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Building stable variable freqency oscillators- Some considerations.

May 14th, 2008 Comments off

A stable variable frequency oscillator is the heart of any home brewed ham radio gear. This article will describe various aspects of choosing circuits , components and construction

Choosing a VFO circuit.
Variable Frequency Oscillators (VFO) are used is almost all ham gear.A stable and reliable VFO is the dream of any homebrewer Any circuit that provides stable oscillations can be used as a VFO.There are several circuits available on the internet. Some of the popular ones are Heartly , Colpits Vacker etc. Each amateur has his on preference regarding the circuit combination.

Choosing components.
VFOs can be built around transistors or FETs. I use BFW10 fet or 2n2222/BC107 depending on availability in my junk box.Any other equivalent transistor can be used. Make sure that Ft(unity gain bandwidth) of the transistor is at least five times your operating frequency. The transistor must have sufficient gain(hfe). The other important components are resistors and capacitors. You can use quarter watt carbon film resistors(with 5% tolerance)). As to capacitors, styroflex capacitors are preferred. However the availability of styroflex capacitors may be a problem for most of the homebrewers. NP0 ceramic disk capacitors can be used instead. You can rescue NP0 capacitors form old cordless phones or TV sets. The tank circuit consists of an inductor and an variable capacitor. You can wind your own air core inductor.This can be done on any former. I have wound coils on tap washers , pill bottles ,PVC pipes etc. Use your ingenuity in finding a suitable former. The gauge of the wire is not much important. I generally use what is available in my junk box. Air dielectric metal gang condensers are preferred for variable capacitors. Availability may be a problem in this case also.(Look out of old valve radios). Polyvaricon capacitors used in cheap radios can be substituted instead.
Building VFO
You can use a PCB if it is readily available. Otherwise ugly or Manhattan style construction can be utilized. Double side pcbs are not preferred and it can provide unwanted stray capacitance. If you are designing your own PCB avoid using long PCB traces as this can add unwanted inductance. It is suggested that the VFO supply voltage is kept low from the supply voltage of succeeding stages. This will provide stability. Use a Zener diode to regulate power to the oscillator stage. Always use decoupling capacitors.near the Zener.Finally , keep the VFO in a suitable metallic enclosure. This will prevent unwanted RF leakage.
VFO drift.
VFO drift means small change frequency from the present value. Two kinds of drift is noticed in VFOs. a) Short term drift and b) long term drift. Short term drift is heating up of semiconductor junctions and other components due to RF currents.Long term drift is caused by a variety of reasons such as humidity,ambient temperature etc.
Short term drift is predominant in the first few minutes of VFO operation. It is suggested that if possible keep the VFO powered even standby periods.Chose high quality components such as NP0 capacitors. Avoid magnetic material as core of the tank coil. Trimmers should me air core if possible. Cheap mica trimmers should be avoided.

Categories: Ham Radio Tags:

How do they recruit teachers in Kerala

May 11th, 2008 4 comments

Today’s Hindu has an advertisement by Kerala public service commission for recruitment of High school teachers. The notification is available online at the commission’s website here

Here is the method of appointment” Recruitment by transfer from among the employees who have completed the 5 years of prescribed service and are working as  Clerk/ Typist/Attender/ Peon in Education Department.”

This is pretty ridiculous. If you join the education department as a Peon and some how attain a degree, you will become a teacher in 5 years.. I have seen this kind of academic degradation in Kerala’s polytechnic colleges.
For example, an 18 year old guy( somehow he passed the high school) is recruited as a watcher or gardener in a polytechnic . He will join a part time ITI ( certificate) course and complete it in 3 to five years depending on his intellectual capacity. The pubic service commission will appoint him a tradesman after some 5 years of glorious service( Here again a notification will come which will select from among peons or gardeners only). Then he will try to get a Diploma in engineering( may be by correspondence or from Tamilnadu). If he gets his Diploma in another five years, he becomes an assistant lecturer( again recruitment among peons/tradesman etc). Then he joins a part time BTech course. Another 5 years he will become a lecturer ( again closed recruitment among assistant lecturers). This process goes on and by the time he retires at the age of 55 , our poor gardener will be heading a teaching deparment. I can point out several such cases.
I am not against people building their careers. But here , a certain percentage of posts are reserved for such people and they need not compete with other outstanding guys outside education department. They become teachers by virtue of being a peon or an attender in a school.
I pity the future generation. May be I will have to teach them multiplication table when they reach our college.
Finally, please take a close look at the notification shown above. The public service commission has floated a short “tender ” for recruitment of teachers. I thought tenders and quotations are for purchase of materials by Government departments . My guess is that the under secretary who drafted the above tender might have been selected from among gardeners peons etc. 😀

