Re: uBitx v5 issue


Jerry Gaffke
 

Evan,

Good to know that your back-to-back diodes present no issues with reception.
Seems a fine solution.  If incoming RF is kept below 4 volts peak-to-peak,
there should be no trouble with Q90 blowing due to Veb-max of 6v being violated.

I would not call the 1n914 obsolete, they still sell them on Mouser.
1n914's are an older part, were not spec'd as tight as the newer 1n4148, but otherwise identical.
I suspect that any parts sold as 1n914's in the last few decades come off the same line
as the 1n4148's, just slapped with different labeling.

>  There was concern about the reverse diode on the base of Q90 having a negative impact on the SSB transmission. 

I'd be surprised if this was an issue, but worth looking at if you have a way to measure IMD.
Transmit RF into Q90 should be down around 100mv rms, so currents through
a silicon diode would be measured in picoamps.
Also, we already have the base-emitter diode in there to cause distortion of large signals, 
adding a 1n4148 going the other way across that junction diode would make it symetrical, 
and may well reduce distortion rather than add to it.

Jerry, KE7ER




On Tue, Nov 24, 2020 at 03:38 AM, Evan Hand wrote:

I have done the back to back diodes on the input to my v4 board and have not noticed any negative effects on the receive capability. 

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