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Hi All,

after having built the Ensemble RXTX for 30/20/17 m, and all tests performed, and compared to LTspice simulation of operating points, everything is just fine: SSB ok, CW ok, tuning ok (Quisk). Except a nasty spurious at ANT, frequency exactly LO, level -20 dBm. It shows up at ANT as soon as PTT is on, and stays there, also while transmitting on a nearby frequency.
On a nearby external receiver, the spur is also audible with PTT off -> QSE disabled.
Disconnecting the four I/Q inputs to the QSE does not help.
All supply and bias voltages are clean.
The only thing that "helps" is shortcircuiting the balancing xfrm T2 after the QSE mixer.
So, does anybody has the same problem, or any ideas, how to get rid of it?
Any help or ideas thankfully appreciated.

73 es tnx
Felix DB9FN

Felix,

a nasty spurious at ANT, frequency exactly LO, level -20 dBm.

It is normal for the LO to be present at the antenna.
On a RX close by it will seem loud.
It will be present at a low level on TX. How do you measure -20dBm? Below full carrier?

I have never found the level of this signal to be significant. If it is really -20dBm is the mixer working correctly? Is TX IQ balance correct?


73 Alan G4ZFQ

Alan,

tnx for the fast reply.
The -20 dBm is measured with a 200 MHz scope: 60 mVpp sine at ANT -> "true" power and is verified with an R&S FPC1000 Spectrum analyzer.
Mixer works fine, and TX IRR is better than -50 dB. And anyway, spur is still present with QSE mixer disabled and even disconnected from following T2 xfrm!
Guess, I will have to live with it.

73 Felix DB9FN

On 14/07/2021 17:45, Felix, DB9FN wrote:

The -20 dBm is measured with a 200 MHz scope: 60 mVpp sine at ANT

At 1W PEP...

20*log(0.06/20) = -50dBc

The -20 dBm is measured with a 200 MHz scope: 60 mVpp sine at ANT

At 1W PEP...

That is therefore 50dB below peak which is not an excessive figure.
It is a long time since I tested mine so do not remember my estimate.
My theory was imbalance in internal capacitances. I seem to remember that I managed to reduce it by more than 10dB at one frequency by connecting short wires (~10mm) to some of the mixer pins and moving them around.
That seemed to confirm it was the mixer, that is all I did. I cannot explain how disconnecting T2 made no difference.

73 Alan G4ZFQ