Friday, 26 May 2017

XENA- A Portable DSB/CW Transceiver

In my post of 24 th March, 2016 I shared a simple direct conversion receiver project built around a home-brewed mock I.C. This project grows out from that design. 'XENA' is a simple, portable DSB/CW transceiver for backpack use. As mentioned above this cute little project evolved around a single home made mock I.C. (made around components shown in a dotted square). It functions both as a transmitter and receiver mixer. For simplicity the design uses a ceramic resonator/crystal based dual band, super VXO for 80 and 40 meters (more about this later). The receiver is developed around ubiquitous parts generally available in one's collection and the complete project is developed in modular form with all modules made using ugly construction technique. The modular construction allows scope for future experimentation and further development of the project. The schematic of exciter cum receive module is given below:

Even to a crass view the circuit of exciter cum receiver module is too simple and hardly needs an explanation. I have added a multi-turn preset in the biasing arrangement of the mock mixer I.C. to aid precise mixer balance in order to achieve minimum carrier leakage during transmission.

Post under construction.

Sunday, 2 April 2017

ZERON -A Super Simple QRP Dual Band Multimode Transceiver. -II

Just back after the hiatus, mainly due to a complicated leg fracture. However in this brief post I will share the direct conversion receiver part of "Zeron".

As I already told you about my obsession for digital chips for use in RF circuits and especially in switching mixers etc. I used an 'HC4053 as receiver mixer. This chip is an excellent choice over expensive diode mixers for many reasons. First as I already mentioned that it is cheap and easily available. The measured insertion loss is just about 0.5dB and the off state RF isolation is better than 45dB. The measured return loss is also better than 22dB for a variety of chips put under test, from different manufacturers. More than that you don't need to balance or match any devices! It is wonderfully simple to use.

The mixer is preceded by a pre-selector stage employing a common JFET in common source mode. For simplicity I used the air core coils but you can replace them by appropriate replacements in case compactness is more desirable. The variable gang capacitor is an ex- broadcast receiver type that facilitates the receiver peaking on the desired band of operation.

The AF component from the mixer is routed to a two stage AF preamplifier, through a duplexer circuit. The duplexer ensures that the mixer must see a fixed terminating impedance of fifty ohms on its output port, for an entire gamut of frequencies, literally from DC to daylight. The two stage preamplifier is modelled as constant impedance amplifiers, built around low noise transistors. 

A passive AF filter is placed after the first AF amplifier to shape the overall frequency response of the receiver. And as you can see it is the best place to place the filter in AF chain. The main reason of choosing a passive type of filter is that it has more dynamic range than its active counterparts. The inductors used are miniature encapsulated ferrite shielded units, generally available from Digi-Key. The two stage preamplifier is designed to eradicate any trace of fifty Hertz hum, generally considered a menace in most DC receiver designs. The two stage preamplifier provides about 60dB of gain. The following bode plot displays its frequency Vs gain characteristics:

I have built several receivers in past four decades ranging from re-gens, reflex, spontaflex, pentaflex, superjets and many others. I was delighted to build some and disappointed to hear others. One common last thing that can mar the overall performance of an otherwise good receiver design is; of course the final AF amplifier design. Consequently, I opted for a distortion free class-A amplifier employing a very low noise op-amplifier as its major gain block that supplies around 44dB of AF gain. The output of this amplifier is about 100mw and can drive a low impedance small speaker or a set of headphones.

The overall receiver performance is amazingly good. It sounded so well on crowded bands. There was no trace of overloading or broadcast breakthrough on forty meters but on eighty I had to switch in the attenuator on some occasions. The receiver performed as expected and was pleasingly sweet on ears.