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.

A Special Thanks To WU3WJM

A special thanks to Om Rahul VU3WJM for making excellent modifications to the actual Bitx by Om Farhan and making the PCB for Bitx Version 3 PCB's for Main Bitx Board and linear.

Hope the details of linear pics posted will help all making the consruction of Bitx 3 version easy now.

Special thanks to Om Leonard also for his worthy contribution to the Bitx Version 3 by VU3 WJM Om Rahul

All the best

Update Linear Cum Power Supply Board Version 3 Bitx [By Om Leonard]

Linear Amplifier section

The version 3 linear amplifier board has 2 very nice additions. The first, the RF signal is switched between rx and tx by relays. This cures a problem I found with the Far Circuits board that I cured by adding relays. The other main addition is an AC power supply. It is setup to accept an AC input and has a rectifier and voltage regulator on board. This allows 13 volts to be fed to the bitx board for it's supply voltages and a higher voltage unregulated to be used on the IRF510. Above is the original board setup.

I made a few modifications to the board. The board looks like this. Power supply on the right and the IRF510 lower left on the big heat sink.

I wanted to use a 22 vac transformer that I had so I needed to use a 35 volt capacitor. I had a 6800 mfd at 35 volts in the junk box so I used it. I had to move the 2 - .1 ufd capacitors to get room for the big filter cap so I put them on the back of the board using some chip caps I had. The spacing worked out just right. Just to the right of them is the original pads for the 2 electrolytic caps.

I used 3 amp silicon diodes so I had to drill out the holes a little and then stand them up vertically. The 6800 ufd cap was hot melt glued to the board after soldering the wires for it into place.

After hooking up the power transformer,, I measured 30 vdc for the IRF510 and 13.6 volts for the bitx. The next step will be to test it after I complete the bitx. I'll be interested in the power out with the higher voltage.

Version 3 Main Menu

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.