilloscope is needed to find dead circuits. The LEDs provide a simple, but very useful, build-in oscilloscope. An LED that is extremely bright, dim, or off means inspection of that circuit for errors.

The mixer LEDs are visual indicators of oscillator energy. Gate 2 of the MOSFETs is at .5 volt, which turns off the device. When oscillator injection is present (raising Gate 2 voltage), the LEDs turn on providing a test for oscillator operation and an injection level indicator.

LEDs are used at the bandpass and crystal filters, VFO, and crystal oscillator to show which filter, VFO frequency, or crystal is selected. Super-bright LEDs can be used in these locations to increase the visual impact of the receiver.

Circuit Description

The receiver is a dual dual-conversion, dual dual-image superhet receiver that covers the 40, 30, 20 and 17 meter amateur radio bands. With minor modifications, the 41, 31, 19, and 16 meter SWL bands can be received.

Two IF's are dual imaged using a relay switched Tesla VFO [1]. The IF frequencies are 455 kHz apart, allowing conversion to a 455 kHz IF strip.

VFO frequencies are 14 MHz, added and subtracted with the 4.000 MHz IF yields the 30 and 17 meter bands, 10.545 MHz subtracted from the 3.547 MHz IF yields 40 meters, and 10.455 MHz added to the 3.547 MHz IF yields 20 meters.

PCB Board 1

The highlights of board 1 are the RF amplifier, variable bandpass filters, and the first mixer. Also, the control center for the infrared switching is located at the input of the crystal filter.

The best high-level RF amplifier ever designed, the post-mixer amplifier for the Progressive Communications Receiver [3], is used as the RF Amplifier. The Bandpass filters [2] tune 4.4 MHz to 21 MHz. A 50K pot tunes both bandpass filters, with relay switching, controlled by IR (infrared) devices to improve input/output isolation. The range of the Bandpass filters allows the receiver to be built as a ham band or an SWL band receiver.

The first mixer is a four MOSFET mixer/amplifier combination. Gain is approximately 14 dB.

A VFO injection level trim pot at the first VFO amplifier is included so that the sensitivity and dynamic range of the mixer can be adjusted. High injection levels increase sensitivity; lower injection levels increase dynamic range.

Two ladder crystal filters (3.457 and 4.000 MHz) [4] clean up the signals for delivery to board 2. The bandwidth of the filters can be adjusted by changing the capacitors in the filter. High values (200-300pf) tighten the bandwidth; lower values (20-39pf) widen the bandwidth. The capacitors included with the kit, 100pf, provide a compromise for SSB or CW listening.

The dual LED input switching of the crystal filters is the control center for the Infrared band switching of the receiver. One of the LEDs is an IRED and the other is a switching indicator.

A high power IRED [5] at the 4.000 MHz filter activates a Phototransistor at the Crystal Oscillator so the output of the Second Mixer is always 455 KHz. Another high power IRED at the 3.547 MHz filter controls the VFO frequencies.

PCB Board 2

The second board includes the second mixer, crystal oscillator and amplifier, 455 KHz IF strip with AGC, product detector, BFO and BFO amplifier, and two audio amplifiers.

The LEDs of two AGC controlled amplifiers will flash with the level of the received signal providing a built in S-Meter. As the gain of the MOSFET amplifiers is varied, it changes the drain/source current, which is visually reproduced in the LEDs.

An S-Meter output is provided with adjustments for meter movements from 50uA up to 500MA. It is highly active, so weak signal reception can be aided with the S-Meter.

A variable gain BFO MOSFET amplifier sets the injection level to the product detector. Running this amplifier at it lowest level, with the LED barely on, yields a very quiet receiver.

An adjustable gain pre-audio amplifier follows the product detector output filter. A "Gain Adjust" emitter bypass capacitor [6] is marked on the board to adjust the drive to a TDA2002. No capacitor is good for home use; a 2.2 mfd is used in a noisy environment.

Building Aids

The PCB is a single sided design with traces .50" wide. All the soldering points were enlarged while trying to keep soldering bridges to a minimum. Almost any size soldering tip can be used to populate the boards.

Solder checks are done by looking for light leaking through a hole (board held over a light source), which means a missed solder pad or an incomplete soldering job.

A solder mask was not placed on the boards. Solder has plenty of room to flow properly, giving a smooth appearance, easy identification of bad joints, and lots of ground plane for modifications and experimentation.

Each board is three sections, with rows of holes defining each section, clearing defining each functional section of the receiver. Small shields can be installed along the holes for isolating the VFO, crystal filter, mixers, etc. The mounting holes in the PCB boards are aligned for easy stacking, either whole or split apart.

The topside of the board has a complete silkscreen, for easy location of parts. Important instructions are silkscreened on the board, so a hurried person would not miss important points.

The most common parts, 95 .01 caps, 32 100K resistors, and 28 100 ohm resistors, all have the part number inside the footprint for easy identification. Spend two hours stuffing these parts, the boards will look half-done, and give the inspiration to keep going on to completion.

Included is a CD-ROM, with plenty of pictures, schematics, circuit details and step-by-step instructions.

The receiver is designed to work without a case. An unetched PCB bottom plate is included for stacking the boards and mounting all the connectors for 12 Volts, speaker, and antenna.

The main tuning capacitor is soldered on the bottom side of the PCB, underneath the VFO, for a very sturdy and stable mounting. With four poly capacitors for temperature compensation and solid mounting for the tuning capacitor, the VFO is very stable.

Conclusion

It is time to put a kit together that you can see, easily place parts, and have the freedom to modify and improve without major hassles. The result is a very good receiver that can be a working base for experiments, improvements and learning.

The LEDs have important circuit contributions, but after the receiver is working, also contribute to an exciting visual experience. Saving 500 mA on a desk lamp is another advantage.

With the large board sizes and distinct outlines of each individual circuit, it is an extensive, but easy kit to put together, while familiarizing you with the circuits that make a receiver "bring in the signals".

See "How Good is This Receiver?" for more information.

Footnotes:

[1] The name for the VFO, "Tesla Oscillator", comes from the April 1999 issue of QRP Quarterly, Chris Trask, N7ZWY, "LC Oscillators - A Brief History". The common name for the VFO is the Vackar VFO.

The VFO is an adaptation of the circuit from the July 1997 issue of QEX, "Meet the Vackar: The Simple, Stable VFO You've Been Looking For", by Mark L. Meyer, WU0L.

[2] September/October 2000 issue of QEX, "Narrow Band-Pass Filters for HF", by William Sabin. NBPF.ZIP at http://www.arrl.org/qex-files has all the information on the filters.

[3] Wes Hayward, W7ZOI, and John Lawson, K5IRK, "A Progressive Communications Receiver," QST, November 1981, Page 11. http://www.qsl.net/aa3sj/Pages/PR.htm

[4] "Designing and Building Simple Crystal Filters", by Wes Hayward, W7ZOI, July 1987, QST, Page 24

[5] Individual IREDs are part number LED1067, Page 8. (T-1 3/4", 1.3V, 1A peak, 100mA continuous, 940nm, 16 degree viewing angle, clear package). BG Micro at http://www.bgmicro.com