I am a new user of the X6100, which I bought hoping to use as an IF rig for some microwave?transverters

I am building.? I saw the reports of spurious signals and also observed them on my rig so I decided to?

investigate by looking at the RF leakage that everyone was talking about coming out of the RF connector.

Not surprising, the main LO leakage is quite large and follow the frequency where the radio is tuned.? In addition

to that there is leakage coming from a 26 MHz source with additional large spurs every 6.5 MHz apart, and many

smaller spurs at random intervals, mostly CW carriers.? Looking at the service manual I see there is a 26 MHz clock

(probably a reference for the PLL) which sits close to a power module.? I think the placement of a PLL so close to?

a noisy source is unfortunate, and it should have been placed further away.? Not to mention there is what looks

to be both RF and a processor on the same board, although on opposite sides.? I don't know how much isolation

there is between the two domains.??

On a zero IF radio there probably isn't much you can do to eliminate the main LO leakage, but certainly if the

noise from LO1 and LO2 can be cleaned up (maybe by shielding and bypassing) that might help.

I do like the features and form factor of this rig though.? Since I bought this as a rig to hack, I will do some experimentation

when I get some time and motivation.? BTW, for comparison, the MCHF has similar LO leakage but not the other spurs that?

plague the X6100.? And the IC705 is very clean, no leakage that I can detect, although I do see a birdie here and there

on Rx but not enough to be bothersome.

Your thoughts please?

73 Brian W6BY

On Wed, Jan 3, 2024 at 3:42?AM <dg1smd@...> wrote:

Happy new year to all! I had the opportunity to measure the LO leakage on my X6100, production dateJuly, 2022.

Here is what I found:

- as expected, the leakage is frequency dependent: the worst case is the 6m band, -41.5dBm . Switching on the attenuator drops this level by 15dB, the attenuation level. Switching on the preamp drops the level by 28dB, that is the reverse isolation of the preamp.(gain 19-20dB).

- my suspicion is that the LO leakage stems from the gate-to-channel capacitance of the multiplexers used as mixer. The leakage comes from the high level (5Vpp!) of the LO signal so that even a very small gate-to-channel capacitance is enough for the leakage levels we see. On the other hand, this level guarantees a good IP3 level. Since the LO is operating directly on the receive frequency, no amount of filtering will remove it. This is a known characteristic of direct conversion receivers - the literature is full of research papers trying to minimize it, because direct mixers are widely used in mobile phones and other mass market applications. If your mobile phone has FM reception, the probability is very high that a Silabs single chip radio or derivative is used - yep, a direct conversion radio with I/Q signals going direct into A/D and DSP on chip.

- direct conversion radios with low LO leakage use a preamp stage with high reverse isolation in front of the mixer. The balance act here is to keep the gain low for a good IP3 level, and not make the signal to noise ratio worse. Good preamps with this characteristic are not exactly battery-friendly...

- what we can do if keeping the LO leakage down is important: switch on the attenuator AND the preamp. This will drop the LO leakage by 43dB, enough to drop it below the band noise. The gain stays at a reasonable 4..5dB so you dont lose much headroom (IP3). Of course, putting an attenuator in front of a preamp is not exactly optimum for S/N ratio, but at the noise levels we suffer from all that consumer crap around us, this is a small price to pay.

Greetings,

Rainer DG1SMD

Happy new year to all! I had the opportunity to measure the LO leakage on my X6100, production dateJuly, 2022.

Here is what I found:

- as expected, the leakage is frequency dependent: the worst case is the 6m band, -41.5dBm . Switching on the attenuator drops this level by 15dB, the attenuation level. Switching on the preamp drops the level by 28dB, that is the reverse isolation of the preamp.(gain 19-20dB).

- my suspicion is that the LO leakage stems from the gate-to-channel capacitance of the multiplexers used as mixer. The leakage comes from the high level (5Vpp!) of the LO signal so that even a very small gate-to-channel capacitance is enough for the leakage levels we see. On the other hand, this level guarantees a good IP3 level. Since the LO is operating directly on the receive frequency, no amount of filtering will remove it. This is a known characteristic of direct conversion receivers - the literature is full of research papers trying to minimize it, because direct mixers are widely used in mobile phones and other mass market applications. If your mobile phone has FM reception, the probability is very high that a Silabs single chip radio or derivative is used - yep, a direct conversion radio with I/Q signals going direct into A/D and DSP on chip.

- direct conversion radios with low LO leakage use a preamp stage with high reverse isolation in front of the mixer. The balance act here is to keep the gain low for a good IP3 level, and not make the signal to noise ratio worse. Good preamps with this characteristic are not exactly battery-friendly...

- what we can do if keeping the LO leakage down is important: switch on the attenuator AND the preamp. This will drop the LO leakage by 43dB, enough to drop it below the band noise. The gain stays at a reasonable 4..5dB so you dont lose much headroom (IP3). Of course, putting an attenuator in front of a preamp is not exactly optimum for S/N ratio, but at the noise levels we suffer from all that consumer crap around us, this is a small price to pay.

Greetings,

Rainer DG1SMD