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

Components, complaints and complacency

Whilst reading a few issues of Wireless World from the world radio museum web site, I wondered about the issue that happened on my 21st birthday month. Reading the editorial rang a very loud bell! If it reads familiar to you enjoy the feeling. JB ZS6WL

We are constantly receiving letters from private individuals who are finding it impossible to obtain supplies of certain components and whose pleas to manufacturers and distributors are met with stony silence. Even the small company, not in the electronics field, which requires a special component for a one-off job—and which has the advantage of a company letter heading—sometimes receives the same treatment.

One of our correspondents, who was starting a small company, claimed he was asked for two trade references and the name of his bankers, and that was only in order to receive a catalogue!

However, component supply is the result and not the cause of the problem, the whole attitude of the electronics industry towards the private experimenter and the amateur is one of non-co-operation to the point of scorn. Why is this, when many of yesteryear’s major innovations in radio and electronics emanated from the results of work carried out on a kitchen table?

In those days the amateur and the professional (often one and the same person) were working on similar problems and there was a mutual respect. The technology has advanced in leaps and bounds since then and industry is staffed with people who more than likely do not have an amateur background and who have no appreciation of the problems and frustrations that can face anyone trying to work on his own for interest, self-education or amusement.

Because of the great amount of publicity given to electronics, and the aura of mystery surrounding it in the eyes of the layman, more people are taking a practical interest. This has led to manufacturers and distributors being bombarded with letters requesting the solution to private electronic problems, many of which are nonsensical or frivolous, and others could have been answered easily if the writer had shown a little initiative or visited a good library.

To answer all these queries would cost a company a great deal and what would they get in return? Perhaps an order for two or three components, the value of which may be less than the cost of the handling.

By making their components generally available on the retail market, to be bought by people who may not be qualified to use them, a company feels that it is inviting the sort of costly correspondence mentioned. The reason for the reticence in this respect can be understood.

All this has led to the present ultra-low status of the amateur in the eyes of industry and the reluctance of many concerns to accept small orders.

The industry does, however, have a responsibility to the public, even if it is only to maintain its own image, and attempts must be made to give assistance in genuine cases. Refusals because of a couldn’t-care-less attitude can never be justified and small losses should be accepted at times.

Manufacturers could easily set up machinery to ensure that their products can be sold on the retail market through a distributor. Because of the difficulty in assessing the possible quantities required perhaps some sort of sale or return arrangement could be operated with the distributor. At the present time many components are completely unobtainable on the retail market.

In addition, all private individuals seriously interested in electronics should put their own house in order, and as a first step may well think of joining a club. If there is not one in the area—start one. The answer to nearly all the problems likely to trouble the experimenter could be found amongst a group of people with a common aim. Particularly difficult problems could be made club projects. Benefits could be reaped in terms of central facilities, pooled test equipment, tools and literature.

A great deal of useful work can be done by a well-run organisation of this nature and the local community can benefit. For instance, club projects could aid local handicapped people, small electronic systems for local firms could be designed and constructed (power supplies, control systems, photo-electric switches etc.). Often these firms can advantageously use electronic equipment, but, because only a one-off is required, it is uneconomic to employ professionals to do the job.

The companies who supply components would, we feel sure, be more than willing to assist such organisations so long as things were done on a business-like basis. A good example of the sort of co-operation that can be achieved is to be seen in the components list for the Logic Display Aid in this issue.

From: Wireless-World 1969-06 pg69