Re: LED Power Output Indicator #ubitx

Jerry Gaffke

I'd recommend placing it before the transmit LPF, not at the antenna port.
The diodes will create a bit of harmonic content that the LPF's can remove for us.

I think you could remove the lower 1n4148 and get the same performance.
Then it's just a diode detector, like found in some AM radios.
The LED should start turning on (in a dark room) at 3.0+0.6 = 3.6v peak.
That's 3.6*0.707 * 3.6*0.707 / 50ohms  = 3.6*3.6/100 = 130mW

I don't think the lower left 1n4148 adds anything except perhaps clipping the low going
tip of the RF more or less the same as the high going tips get clipped by the other diode.
This might reduce some of the harmonic content, but the LPF would do a much better job. 

If the 5.6k series resistor also had a 0.01uF cap in series with it, then you should
get double the DC voltage to the LED making this usable with lower power rigs. 
The LED would start turning on at around 3+0.6+0.6 = 4.2 volts pk-pk, or 2.1v peak, or 2.1*2.1/100=44mW.
A low going RF tip will cause the lower-left 1n4148 to conduct, charging up that
new series cap.  So the voltage into the second 1n4148 will be an AC waveform
that rides up on top of ground instead of being centered on ground.


That lower left 1n4148 doesn't really do anything except maybe clip
the low going peaks in a manner almost symmetrical with the high going peaks 
that get clipped by the other 1n4148.

On Tue, Jul 3, 2018 at 06:41 pm, Lee wrote:
I ran across this when looking for an transmit indicator for my Small Wonder Lab PSK31.  The original resistor was 2.2K but our radios put out more than double so I changed the resistor to 5.6K.   Maybe someone with more electronic knowledge than me has better suggestions.   It lights up bright and solid for CW and you an watch your modulation on SSB.   I can really see the difference when I switch in my SSM2167 module.

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