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.

1 comment: