Sunday, 16 July 2017

VXOs-Simple Minimalist's Signal Sources.

With the recent advancement of technology, the availability and accessibility of stable signal sources has become within reach of an average radio home brewer. The newer versions of super stable and very dependable synthesizers based on both phase lock loop and direct digital synthesis are available for a moderate price. In addition to their accuracy, they usually come with an ornamental frequency display as well, to decorate the front panel of your rig.  

But for the minimalist QRP enthusiast X-tals are usually considered the cheapest stable alternatives. In practice though; they are undoubtedly cheap and stable but they do not offer the needed agility. Recently some good VXO designs have been published by some ham friends those offer the desired agility to work.  

In almost all of the projects I described, I use VXOs and I preferably advocate them as a low cost Scrooge's first choice. In this post I describe two ceramic resonator dual band VXOs, those can be adapted for X-tal operations as well.

1. BIPOLAR VXO: The bipolar VXO circuit uses three transistors, first of them wired as a colpitts oscillator. A common 3.58MHz ceramic resonator is used to produce oscillation. A varactor type tuning method is employed. I use a 5mm red LED as a varactor diode. Alternatively 1N5808 type diode can be used for this purpose as they exhibit linear tuning. A good quality ten turn potentiometer is recommended for the tuning control, preferably mounted with a calibrated rotary dial.

Q2 and Q3 are wired as a buffer amplifier that provides a reasonable output of about +17dBm. L2 should be wired on an eight mm dia slug tuned former with about 13 turns of primary and five turns of secondary. I used 28 SWG copper wire for the windings. If you use a toroid core for L2, add a trimmer capacitor parallel to C6 for fine tuning. C1 and C2 should be NP0 type or you can try styroflex type capacitors. L1 should be a moulded RF choke of about 10uH or more. In my version I just used the primary winding of an old 455KHz IF transformer with its internal capacitor removed. It measures 33uH. The VXO provides exceptionally stable signal even from the cold start. It provides about 80KHz tuning on 80 meters and about double on the forty.

2. DIGITAL VXO:  VXOs can also be built using digital invertor chips as these are cheaply available, these days. I have built many of them using 74HC04, 74HCU04 and CD4069 etc. The "U" suffix represents unbuffered version of the invertor and is usually preferred version for such applications. Though these chips are designed to be used at 5 Volts supply but they can work happily till 6 Volts and deliver a little more output; about 24mW. This level is sufficient to drive a diode ring type double balanced mixer. The schematic diagram of such a VXO is shown below:

C4 is an old ex-BC receiver component that can be replaced with a varactor arrangement for tuning, if required. L2 is wound using 32SWG, 11 turns close wound on a former made from the body of an old ball pen. The output low pass filter is included to filter out harmonic products that can cause erroneous harmonic mixing in the mixer, since being square wave the output of this VXO is rich in harmonics.

Apart from ceramic resonators, three to four crystals can be used in parallel. I am using four 5MHz crystals in a VXO to provide coverage of 40 meter band on bitx. An FLL can be used for more frequency precision. If you like to add a simple frequency counter, you can try Fredy's (DJ3KK) SPRAT counter. It uses an eight pin u-controller 12F675, thus it is very small and cheap and announces frequency in Morse, through a small piezo buzzer.

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. All broad band RF transformers are wound on pig-nose balun cores using 30 SWG enamelled copper wire.
The RF amplifier circuit is quite self explanatory. The schematic diagram of RF final is given below:
The three stage amplifier ensures almost seven watts of RF output. I used 2N4427 for the driver as it was available in my collection but many other suitable candidates like 2N3866, 2N5109 and 2SC1175 seem to work as well. The receiver band peak capacitor C21 is a 330pF type variable tuning capacitor. It eliminates the need of mechanical band switching arrangement in receiver front end using switches or relays etc. An attenuator ahead of it has been included for dire reception phases. It can be switched in during presence of receiver overloading.

The entire project is developed on 4"X6" PC clad pieces using ugly construction and each board is fixed to the sides of the enclosure body. The control circuit being in the middle. The control circuit provides the required QSK delay during CW operation, does all switching and provides side-tone during CW transmissions. I used a small piezoelectric buzzer, the type used in computer motherboards, UPS's and microwave ovens etc. to provide CW side tone, since the idea was simple to implement. The schematic for control board is given below:

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.