Re: Modular uBitx - "Ex: Harmonics"


On Wed, Aug 15, 2018 at 01:41 AM, Henning Weddig wrote:

first IF > 45 MHz (to avoid the 2*IF - LO problem) using four pole xtal filters  e.g. 70 MHz (if easily avaliable)? Another "popular IF could be 58 MHz or 58.1125 MHz (used in Hagenuks cordless phones) My be enough NOS filters are laying around.


The magic frequency is over 60 mhz. At 58.1125 the half IF is ~29.06mhz 
or the point where the 2IF minus LO will be one loud birdie (RX and TX).  
It moves the current issue up but not out.  Works for cell but not a 3-30mhz radio.

Band pass filters are easier.  As is we are asking a 300mhz transistor to work
marginally at 45mhz.  Going up makes that worse.  Better yet a better transistor.

1) receiver/exciter pcb

Makes the core a module and replaceable with improved or different technology
without having to toss everything else out with it.

2) xtal filter board for the second IF (e.g. 5 MHz) with filters for SSB; AM; FM(?), CW

FM again, why?  For 10M, easier to make a dedicated radio for that with 
a decent limiter for the RX.  Same for AM, 10W radio with full carrier AM
is 2.5W, not a power house.   It forces a decent power amp as well one
that an sustain full power.  Better to limit to SSB/CW.  Switched filters
makes sense.

2) input lowpass filter (corner freq. 1.6 MHz) plus  overlapping bandpass filters (e.g. 1.6 - 4MHz; 4 - 8 MHz, 8 - 16 MHz; 16 - 30 MHz)

Input highpass at 1.6mhz? Correct?  
Also not octave filter I presume you mean half or 2/3 octave filters so the second harmonic is not able to escape.
The the 2/3 octave lineup results in a large umber of filters unless their corners are suitable to stop harmonics
in the next band up.

3) driver board

5) PA board

This is an approach the driver has to be designed with 50 ohm outputs.  The interconnect can be short wires.

6) lpf´s

Goes without saying easy and its more about layout and switching.

7) VSWR bridge

Should be there and its a very useful and simple thing.

8) VFO (several SI5351 ? to avoid crosstalk between outputs of a single SI5351 driven from a single reference (TCXO; or VCXO which
could be synchronized to a GPS reference?)

Leave out the gps locked oscillator.  we are not designing the next generation Hilberling.  More than one LO device
makes a lot of sense and allows for a better RF layout to put it where its needed.  The 5351 is inexpensive.

RF signals to be routed via SMA connectors

No, as much as I use them and like them they are not cheap and for those that
are mechanically challenged or lack dexterity the word torture comes to mind.
They are not inexpensive  Cables with the connector on them are not cheap.
The base idea that RF should be routed with RF connectors is a vhf and up
thing.  With care at HF pin connectors can work but the board layout has to 
not create RF current loops.


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