Updates Bitx Versio 3 By Om Leonard [Second IF Section]

2nd IF Section

Before we start installing components, now is a good time to install all of the wire jumpers on the top of the board. It will make testing easier as we will connect to the rx or tx line plus the 12 volt line to power the appropriate circuit during test.

This section is almost identical to the the RF Amplifier section so it shouldn't present any difficulties.

I found a board problem on mine. On the left picture, notice there is no right hand hole for the 100 ohm resistor in the lower left corner. It's easily fixed simply by soldering the right lead of the resistor to the wire adjacent to where the hole should go.

Your board should now look like this.

Lets test it!

Testing the receive side is a problem as there are many frequencies present after the mixer and when looking at them with a scope, it is confusing. Lets look at the receive side using a spectrum analyzer.

Here's some of the frequencies at the input to the crystal filter. It's a good thing that most of these will go away because they won't go through the filter. Part of the reason there are so many frequencies here is because we are overdriving the antenna input circuits with 10 millivolts input. Lets look at what happens when we reduce the input to the mixer. This is normally done by decreasing the RF gain, either with a manual control or agc on the rf amplifier.

This is a reduction of 10db of input signal. That made a big difference. The VFO harmonics are still there but a lot of the mixer products are drastically reduced. Remember, each vertical division is 10db and a reduction to 1/10th of the power. Lets reduce the input another 10db.

That almost eliminated all of the excess mixer products. This is a 20db reduction from the first picture. 20db is 1/100th of the original power or 1/10th of the original voltage so we now have 1mv input at the antenna connector. Under normal operation, that's still a big signal. Our receiver should see signals down to around .3 micro volts.

Version 3 Main Menu

Bitx Version 3 Updates By Om Leonard [First Mixer Section]

First Mixer section

This section is a little harder to understand. I've included a picture to help show the diodes placement. The colors below are the wire color connections if you use red, brown, and green wire. If you use different colors, substitute them for mine.

We need to wind the trifilar transformers. Use the small binocular cores and wind 13 turns of trifilar wire onto the cores. 1 turn is a trip through both holes ending up where you started. We need 2 transformers. I had red, green, and brown #28 wire. The cores aren't big enough to use larger wire. The original drawing called for #32 but the size doesn't make much difference as long as it will fit into the core. The different colors make keeping the windings straight easier. When pulling the wire through the holes in the cores, be careful not to scrape the insulation off. The binocular cores can have sharp edges. When you are done, it should look like this.

Now you can place them on the board as shown above. The final placement should look like this.

Notice the direction of the bands on the diodes. Make sure the leads don't short together.

Bitx Version 3 Construction With Om Leonard Of U.S.A

My Friend Leonard from U.S.A has started building the Bitx 20 verion 3 and he has posted some pictures and results for the version 3.

To make it helpful for all building the Bitx version 3 I am posting these pictures here and feel
very grateful for the contribution and a fantastic effort by Om Leonard.

BITX UNDER LENS OF LEONARD

Bitx20 Version 3

After building a Far Circuits Bitx20 and a Hendricks's QRP Kits bitx20a kit, I decided to build another bitx after seeing this posting:

A new batch of Bitx PCB, in glass epoxy ,masked, roller tinned and with
component place-ment is in ready stock for delivery world-wide.
Also available is the pcb for Avala 01 Sdr monoband tcvr.

I am giving a very special discount on the pbc's. To know further about
the discount, kindly send me a mail at suillakhani123@ gmail.com

I sent off my email and received this back:

Dear Leonard,
The Bitx Version 3 PCB is for $10 PCB +Coils+1 No 2570 transistor.
The Avala sdr pcb is for $5.Shipping is extra $ 10.

I looked at this web page to get an idea what the Avala sdr was and decided that I couldn't pass up a deal like this.

After a trip to paypal, the boards for the bitx and the avala were ordered. Soon I received a package in the mail. In it was 3 pcb's. This was the bitx board along with some of the specialized parts. The other boards were the PA board and the Avala board,

There was enclosed, a schematic, a component layout, and a few other pages of documentation.

