Bitx By Mark G0MGX [Pictures]

Mark has been very quick in assembling his Bitx version 3, and Mark you have done a great
job,posting your great Bitx pics on CQBITX.

Bitx Version 3 By Arjan Holland DSC02145

Posting some spendid pictures of Bitx version 3 home-brewed by Om Arjan Holland

DSC02145

Frequency Counter Picture

Here is the picture for the frequency counter in power safe mode. I had to take the counter out of the shack to get a good picture of it as I do not own a good camera as yet :))

This is apparently the beta test Pcb,and does not have a switch for programming nor does it have a 5 volt regulator.

A new set of Pcb for counter with a programming switch and a 5 volt regulator will be available for members with-in a few days time.

Frequecy Counter For Bitx

BITX HAS COMPANY: A FREQUENCY COUNTER

The Bitx Version 3 has company now.A frequency counter designed by Wolfgang "Wolf" Büscher, DL4YHF.http://www.qsl.net/dl4yhf/index.html
is now available with me for Hams who would like to add it on their Bitx
or use it in their rigs or set it up as a lab instrument.

All the credit goes to Om Rahul VU3WJM for making a fantastic PCB
and also allowing me to offer it to Hams world-wide,we are also full of appreciation to DL4YHF for putting up such a simple design.

I will post the pic of frequency counter soon.

In meantime any interested may contact me.

Many Goodies for Bitx on the way.QRX for them

Wide Band Transformers

Wide-Band Transformers In the Bitx Mixer Continued

In the previous web page, we looked at the difference that the number of turns made at the input of the mixer transformer winding. Lets change the viewpoint and look at the input to the diode bridge.

We are going to look differentially. That is, across the 2 points rather than from 1 point to ground. The scope will be connected like this using 2 scope probes. This takes a special differential amplifier plug in.

First, lets look at the 4 turn, 5.25uH transformer input signal that we left off with in the previous page.

The left picture is the signal at the collector of Q7. The right picture is at the input to the diodes after going through the input transformer. The difference in amplitude is due to using a differential input to the scope instead of going from 1 end of the transformer to ground, we are going across the + and - leads of the transformer so we get twice the amplitude. There's not much change going through the transformer.

Lets look at a 13 turn, binocular, 69uH transformer again. Once again, the left picture is the collector of Q7 and the right picture is across the transformer leads going to the input of the mixer. The input to the diodes looks very similar to the 4 turn waveform above. The corners may be a little sharper but there appears to be no major difference. The waveform inversion is caused by the scope probes being hooked up reversed on the transformer leads. If I would have noticed it initially, I would have reversed them and the the waveform would look exactly like the one above.

What conclusion can we draw from all this? What I see is that as I've been told by many people, unless you are trying to match impedances where you have a turns ratio rather than a 1-1 ratio, the number of transformer turns really isn't particularly critical. The minimum amount of turns should be selected to give an inductive reactance of at least 4x the circuit impedance. In this particular application using FT37-43 cores or something with like characteristics, anywhere between 4-13 turns seems to work very similarly.

Lets look at one more thing while we are here. Lets move the scope probes to the output side of the mixer. The left picture is with no signal input. The spikes you see are from the vfo. Also notice the amplitude setting is 50mv/division. This is a double balanced mixer so both the vfo and the input signal should be attenuated. In this case, there is no input signal. The right picture shows an input signal at 14.2mhZ. The amplitude is now at 100mv/division. What we see is the combination of all the new frequencies created and the 2 original frequencies attenuated. If you look closely you can see a repeating signal that is spaced 1 division apart. That is the 10mhZ IF frequency that is used in the original Farhan design and implemented in the Far Circuits boards and the Version 3 boards. It gives us a waveform spaced every 100ns apart. In the bitx20a kit, it would be spaced every 91nsec apart as the kit has an 11mhZ IF.

By now you should just about be a mixer expert and probably wonder if you really need to know all of this "stuff". That you will have to answer for yourself.

Stage By Stage Construction

What impressed me most is the way OM Leonard has planned making the Bitx stage by stage.
This method is very secure as you know the particular stage is working after you have constructed and tested it,there is no cause to worry later.

BITX CONSTRUCTION PLAN: By Leonard

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.

RF Amplifier Section [Leonard]

RF Amplifier section

The Printed Circuit Boards are furnished with a 2SC2570A RF Amplifier Transistor for Q1.

The pads on the board are for that RF transistor and are C-E-B left to right and the board is correct for that device.

Using it, the board is correct. If you desire to use a standard C-B-E transistor configuration, go to C-B-E mod for an explanation of the board changes.

The amplifiers can be tested individually by connecting +12 volts to the end of the appropriate 100 ohm resistor. Do Not power up both of them at the same time!

We have 100 millivolts in and 920 mv's out. db=20log E1/E2 so db=20xlog 9.2 so we have a measured gain of 19.27db.

Lets measure from the antenna input to the same output spot. This will send signal through the band pass filter. Simply move the signal generator over to the Rx Antenna point, double check the signal generator output and measure again.

100 mv's in and 720 mv's out. db=20log7.2 so we now have a gain of 17.15db. The difference between our earlier gain of 19.27db to our current 17.15db is due to the loss of the band pass filter. Filter loss is -2.12db.

Here we are sweeping from 10 mhz on the left to 20 mhz on the right. Each horizontal division is 1 mhz. It looks like we go from about 13.8 mhz to 14.7 mhz. We might be able to tweak it a little narrower but I can live with this.

Testing the other outgoing amplifier is the same as this one was. The values are the same so the gain should also be similar. All we have to do is to move to the other set of points and shift the +12 volt line to turn it on. We connect to the lower left input point and lets take the output from the "to PA" point. That way we are going through the Q13 amp, the band pass filter, and Q14. We are testing almost 1/4th of the output string.

With 100mv's in, we have 3 volts of output. This is open-circuited and will be lower later but this is a good measurement of how the circuit is working.

db=20log e1/e2
or 20log 3/.1
or 20log30=29.54db

The actual gain of the 2 circuits will be about 2.1 db higher or 31.64db because of the -2.1 db loss of the band pass filter.