First off, the uBitx as is does pretty well on 80/40/20 meters.
Power drops off on the stock uBitx as we go up to 15 and 10 meters.
There's 6 months worth of discussion in this forum about how best to?
get power up to where it should be on those high bands.
Tough to do, most attempts failed when modifying the existing uBitx power amp.
Grafting the QSX amp into the uBitx could mostly solve that.
Could also solve issues of power amp energy coupling back into the IF amps,
especially if the power amp is off in it's own box.

But the current signal levels in the IF amps and mixers create trouble with spurs
on those upper bands.? Not too much point in going to the QSX power amp unless
we clean that stuff up first.? And while we're at it, improve IMD and carrier leakage
in those IF amps and mixers as well.
?
If we assume we increase local oscillator CLK2 into the first mixer at D1,D2 up to 7dBm,
then the maximum signal into it from the 45mhz IF during transmit should be at least 10dB down
from that, or about -3dBm.? The mixer probably has a loss of around 6dB, and the 30mhz LPF
might lose another dB.? So if we replace everything from Q90 on out to the IRF510's with a
new power amp, that new amp will see a max of -10dBm, or 0.1mW going in.
This new QSX amp has 26 dB of gain for 10W out (where 10W out is +40dBm),
so needs 40dBm-26db = 14dBm going into the QSX amp (as Henning has said).
We need at least an additional 24 dB of gain to go from -10 dBm out of the 30mhz LPF
to +14dBm into the QSX amp.? That could easily be done with two MMIC stages,
and there are many such MMIC's capable of delivering 14dBm of power to the QSX amp.

Some extra gain in those two MMIC's would be good so we?can keep the signal through
the mixers even lower, and have some headroom for an adjustment such as RV1 of the uBitx.

Mouser 630-ADA-4743-TR1G

If you plan to bring the power up to 20W by boosting the supply voltage into the QSX amp,
then you will need another 3dB of power going in, so 14dBm+3dB=17dBm into the QSX amp.
And at 20W, you would want heavier transmit LPF's than the ones provided on the uBitx.

Also, it may be a good idea to have a 3dB attenuator between the MMIC's and the QSX amp
to keep it stable.

It will be interesting to see how clean we can make the uBitx as an exciter.
May need better transistors in the 45mhz stage, certainly want less gain
on the two transmit side IF amps.? Want 7dBm of drive from the local oscillators.
Perhaps the transformers in the mixers are not ideal, especially at 45mhz.
Raj has made considerable progress in sorting this some of this out.
?
I've got a QSX amp on order.?
Looking forward to playing with it.
But the uBitx has other issues to fix before we make use of the QSX amp
to increase power out on those upper bands.

Jerry, KE7ER

On Tue, Oct 9, 2018 at 07:54 AM, Henning Weddig wrote:

Hans,

I just ordered one of Your 10 W PA kits and quickly read over the assembly instruction.

Concerning the UBITX: the PA delivers 26 dB of gain, so for 10 W output the input power for the PA is +40 dBm - 26 dB = +14 dBm This power can be easily achieved with only one MMIC e.g. one of the GALI amps--- or a discrete verions (BS170)?

As You also offer LPF kits, the problems of the uBITX can be obsolete, but requires extra space for these extra boards.

One thing I m missing are the information about the P1dB pointz ( 40 dBm)? and IMD performance.?

Henning Weddig

DK5LV