Re: uBITX - A reboot of the old BITX

Jerry Gaffke

You pretty much addressed Allison's entire list of things to improve on the Bitx with this new uBitx design, and did so before she put out her list.  I like how you chose to do CW.  

A few questions and comments:

How much of a compromise is the Si5351 on the uBitx?  Would you expect to hear significant difference between an Si5351 and going to three Si570's due to phase noise or crosstalk between channels?   Perhaps put pads down for the Si5338 as a stuffing option, the Si5351's higher performance big brother.

Would a four FET H mixer or similar give better performance up front on receive?  If one were to chase this path, would it be best to have separate transmit and receive mixers?

This might be an interesting option for a high performance first IF filter:   They say typically 3db loss, but we might find a 3khz slice that is even less.  Are those diode ring mixers and BiDi amps good enough at 87mhz?

How close are we to being legal on the second harmonic using just the two transmit LPF's?  I'm ok with offboard filters in the antenna line, perhaps have a suggested antenna tuner for use with an end-fed-half-wave.

For shielding, I'm looking at 1.25 inch diameter thin walled copper pipe at the local hardware stores.  Perhaps cut it into 1/4 inch high pieces, have four ground test points in a square around each RF section of the board such that the pipe fits just inside the test points.  When complete, a stiff cover of PCB material gets bolted down on top of the copper pipe sections, use conductive foam sheet as gasket material. 

I'd prefer the Si5351 on the main board, keeps the Arduino processor and display choices flexible.  Also, use board mount connectors and pots and switches as much as possible, all with panel mount bushings for sturdy mechanical support so they don't rip up PCB pads with use.  Those that want different panel arrangements can choose not to stuff them, and run wires to their chosen connectors and pots and switches.  Most Bitx40's are getting built with an ugly mess of wires boiling up off the board, a significant part of the problem around birdies and clicks and such.

Keep in mind any checkout aids you can include in the design.  I'm thinking AF and RF signal sources with appropriate attenuators, diode RF probe, voltmeter through the Arduino ADC that normally just monitors battery voltage.

Some kind of SWR monitor would be nice, and perhaps a temp sensor that shuts down the IRF510 bias.  Would save lots of people lots of grief.

Jerry, KE7ER


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