Wednesday, December 14, 2016

Building the BITX 40 v 3 A SI5351 VFO update 1/22/17

Between getting ready for the Holidays and a couple things i need to do around the house, I have not been spending much time working on the BITX.  Except for the drift issues I still have not been able to get resolved, I am very happy with the performance so far.  The simple AGC seems to be working well, and makes listening much more comfortable.  I took a look at the output of the detector on the AGC board, and it looks like it will be usable for a signal strength meter.

With the drift problems, and desire to add a signal strength meter I decided to go ahead and build up a digital VFO. I have a 9850 module left from the SNA Jr.  and a couple  Adafruit SI5351 modules from earlier projects. I had ordered 2 of the BITX boards, and had planned on making the second one into a multi-band rig.  With this in mind I decided to go with the SI5351, because I could use a second clock in the multi-band version for side band selection.

Over a year ago I had built a 5351 based VFO for use with Frog Sounds 40 Meter CW transceiver.  I called it the Canned Frog, because I put it in a canned meat can.

I had used a separate clock for the receiver and transmitter LOs, so had about everything I needed in the board I had laid out for it.  This VFO used a Arduino Nano, and a small OLED display.  This will be fine for the single band version, but want a larger display when I get around the the multi-band version.  I used the basic layout I had, and brought out most of the Arduino pins for use with a different display, and other options I am thinking of adding.  When I did the original VFO. I found the regulator on the Nano got quite warm when running on 12 volts.  So, I added a 9 volt regulator to the board to keep the Nano cooler. 
I also changed most of the passive components on the board from leaded to SMD versions, that will be mounted on the back.  I have started to try to use SMD components where ever possible.  Drilling holes is the most tedious problem of making my circuit boards.  Only problem with home made PCBs and SMD components is watching out for solder bridges.  I have been playing around with UV curable solder mask, and have gotten the process down to something fairly easy with consistent results.  So after etching and tinning the board, I added a solder mask before drilling.   Everything looks very nice, now to build the board and modify the existing sketch to take care of a IF offset.

12/18/2016 board and software mostly finished

Finished getting most of the board populated except for the filter and attenuator circuits, and the basic software working.  Now I can try setting different levels and filter values on the board to see what values work best.    The software is basically what I had used for the 'Canned Frog', with the CW keyer and CW offsets, and RIT removed.  Since this used a direct conversion receiver, the signal output was on the transmit frequency. The BITX is a super-hetrodyne, and has a IF  frequency around 12M Hz. Therefore the VFO signal must run at a different frequency.   Because you are receiving the Lower Side Band  the exact frequency used is the BFO frequency. Basically the VFO frequency is the BFO frequency minus the operating frequency. 

 For the 7 Mhz. band this is around 4.7 Mhz. to 5 Mhz. The original software used the actual operating frequency for display and setting the si5351.  I measured the frequency of the BFO and used that value in computing the required VFO frequency. This value was used to set the si5351 clock 0 output.

After making the changes to the software, I ended up spending an hour or so trying to find out why the frequency was not changing when I turned the rotary encoder.  I finally looked at signals with a scope, I found I had a bad rotary encoder.  After changing that everything worked the way I expected it to.  

Update 1/2/17

I spent most of the holidays visiting relatives, and did not have much time to work on any of the projects I am working on.  I did bring my laptop along and had some timeto work on a BITX front panel that will work with the SI5351 VFO and Display.  The original front panel was designed to work with a small counter and had a grill opening for a small speaker.  After trying the BITX  receiver, I found that even with an AGC circuit added, the audio sounded much better with a larger external speaker.   This version has a built in bezel for the display, mounting posts for the circuit board. 
I also had to include a recess to provide room for the stacked Display and Adafruit SI5351 board.  To center the display, I had to move the microphone and volume control to the opposite side of the panel.  

 Update 1/22/17

I 3D printed the new design front panel and tested the VFO out when connected to the BITX board. I found some of the same kind of receiver noise I had at first with the frequency counter.  It looks like the BITX board  is very sensitive to any noise on the DC supply line.  I had a reverse polarity diode on the board, that I replaced with a 10MH. choke and added a 100 uF. capacitor across the input to the regulator on the board.  With this I could not hear any noise unless I turn the volume all the way up and disconnect the antenna. 

 The recess I put in the back of the front panel helps keep the display and si5351boards in position, along with reducing the overall depth in the case.

Just a little more work on the software to make sure the RIT is working correctly and possibly add a  S-Meter

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