I recently blogged about the BITX40 and all the details and errata here: http://amoghdesai.com/technology/electronics/hfi-2014-bitx40-design-everything-you-want-to-know/
Today I have started my own build of BITX40, and decided to keep it posted here as I progress. This can serve as a guide for others and builders can learn from my mistakes
Before I start, some experienced homebrewers have told me these important points while building any project:
- Break the projects into stages, test each stage before moving to next
- Inventory all the parts and componets, and test each components before mounting
- Test transistors using HFE meter from the Multimeter, it should show the gain of transistors (a value around 300 or similar). Check the correct Collector base and emitter while mounting. CBE of 2N3904 and BC547 are reverse.
- Test each resistor for correct value before mounting
- If you have LC meter, also test capacitors
- Follow point 1 and 2 faithfully.
I am planning to make my BITX a multiband Tranceiver, with 10MHz IF and a DDS VFO. I will not be populating the analog VFO and BPFs. Will build separate BPFs with auto switching on band change from DDS VFO. Thats the plan, lets see how successful I can get.
Part I - RX Chain BFO --> Audio Amp --> Product Detector --> Crystal Filter, IF AMPs --> Double Balanced Modulator --> VFOBFO
I started with the BFO section, so that I can check all the crystals for their exact frequency and use the closest matched ones in the Crystal filter section. Here is the BFO circuit:
This is my build: Note: I am using BC547 and they are mounted in reverse, as this PCB is meant for 2N3904
First connect the Jumper J3, it supplies 12v to the BFO (blue wire in the above picture). Use a IC socket or similar plugin for the crystal, so that we can plug all the crystal for testing. Do not connect L5, instead Ground the L5 side of the crystal. L5 is used to pull the bfo frequency for getting the correct passband (LSB/USB) in the product detector. It will be connected later once the product detector n crystal filter stage is working n tested. Build the BFO and check the frequncy output at C50. I tested it using the RTL SDR dongle (and a spare 16Mhz crystal) as shown below: Matching Crystals for filter and Diodes for Mixers
Once you have built the BFO and tested for working, Test all the crystals for their Actual Frequency. Buy 10 or 20 crystals of same IF frequency you have chosen. You need to find out their actual frequency and use 4 crystals in the filter with their actual frequency within 100Hz from each other and one Crystal in BFO with around 750Hz + or - (to achieve good SSB bandwidth). I have chosen 10Mhz as my Crystal filter frequency. I bought 20 10Mhz crystals. Now connect each Crystal in the BFO, where IC socket or similar was used in above step. Take the output of BFO from C50 and measure the exact actual frequency of each crystal. You can use a Boradcast receiver with digital dial, or take fellow HAMs help and test it on their Commercial TRX, or use a Oscilloscope from college or like me test it on a RTL SDR dongle like below.
Make a table in a XL sheet or on paper. With each crystal marked with a Number (use marker) against its frequency as below:
If you check I got few sets of closely matched crystals but I will use the one with four 9.996.000hz in filter X1 - X4 and 9.995.700hz in BFO X5 (which can then be pulled down to desired BFO frequency using L5)
Diodes in the Product detector D11 and D12 needs to be matched, and if matched for equal forward voltage you can omit the Preset between them. Diodes in Double balanced mixer D3 to D6 are not critical to match, but it is better if you match them too. To match the Diodes, take a 2.2k resistor put it in series with the diode with 12v and ground. Measure the voltage across each diode (+m & -M in circuit below) and put it in a table. Circuit: 12v O----^^^^^^
O Ground 2k2 +M Diode -M Put it in a table like this:
I used two 689mv Forward voltage diodes in Product detector (D11,D12) and four 688mv Diodes in DBM (D3-6). Keep this matched components safe, till they are used. Audio Amplifier
Audio amplifier is fairly easy to build. Solder all components as per PCB and circuit. Use 10 Ohms for R64 as per Errata. Also mount Q13 as per correct polarity of CBE. Once built, connect the 10k volume pot and Speaker to test the Audio amplifier by passing any music (from mobile,computer, mp3 player, etc) to -ve of C68. Audio Amp is of 2 stages, using LM386 and pre-amp using Q13. So you can test each stage by passing music at either C68 pr C66 to test both stages. (Keep the music volume from source low to avoid over loading.) You should get loud and clear music from speaker. Also another simple test is to touch the top metal lid of C68 or C62 (electrolytic 1uF caps) with wet finger or use tweezer. You should get loud Hum or local broadcast station. I get the local station pretty load and clear with the wet finger test. Trifillar Winding for Product Detector and Double balanced Mixer.
Take 3 pieces of 28 or 30 Gauge enamelled copper wire of around 1.5 meter length together. attach one side to a fixed point and other to a hand drill or use a pen/tweezer on the other end to turn the wire. Give good and even amount of turns across the length of wire. Take the tv balun core and wind 13 turns on each of the balun. (Make space in the balun holes using a toothpick if turns won't fit). Make 3 such trifilar windings. one for product detector T2 and 2 for Double balanced Modulator T1 and T1A. Check the video on this link
for understanding how to make the trifilar wire and how to make the winding. Once 3 trifilar windings are made, test each winding with a continuity tester (low ohms setting) on Multimeter for short or cuts while winding. Also align the 6 wires from both sides of each winding with continuity for mounting on PCB. Please note each wire starting from one side and ending on the other side should match and should be exactly opposite while mounting on PCB. Example: Product Detector
Solder all components including T2. Use diodes D11,12 matched earlier. Short all 3 pins of Preset between D11,12. (You can omit C52 trimmer and C53 for now). As per Errata C73 should be .001uF. IF Chain and Crystal Filter
Mount and solder all components as per the circuit below. Omit C32 and use 10R for R29 as per Errata. Solder 4 matched crystals. Solder transistors as per correct CBE pins. Double Balanced Modulator
Solder the T1 and T1A, with 2 trifilar winding made earlier. Test wires from both sides of each winding for continuity while soldering. As per Errata Ground the T1A correctly. Solder 4 matched diodes at D3 to D6. VFO
Im not building the On board VFO, and using this DDS VFO
. Build the VFO buffer amp using Q8 to use with DDS. As below: Testing The RX Chain.
I will be mostly using an RTL SDR dongle for most of my testing. You may well use Oscilloscope and other tools for your test. You can even use a broadcast receiver for some of the testing. We have already tested our BFO and Audio Amp. We will now test the remaining RX chain.You can use your own methods for testing. I disconnected the C50, and used a 16Mhz crystal for X5 in BFO (which I had spare from a arduino), you can use any crystal for testing and do the math. Take the output of BFO and feed it to C11 (as antenna input). Do not forget to test if the BFO is oscillating at 16Mhz. Now give a 6 Mhz signal from VFO (I use DDS) at C43. The DBM should mix your Antenna signal (16Mhz) and VFO signal (6Mhz) to give 2 products.
- 16Mhz + 6Mhz = 23Mhz
- 16Mhz - 6Mhz = 10Mhz
To test this connect the RTL Dongle (or oscilloscope or a broadcast RX with digital dial) to DBM output at C19. You should see a signal at 10Mhz and 23 Mhz.