RAN Technology

Using rectifier diodes as varactor/varicap substitutes

Technical 0 Comments 03/02/2019 

Modern-day solutions for RF part obsolescence

Posted By: Robert Nickels (ranickels)

As anyone who likes building radios knows, it's getting harder and harder to buy RF components.   One of the last casualties was the 3-section pi-wound RF chokes made by Hammond, a company that has done more than most to support the vintage radio and audio hobbies.    The Toko variable inductors long favored by QRP enthuisiasts have become all but unavailable, and variable capacitors are expensive, scarce, and only a few values are still being made.   Collins shut down it's mechanical filter business about the same time International Crystal closed it's doors, and more modern components such as ceramic resonators and filters are rapidly being discontinued by companies such as Murata.    And while you can still buy some popular parts such as varicap tuning diodes and J-310 MOSFETs in SMT packages, thru-hole packages have become obsolete.

So what's the homebrewer to do?   Of course it's always possible to scrounge up the parts needed to build a  one-off homebrew project for yourself,  but not everyone has a deep junk box or the ability to find vintage parts.   That's why my designs are always based on parts that can be purchased new or at least are reasonably available from present-day sources.   To do so often means innovating alternatives to vintage parts  or applying new technology to do what used to be done in different ways.

Tuned circuits for receivers and low-powered oscillators and amplifiers are a good example.   While inexpensive trimmer capacitors can be found, slug-tuned inductors and transformers aren't being made anymore except for low-voltage rated types for solid state circuits.   Only a very limited selection of air variable capacitors can be found outside of junkboxes, surplus, and hamfests.      Varicap tuning diodes are a good alternative for tuned circuits, but currently stocked parts are in the low picofarad range and only available in surface-mount packages.

Regular power diodes like the 1N4007 and 1N5408 can be used effectively as varicap or tuning diodes, taking advantage of the inherent voltage-dependent capacitance property of a back-biased PN junction.   Applying a DC voltage causes the thickness of the depletion zone to increase, increasing the capacitance proportionally.    Unlike varicap diodes that are sold for this purpose, the base capacitance and amount of capacitance change that is possible are not specified by the manufacturer, so these are not precision devices, but homebrewers can compensate for the minor part to part variations that exist.

I recently characterized a dozen each 1N5408 and 1N4007 diodes to get a sense for how consistent their properties are.   Rather than measuring capacitance directly which is somewhat error-prone,  I put each diode into a Colpitts oscillator circuit and measured the resulting frequency at 0.5 volt increments from 0 to 5 volts DC.    The resulting data can be seen below.   (A 10uH inductor and some additional parallel capacitance was present in the test fixture).

Since I had two different lots of 1N4007s from different suppliers I tested both lots to see how closely they track.   While it is obvious that the part to part variation is not what would be expected from a precision device, both of these common rectifier diodes provide a useful tuning range and both  are so widely used it is likely they will be available for a long time to come.

Lastly, since I'd read about using LEDs as tuning diodes I compared a variety of types against the two rectifier diodes.   While some do work fairly well, I also tested a few "short lead" T1 LEDs from a recent purchase and found that they produced no capacitance change at all!   For this reason I decided to stick with the rectifier diodes, but LEDs and other diode types may be worth evaluating further.

    In his book "In Touch with Leo", the success and growing backlog of CB-100 orders is what drove Leo to his banker, Lou Ross, who asked him why he didn't anticipate the number of orders and required capital.  Leo responded "Lou, never on God's green earth could I have believed such an influx of orders!".    More funding was needed than the bank ...  READ MORE
- Robert Nickels (ranickels),  03/18/2019 

Spectral Purity using Epson Programmable Oscillators

Do harmonics matter if there's no way for them to be heard?
Category: Crystal Replacement
 In the 5 or so years I've been using the Epson oscillators I've looked at the spectral output many times.   Without a doubt the harmonic output from the Epson oscillator is high as would be expected  from a square wave device.   The question to me has always been "does it matter"?PLL controlled digital oscillators have long been used as LOs in recei...  READ MORE
- Robert Nickels (ranickels),  03/12/2019 

The Foxhole Radio

aka "Razor Blade and Safety Pin Radio"...from the Boy's Fun Book
Category: Vintage Radio
 When I was a kid, my dad gave me a thin hardcover book called "Boys Fun Book - Things to Make and Do".   It was printed during WWII on very thin paper as a wartime conservation measure, but was chock-full of interesting projects, ranging from hobbies and magic tricks to sports, puzzles, camping...and the chapter I was drawn to:  "Unusual Radios You Can Build You...  READ MORE
- Robert Nickels (ranickels),  03/08/2019 
   The following obituary appeared in the Freeport Journal-Standard on Febr. 9, 2019.      I attended a lecture by Mr. Anderson in the late 1990s after moving to Freeport and working for Honeywell where despite being a supplier to DEC, few realized the local connection existed to the founder of what once was the second-largest computer company in the world.   Mr. Anderso...  READ MORE
- Robert Nickels (ranickels),  02/09/2019 

The Case of the Phantom Voltage

“When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.”
Category: Technical
A Heathkit HW-12 gave me a troubleshooting challenge the likes of which I've not seen before - or even heard of!A few days ago the transmitter wouldn't produce output, and in checking voltages and I measured -75 volts on the ALC line which should be -20.   But it corrected itself and I figured it must have been a solder splash or something that cleared by itself.  That theor...  READ MORE
- Robert Nickels (ranickels),  02/08/2019 

The Solvay Conference, 1927

The Smartest Photograph Ever Take
Category: Historic
The Solvay Conference, founded by the Belgian industrialist Ernest Solvay in 1912, was considered a turning point in the world of physics. Located in Brussels, the conferences were devoted to outstanding preeminent open problems in both physics and chemistry. The most famous conference was the October 1927 Fifth Solvay International Conference on Electrons and Photons, where the world’s most...  READ MORE
- Robert Nickels (ranickels),  12/27/2018 
   Put yourself in 1963...A stamp was a nickel, bread was 22 cents a loaf, and you could buy 3 gallons of gas for a buck.   The average wage in the US was $84 a week.  If you were a ham, you might have been able to build and trade your way up to a really nice AM/CW station - maybe a Valiant or DX-100 transmitter with an NC-300 or HQ-170 receiver - a huge improvement over the S-38 you s...  READ MORE
- Robert Nickels (ranickels),  12/17/2018 

Astro...Don Stoner's legacy

How many of these rare transceivers have you seen?
Category: Vintage Ham Radio
A recent post on the Cubic-Astro mail reflector noted the similarities between the rare CIR Astro 200 and the much more common Cubic Astro 150 transceivers.   And therein lies an  interesting story...I have done a little research on the CIR Astro 200, and the commenter was right - it is the progenitor of the Astro line we are much more familiar with as a result of Swan (already owne...  READ MORE
- Robert Nickels (ranickels),  12/09/2018 

The DX-60 Family

what hath Benton Harbor wrought?
Category: Vintage Ham Radio
When an engineer at Heathkit in 1962 turned his attention to designing a replacement for the somewhat flawed DX-40 novice transmitter, little would he have expected the DX-60 to remain in the Heathkit catalog for 14 years.    From the new solid-state power supply to the streamlined apeperance and dependable performace, the new transmitter was instantly popular with novices who had t...  READ MORE
- Robert Nickels (ranickels),  12/09/2018 

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