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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.
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.