After examining them, I decided to make an "exploded" layout drawing using a section-by-section approach to the assembly and testing. I will function test each section as it is assembled. This will almost insure that the board will function when everything is completed. I already had an head start on the project as this would be the 3rd Bitx I had built. I already knew what the signals should look like in each stage.

Bandpass filter section

RF Driver stage

VFO Section

Bandpass filter section

I decided to start with the antenna end and soon I had the following parts placement diagram for the bandpass filter section.

Next I assembled it and connected a sweep generator to the input and my oscilloscope to the output.

After a little tweaking, this was the final result. The scope is displaying from 10mhz to 20 mhz. Each division is equal to 1mhz.

Rf Driver Section

Following my plan to work from the antenna back, the next section was the RF driver stage. It was the next logical section of the board to populate also.

With the drawing done, the next step is assembly, then add a little power, no smoke, that's good, and a signal input.

Then on to test with the scope. 100mv 14.2mhz in, 1.5 volts out. That was open circuit and we probably won't get that when we hook to the PA, but for now, thats a gain of 15. On to the next stage!

VFO section

The VFO section stands on it's own and will be easy to test so I choose that as the next section. A transformer was furnished with the boards but it would have to be rewound as the VFO needed about 9uh's of inductance and the transformer maxed out around 5uh. Depending on the variable capacitor you tune with, the turns on the inductor may have to be adjusted but 50 should be plenty and then you can remove some to adjust the frequency up. My test capacitor tuned from 15-377pf and the VFO had almost a 500khz tuning range so setting the frequency shouldn't be really critical. I didn't tweak the range on mine as I will do that when I install it into a case.

Next I assembled it and connected my oscilloscope to the output. You can see the power connection for the test

I checked it out with the scope. A little over 3.5 volts. This will be less when the transformer is hooked up due to it's loading. There is some distortion of the sine wave so we know there is some harmonic content.

A quick look with the spectrum analyzer to see what the 2nd harmonic looks like.

1 Mhz/div horizontal. The left is 0hz and the right is 10mhz. Each vertical division is 10db. It looks like the 2nd harmonic is down about -25db.

Time to drink a cool one and then pick a new section to do next.

Thanks Sarath

I would like to convey my special thanks to Sarath for including details of Bitx and Avala on his blog great blog http://www.linuxtechs.net/sarath/
Just a correction for the price Of Bitx for Indian and International Hams
The Bitx Kit in India is for Rs 300 but for International Hams it is $10 this is because of fees charged by Paypal and again transfer fees of Paypal into my local account.
The Shipping charges are separate for International hams.

link to Bitx purchase
http://www.esnips.com/web/sunilsPhotos

Some more pics of my kits will be added soon.

All the best

Avala 01 Monobander SDR TCVR

The PCB for Avala Sdr Tcvr are available.

BITX NEW PCB VERSION 3 VU3WJM

New Bitx Pcb

Due to being very busy,during the last few months in 2007 I could not put in much activity for the Bitx.
The new year 2008 will bring in some more modifications for the Bitx,from members in yahoo Bitx group.The Bitx has become into a type of open source project,where you modify and modify,each modification bringing an improvement in the tcvr.

A new set of PCB of Bitx is ready,with complete masking,roller tinning and placement of components on the pcb,this will help as it makes easier to place components much faster on the pcb board.

Work is also under way,with Om Rahul Vu3WJM for bringing out a frequency counter pcb and a new linear pcb,that will operate up-to 30 Mhz,with 12 volts,and give power upto 5-7 watts,with a higher voltage the power could increase drastically.

I wish to thank Dan for the first person putting up a comment on my blog :0)

The Avala sdr 01 monobander pcb is also available with me,so if you wish to go on the air with sdr,then just drop me a mail,so I can get the pcb sent to your qth :0)

Stay Tuned...............
73's
VU3SUA
Sunil L