Home > Ham Radio > Bitx 20 update: The band pass filter.

Bitx 20 update: The band pass filter.

May 3rd, 2009

The  bitx, I  built using the ugly construction method was too ugly to look at.   The board  was rebuilt using Manhattan style construction last week end . Now the board looks tidy and neat. The added advantage is that  I can plug and play with any section.  Individual sections are built on smaller PCBs and all sections are  glued to a larger copper clad sheet.

Bitx Rebuilt in Manhattan Style

Bitx Rebuilt in Manhattan Style

However, the power output in transmit mode was very low.  Up on investigation , it was noticed  that the band pas filter was not performing well.

Bitx version 3 band pass filter

Bitx version 3 band pass filter

This morning, I decided to  experiment with the band pass filter.  My board is based on version 3 schematic of the bitx and the band pass filter was built from old IFTs recovered from a junk radio reviver.  The filter was  not tuning properly. The output at the end of the filter was very low ( 100 mv)  May be the ferrite  used  in the IFT is of inferior quality.

The  band pass filter that was built with IFT

The band pass filter that was built with IFT

I looked around my junk box and found out some small resistor looking  inductors. I decided to try the  band pass filter  from the original design. This design uses 2 ?h inductors wound on tap washers. Fortunately, I had a few 1?h

Bitx version 1  band pass filter

Bitx version 1 band pass filter

I built the circuit as shown below and soldered it to the board.  Oops. The performance went down. I was getting only 50mv  on transmit and the receiver performance also suffered.  Again ,I suspect the  junk box components. I thought of rewinding the coil using tap washer.   Ferrite slugs and torrid cores  are not easily available in this part of the world.

Bitx Version 1 bandpass filter

Bitx Version 1 bandpass filter

. I looked around the junk box again and found out a set of coils used for 3 band broadcast radio.  I decided to rebuild the  version 3 band pass filter again

The radio coil rewound

The radio coil rewound

The coils were rewound .  I  used 11 turns for the main winding and 3 turns for coupling winding The coil inductance was varying form 2.7 to 1.65 micro henries.  The completed filter was installed and bitx powered up . Now I was  getting around 700mv RF  from the filter stage.  I am yet to test the power amplifier section . I hope I will get 5 watts as advertised.   With the new filter in place the receiver performance has improved tremendously. I can hear lot of stations both on CW and  SSB.

Bitx with the new front end filter

Bitx with the new front end filter

The signal I got while whistling to the mic is also shown below.

Scope waveform after filter

Scope waveform after filter

  1. P R MEKAP
    October 7th, 2009 at 22:47 | #1

    I have read your lab notes on experimenting BITX BPF. I appreciate your patience in repeatative work. congratutions for extracting 700mV TX RF. Keep up good work.

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