I have no idea how you get +1dBm.
A 700mv pk-pk square wave centered on ground is 350mV rms (since it's square not sine)
Into 50 ohms, that would be 0.350*0.350/50 = 2.5mW,? so? 10*log10(2.5mW/1mW) = 4dBm.
But the CLK0 LO is driving a 1:2 transformer which then drives two series Schottky diodes,
definitely not a 50 ohm resistor.
?
I believe standard practice is to evaluate dBm for the LO into 50 ohms resistance.
A? mixer designed for that dBm level should then be able to make good use of the LO.
With two Schottky diodes in series and with this transformer arrangement,
we should get best performance (maximum signal handling capacity) at 7dBm of LO power,?
Signal level going in (from the mike amp) should be at least ~10dB less than the LO, or distortion occurs.
The si5351 has 3.3v CMOS output drivers capable of driving up to 8ma,
so even without the attenuation of R110,111,112,113, it's not capable of giving 7dBm into 50 ohms.
While not ideal, it's adequate for a $130 HF rig.
More LO power (especially at the first mixer) would allow the receiver to work with stronger?
out-of-band signals present, which hasn't been a hot button issue for the uBitx.
It would also allow larger signals through the IF stages when transmitting,
but that could also be addressed by adding an amplifier stage to the PA.
Jerry, KE7ER
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On Mon, Mar 18, 2019 at 09:48 AM, jim wrote:
The output side of the CLK 0 pad ...700mV p/p? =~+1dbm? ..should be enough
?
