Re: Modular uBitx - "Ex: Harmonics"

Henning Weddig


I already started to do "my" improvement on the µBITX, but then stopped, after reading and trying to understand all the issues involved and going back to ideas about a spec and blocks to be designed.

As I am an retired electrical engineer specialized in RF and communication techology (I started my profesional career in 1980 at the company Hagenuk in Kiel Germany designing part of the ship´s main communication receiver RX 1001 and later RX1001M, then working on the RF part of the ST900 cordless telephone) 

As I posess a "good equipped home lab" witrh used commercial equipment (spectrum analysers, networt analyser, signal generators and so on), I can test my disgins properl.

Before going to design something  we first should think about a specifiaction and wirte it down. Yes I know seldom engineers like it,  but this will give some insight into what has to be designed. Very important are also level diagrammes of the bolocks involved to see if the to be designed unit will fulfill the desired spec.

From this point of view Glenn´s way of doing a new design in  first to test sub modules is a brilliant idea as it was also used in the industry (and my last occupation within a physics high energy research lab). We often only developed a single unit wioth the use of connectorized "Mini Circuit modules" and built into a case.

As SMD compoents are used the size of the "cheap" chinese pcb´s should be < 100 * 100 mm.

I believe that this forum including Farhan can and shuld be a good potential for an optimized new µBITX!

My previous ideas:

modular design on pcb´s (<100 * 100 mm)

frequency range 10 kHz to 30 MHz

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.

1) receiver/exciter pcb

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

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)

3) driver board

5) PA board

6) lpf´s

7) VSWR bridge

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?)

RF signals to be routed via SMA connectors

Of course AGC should be inclueded!

For the mic amp I remember that for maritime ship transmitters there is a requirement to limiting the max RF output via a Mic compressor. Chips like the SSM2167 could easily fulfill such a (similar) spec!

Comments are welcome
Henning Weddig

Am 15.08.2018 um 02:37 schrieb Nick VK4PP:

Hi Allison/ Kees/ Glenn/ other technical competent hams...

Would you consider working on a uBitx v2 so to speak, advice, criticisms and such?
Tweaking the current design as much as possible to perform better, address the major issues (spurs + harmonics)? 
Better components (2n2222a) ect,
Extra driver stage?
LPF layout..
proper BPF using QRP Labs Modules..

I would like to make it very modular, so its easy to fix/upgrade a section.
I will make some boards, happy to send to you for testing and evaluation at no cost.
Also smaller boars fit in a DL envelope (110x220mm) for DX shipping at $3.50. so that kind of defines the foot print.
Would be cool, 4 boards each about 105x105mm Stacking on one another.
Nano socketed on one board, SI5153 integrated, either SMT or Adafruit module ($$$ vs convenience)

I am not aiming to build a cheap copy, just an upgraded uBitx that you can assemble your self if you don't mind spending a bit more.

I want to enjoy the process and the radio, learn stuff along the way, contribute to the community.
As soon as this is no longer happening, there is no point in carrying on for me.

What are your thoughts people?

Cheers & 73.
Nick VK4PP 

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