More historical stuff

April 26th, 2008 Comments off

Today I had the opportunity to visit some of the buildings which used to house the College of Engineering Trivandrum in world war 2 era. I
thought it will be nostalgic for many of the old timers to see their
laboratories again. These building are located behind PMG. They are
being used as go down by Government of Kerala. (My photography is pretty bad, I know.)

Kollam Ham fair 2008 Photos

April 21st, 2008 1 comment

Yesterday I attended the hamfair at Kollam. Nearly 125 hams from different parts of Kerala participated. I bought a 30W dummy load, 2 mechanical encoders and a few toroids from OM VU2PTR. Here are some snaps I took during the function.

hamfest kollam

hamfest kollam

hamfest kollam
hamfest kollam
hamfest kollam
hamfest kollam
hamfest kollam
hamfest kollam
hamfest kollam
hamfest kollam

Categories: Ham Radio Tags:

Summer Projects for students

March 29th, 2008 Comments off

This post is for my students at college of engineering Attingal . We are going to get vacation during May and June. I think your semester examination will be over by April end. I have a small project which some of you can take up during this summer vacation. The project is related to amateur radio. If you are able to do it, this will be a fine minor project for your sixth semester. However, my aim is to introduce some of you to micro controllers and embedded systems early in your course. I don’t find many home brewers among students nowadays. This will be an ideal opportunity to learn a few tricks.

Now here is the idea. I have a few fixed frequency VHF transceivers. They are Philips Portaphone SXA rigs made around 1977. The figure below show one.

The transceiver can take 6 fixed frequency crystals on the receiver as well as transmitter side. That means it has only six fixed channels. Now ,I have a pair of crystals cut for the Trivandrum amateur repeater. (RX 145.600Mhz TX 145.00Mhz ). That means I am operating it only on a single channel.

I want to convert it to a variable frequency transceiver by adding a DDS ( direct digital synthesis ) controlled oscillator, replacing the crystal oscillator.

Your Task will be to build a micro controller board and a DDS board and drive the tranciever for covering the amateur band of 144-146 MHZ. You can choose either a PIC or an Atmel processor. For the DDS , an Analog devices AD9851 will be sufficient. There are some similar projects on the net. You can look here.

Do a search on google for DDS VFO .You will find more links. This project will be on similar lines. But the programming and driver circuit for the transceiver will be different . The output of DDS should be switched between receiver and transmitter frequencies. I will give you more details later.

You are expected to do the following

a) Study of the chosen micro controller.

b) Design the board and simulate it .

c) Study about DDS.

d) Design a DDS board.

e) Design PCBs for both the boards and build

f) Test the prototype in lab.

g) Make a suitable cabinet and build the final product.

A detailed requirement specification will be given later.

I have the following stuff with me .

a) Some PIC micro controllers

b) DDS chip AD9851

c) Some LCD displays

d) Assorted components.

I can provide you with a PIC programmer if you choose PIC. I can fund you for any costly ( say greater than $2). Rest of the stuff ,you will have to find out.

I can take 3 guys . Each one should do it independently.

You must be willing to work during vacation. I will be coming to college on a few days to college during vacation. Otherwise, you can interact with me on internet . A PC and an internet connection at home is a must. All designs and programs should be posted on internet and must be under a suitable free license.

I will give some preference to second year guys. (but first years can also be considered.). If you are interested , drop me a mail at vu2swx at gmail. com.

Categories: Engineering Education Tags: