There it is guys: A blank canvas of copper-clad board. A clean slate ready to be filled with the components for a BITX transceiver. As you can see, I am fighting my "build first, design/plan later" tendencies. No real design work for me on this one (thanks Farhan!) but I am trying to plan where everything will go on the board. (Thanks to Jim, W8NSA for the board.) I'm going to build it Manhattan style (perhaps with an ugly dead bug or two). I'm starting with a big board because I always seemed to end up with a shortage of space. It looks like I can easily get all of the circuit (minus the PA) on this board. I'll build the PA on a separate piece of copper-clad.
I'd like to build it for 17 meters, but if I stick with the 10 MHz filter that means I have to build a VFO at around 8.1 MHz. That's not impossible, but in my experience it is easier to build simple, stable VFOs at lower frequencies.
I notice that there are a lot of cheap crystals available at higher frequencies. So, instead of keeping the filter at 10 MHz and trying to get the VFO stable at 8.1, what do you guys think about keeping the VFO in the 4 MHz range and building the filter with crystals in the 14 MHz range?
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Bill - a 17 metre rig is crying out for a 'super VXO' - ie 2 crystals in parallel to give a 50 - 100 kHz pulling range.
ReplyDeletehttp://www.rockby.com.au/ have many crystals of useful frequencies.
As an example consider using a 'Knobless Wonder' as a 7.159 MHz IF and an 11 MHz VXO.
Or an 8.192 MHz IF and a 10 MHz VXO (you should be able to pull down to approx 9.9 MHz).
There may be other combinations as well. The higher the VXO frequency the better the shift.
73, Peter VK3YE
Bill,
ReplyDeleteDoug Hendricks uses 12.96 Mhz computer crystals in his 17 meter BITX kit ( presumably with a 6 Mhz VFO ).
He uses 68pf/82pf/68pf caps in the ladder filter, which may be a good starting point if you go that way.
The Chinese AD9850 boards are so cheap that you could use one even in a single band rig without feeling guilty about waste and follow it with a relatively narrow BPF to cut down on spurs. Use something like Boarduino ( http://www.adafruit.com/products/72 ) to control it or roll your own with a bare ATMega with pre-installed bootloader ( my favorite ).
Paul - K0EET
Make that 5.1 Mhz VFO ...
ReplyDeleteUse varicap tuning on a multi-turn pot instead of a polyvaricon and a dial from Doug N3ZI.
ReplyDelete...And visit Leonard's golddredger website for some hints and tips on the Bitx.
ReplyDeleteBuild and test a stage at time.
Bill,
ReplyDeletechoosing a vxo frequency ABOVE the signal frequency and choosing the IF well will reduce spurs and make vxo pulling easy...