A 'Starter Project' for 'Newbies/Interested Constructors' - A DC RX

A "Starter Project for after the RAE"

I was thinking about a 'starter project' for the 'newbies' after the RAE. It used to be something like a 'Crystal Set'. Because the AM transmitters are few and far between nowadays, it should be a 'simple DCRX.' And to make it more 'interesting' it should receive SSB, CW and PSK31 and RTTY.

Direct Conversion Receivers have been around for several decades.

Lets try putting an AI on the task...

Based on a search of technical literature and recent hobbyist projects, there are many articles and resources dedicated to direct conversion receivers (DCRs), ranging from historical overviews to modern, high-performance designs.

Academic and Technical Articles:

Numerous technical papers and articles exist on ScienceDirect, MDPI, and ResearchGate covering DCR design, specifically addressing challenges like DC offset, LO leakage, and I/Q imbalance.

Main Themes in Recent Articles:

Simplification:

Many designs focus on minimizing component counts, often using under four transistors.

Performance Improvement:

Articles discuss techniques to overcome traditional DCR limitations (hum, Microphonics) using I/Q demodulators and modern DSP.

Direct Conversion vs. Superhet:

Several articles compare the cost and complexity benefits of DCR, especially for integrated, zero-IF designs.

Overall, the number of articles is substantial (over 20+ prominent technical, academic, and hobbyist resources found in the search results).

So what do 'we' want to build?

Some design features:-

1) The ZS article used a 420mm loop antenna

2) Use an 'active antenna'

3) An IC "Balanced Mixer" to suppress the local oscillator leakage to the antenna.

*** Whoops! That might be a problem...

4) Which 'Band' do we want to use?

5) How sensitive should it be?

Example Picture showing simple construction method 1.

You can't get the wood, you know!

There 'used to be' many integrated circuits for the Direct Conversion Receiver.

SO42 - SL6440 - TIxxx - CA3086 NE602/NE612 and so on. Most are no longer in production or available for purchase. [Also fake ones are available!]

See the note 'Dilbert Cell.' [Not a typo]

A recent Youtube described how a 'Gilbert Cell' worked. Take a look. The simple version would do the job nicely - using 'readily available' - 'general pupose' transistors!

The Gilbert Cell (084d)

https://youtu.be/mQ36yy7mloA?si=2W3HRTWym6j-LcsR

Now we are scavengers

This is where I get on a 'soap box' and tell you about how many transistors I have recovered/scrounged/scavenged from old/wornout electronic circuits. The list includes ATX Power supplies, VHS recorder/players etc.

Just Found this:-

"There are many conflicting technical requirements for a good-quality front-end in an SW receiver. The noise figure and the intermodulation level should be low, the RF insulation between ports LO, RF and IF should be high, and some amplification is desirable. The Type SL6440 high level RF mixer from Plessey ensures a noise figure of around 10 dB, and offers sufficient suppression of the LO signal." - Oh dear! No longer available.

Dilbert Cell ???

In electronics, the Gilbert cell is a type of frequency mixer. It produces output signals proportional to the product of two input signals. Such circuits are widely used for frequency conversion in radio systems.[1] The advantage of this circuit is the output current is an accurate multiplication of the (differential) base currents of both inputs. As a mixer, its balanced operation cancels out many unwanted mixing products, resulting in a "cleaner" output. Gilbert cells can also be used as variable-gain amplifiers (VGA).[2]

It is a generalized case of an early circuit first used by Howard Jones in 1963,[3] invented independently and greatly augmented by Barrie Gilbert in 1967.[4] It is a specific example of "translinear" design, a current-mode approach to analog circuit design. The specific property of this cell is that the differential output current is a precise algebraic product of its two differential analog current inputs.

NE602/612 ICs

The NE602 and NE612 (including SA602/SA612 variants) are essentially identical, interchangeable Gilbert cell mixer/oscillator ICs often used in RF applications. Originally, the NE612 was introduced as a slightly updated, redesigned version of the NE602 for better high-frequency performance, but they are often identical in modern production and share the same specs, datasheet, and Pinout.

Both are designed by Signetics (now NXP/Philips). The distinction between "NE" (commercial) and "SA" (industrial/automotive) is more relevant than 602 vs. 612.

40m Direct Conversion Receiver

https://community.element14.com/technologies/open-source-hardware/b/blog/posts/simple-dcr-assembling-a-7-mhz-40m-direct-conversion-radio-receiver-part-1

https://qrp-labs.com/images/news/dayton2019/FDIM2019ConfProceedings.pdf

A 40m Direct Conversion Receiver project to upgrade from ZR to ZS

Hannes Coetzee, ZS6BZP, B.Eng Elektronic (Pretoria)

Also in :-

https://zs6wr.co.za/anode/AnodeJuly2010.pdf - Hannes Coetzee ZS6BZP

A 'Work in Progress' JB ZS6WL 2026-05-12

One of the 'things' I was going to ask the students

Just a thought...

You might have noticed I used a Solar Garden light in the session on QRM/EMC, well it occurred to me to check the design in the UK. Slightly different but essentially the same simple circuit. Which oscillates around 1 MHz!

What? Yes it does. The question I was going to ask is this; "How do you get a L.E.D. to light when you only have a 1.2 Volt battery?"

When you did the section on transistors did someone tell you about 'inductive loads'? Or why you need a 'protection diode' across a relay? The same applies to a r.f. circuit with a coil in the collector circuit. Tuned or not it will swing above the supply rail voltage to double the collector voltage rating. So if you connect a transistor to a 12 Volt rail, remember the Vce rating must be 24 Volts or greater.

Remembering that a coil stores the energy in the magnetic field. So when you turn off the current the coil tries to maintain the field. Generating a 'back emf' that in some cases can be kiloVolts! Just arrange a relay as a 'buzzer' with the contacts making and breaking the circuit. Don't touch it! Depending upon the coil inductance the voltage can reach a very high voltage.

So the simple circuit these days is a single integrated circuit. A coil, the solar panel and the L.E.D. With the i.c. doing the charging and switching on the L.E.D.

[I just a fruitless search for a circuit that I know I downloaded months ago! Now thanks to Bert's blog I have one. https://bertsblogs.com/garden-solar-light-circuits/]

He explains the circuit as well. As he and I was going to here is the simple fact. The LED is a 'super-bright' white LED which needs well above the typical 1.8 Volts to turn on. L.E.D.s are much better these days from the 60s and 70s versions which were always red and quite dim in comparison to the filament bulbs/globes of the day.

The one thing I noticed as a difference between the ones I had bought in South Africa and the ones in the UK, the batteries are NiCd in SA (cheap). The ones in the UK are Nimh. Nickel-cadmium versus Nickel metal hydride. Small pricing difference but on thousands of models makes a big difference.

So are they going to interfere with your h.f. reception? Probably not unless you have a few hundred in the garden.

They don't last long. Water ingress usually rusts the connections and screws. Treating the plastic with sealant is just not worth it. Replacing the NiCd battery with a shop bought Nimh helps. But then sealing with marine silicone will keep maybe a few years longer.

73 John ZS6WL