Re: Are the uBITX receivers ripe for improvement? -- And some other miscellaneous thoughts.

ajparent1/KB1GMX
 

Receiver performance has many dimensions.
 Sensitivity
 Bandwidth
 Overload performance and dynamic range.
 Stability, Especially important in the days of VFOs.
 Spurious signals, birdies and unexpected signals.
 
 This one is unique to a new generation of simple radios with microcomputers to
 enhance and provide a user interface.

 From my perspecitive and work I've done the ITX 40 had too much gain and required
 an attenuator most nights.  An easy fix.   The ubitx I have as a RX only test bed from
 about 3-4 years ago ding the Minima development is a good receiver in the 1-10 mhz
 range but I prefer a bit of RF gain added for 10 through 30mhz.    As you go up there
 is more weak signal propagation and reduced manmade and approaches galactic noise
 floor.  That said at 40M a 1uV senstivity is plenty, at 28mhz .2uV is more useful.

 For selectivity, I prefer a tight filter.  2.1 to 2.4khz is fine as the would of SSB is
 about that wide on transmit save for the ESSB people where a 3khz filter would
 be better.  I however prefer steeper skirts and that requires more crystals to do 
 with 5 crystals being the bare minimum and 7 seems to be approaching very good.
 Why is this important, strong signals down the skirts[edges] are audible of not suppressed
 enough and if the same filter is for TX it assures unwanted sideband is suppressed.

Overload, this is a big area for bitx40 users and same for bitx20 as there is a lot of RF gain
and those bands are known for big signals.  Attenuation or circuit changes help greatly.
The uBITX runs with out RF gain and was optimized for a decently high overload point.
So fewer people complain of overload but for both flavors AGC is a common wish list item.

Stability has been mostly solved by going with NCOs Si5351 and Si570.  The price is adding
an Arduino microprocessor, LCD display and a some form of encoder to tune.    The other prices
is there will be signals generates by the microprocessor and its communicating with external
devices like the display.  It does add and open a whole new arena of user interface that didn't
exist in earlier analog designs.  An example of this is KB1OIQ's version of uBitx that is blind
user friendly (speech synth output and keypad controls) as its really well thought out.

An aside, adding a Rpi or similar [STMFxx series] to do signal processing is on face a good idea 
but the cost is considerable software development, more sophisticated user interface and power.
The last item is important to a compact portable radio, many wish to use batteries and a Rpi eats 
about  3-4watts.  Adding touch screen adds another 3-5 watts to that.  At some point its not a simple
radio nor inexpensive and has become battery unfriendly.  Some problems noted are analog in
nature and easily solved without resorting to a computer. 

FYI look at working being done for the various SDR radios.  If you are going digital its smarter to
start with a new architecture and build in the computer rather than hang it on like a laptop on the side.

With all that said, yes a decent receiver.  Can it be improved?  Yes.  Define, improved first
as many have a different idea of what that may be and in some cases its not even a receiver issue.


Allison





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