Thursday, 24 March 2016

A DC Receiver Using Home Brewed Mixer IC

I am always fascinated with the sound of a direct conversion receiver. That sibilant, pure and dynamic sonic impact is just mesmerizing. I remember my first home brew receiver from "Solid state design" by Wes Haywards, using a CA3028A as mixer. It was almost three decades ago, in 1982 and still in use. Last weekend I decided to build another version of it using my home brewed mixer IC (as described in one of my previous post).

RECEIVER: The schematic diagram of complete circuit of the receiver is presented below. Input signal from antenna through a tuned circuit is fed to the mixer. I used an air core inductor with an ex-BC receiver tuning capacitor for the tuned circuit. However in case of nonavailability of the tuning capacitor, a varicap diode can be used as a replacement. The parts in the dotted square represents the home brewed mixer IC, I already have talked about. The mixer is simple, provides reasonable gain and requires little V.F.O. drive, which seems to be the primary requiments for such a portable QRP design. The output AF signal is filtered by a simple RC filter, which also acts as the only selectivity components in this design and the resultant signal is thus amplified by an amplifire built around Q4. This stage provides around 42db of audio gain. The amplifier can directly drive a high impedance headphones or a crystal earpiece. The over all sensitivity is very respectable and receiver can easily discern signals below 1uV.

However if you require to employ a speaker a small amplifier wired around LM 386N or a similar circuit can be used. A suitable circuit for such implementation is given below.

V.F.O.: The schematic diagram of the V.F.O. circuit is given in the following diagram. A single stage Hartley configuration employing a common BJT is used and it worked as expected, without any problems.

However I would like to state few points for V.F.O. stability:

1. Always build a V.F.O. in a separate shielded enclosure. For easy home construction you can make such enclosure from scrap double sided PCB laminates.

2. Always use a feed through capacitor and a series RF choke of about 200uH in series with the positive supply rail of the V.F.O. supply.

3. Anneal the V.F.O. inductor initially in boiling water for about 5 to 8 minutes and then allow its proper cooling before its use in the circuit. This eradicates the initial effect of mechanical stress on mettalic wire during wiring and let them set properly.

4. After building and initial testing seal the circuit with Araldite or similar adhesive to protect it from effects of humidity, temperature and vibrations.

5. Always make your capacitors in tuning and feedback circuit, to be a parallel combination of two or more units. This evenly distributes the RF current through them and reduces their heating and resultant thermal drift.

6. Always use black dot NP0 (C0G) or polestrene (styroflex) capacitors in your. V.F.Os.

I am sure that if you follow this basic philosophy many of the basic V.F.O. problems can be eliminated.

Wednesday, 9 March 2016

An IF Strip Using Homebrew ICs.

In a recent post I described a way to homebrew your own mixer ICs. It actally contain a differential single ended mixer which can be used in many ways as it has been stated in the end of that post. So, why not build a simple cascode amplifier with these sub circuits? It seemed to be a good idea and I tried it. A two stage IF stage amplifier with AGC was wired as shown in schematic diagram and was tested.

The parts in dotted squares are homebrewed subcircuits. Using this two modules, I tried to prototype 4MHz IF amplifier strip of a two-stage cascade IF amplifier. Output tuning circuit of each stage is consists of an RFC of 4.7uH alongwith a capacitor of 330pF. These values can be altered for the frequency of interest or use. I used standard axial chokes for inductors but obviosly toroids are a better option. AGC is derived through a voltage doubler rectifier a portion of the output in 1N60 or 1N914 diodes,and a common PNP transistor. 

The initial tests show a maximum IF gain of about 68dB, AGC  effectiveness started from an input of about 20dBuV EMF, that is quite an impressive figure for such a simple circuit with inexpensive devices. AGC versus gain characteristics are plotted in the following diagram:
Indeed good designs can be evolved empirically with inexpensive components and simple thoughts.

Friday, 4 March 2016

SPOTTO:A High Performance DSB/CW Transceiver.

Over the past few months, attracted and inspired by the simplicity of direct conversion and DSB techniques I built several simple transceivers like that of Wee Willy and similar NE612+LM386 flavors including all the stuff I could Google on the net. Though technique of direct conversion appeals me for its added incentives of high performance versus simplicity and of its being less subjective to spurious responses and those designs don't entail the complexities of those high end superhets. But most of the designs including those mentioned earlier are of that Neolithic NE 602/LM 386 variants, more popular for their bad reputation and non-convincing performance. I, myself eschew the use of LM386 in serious receiver deign as much better and quieter alternatives are now available for cheap. Moreover I feel that for a given performance level, the fewer the components you use; the more critical to design each components becomes!
When I incepted the design of SPOTTO the focus was mainly on a simple and quieter high performance receiver with excellent sensitivity and dynamic performance combined with a simple DSB transmitter offering cleanest and moderate output for QRP use. The whole project is a collection of bench-marked building blocks put together to form a universal all band transceiver with the inclusion of a VFO/Synthesizer of user choice. The receiver uses a high level diode mixer which both is cheap and robust performing. It is usually imperative to properly terminate all ports of such mixer. Consequently I tried to do that in the simplest procedure (KISS way...!). Though it is not acceptable by the purists; but it certainly works for the QRPer. The mixer is followed by a W7EL low noise af amplifier with 50 ohms input impedance. A TL072 is chosen for the receiver audio for its low noise and small foot print, the prime essentiality for any serious and compact QRP design. A quieter TDA 7052 performs the final af amplification. The entire receiver is very lively with plenty of gain to give room filling audio. Further the TX/RX switching is kept to be minimum and simple. A conventional RF linear amplifier gives a reasonable 2.5W of RF output using a common BD139 on 40 meters but for higher bands a better device like 2SC1952 can be a much better choice. An additional module for CW enthusiasts is also available. No values for output RF filters have been given as there are G3RJV filters with values for all bands; already available on the club site. For those who expect multi-band operation, such filters can be constructed as three pin plug-in modules for easy band shifting. Many good DX were possible with the great sounding receiver during initial few evenings of use and several good compliments for its clean sounding signals were reciprocated. The transmitter is capable to produce Hall Effect that some amateurs seem to love a lot. I have assembled several copies of the project using VK3XU patchy board construction technique. It is easy to give it a go that way or you can ask editor, if the PCB for the project is available with the club store. PCB templates are given for those who wish to build their own. No PCB template is available for LPF and CW modules. They can be made on pigmy Vero board modules. LPF modules are made as three pin plug-in type modules for easy band changing. The use of suggested line filter circuit is strongly recommended to avoid common mode hum issues in case the rig is powered using a wall wart etc.