Re: #bitx40 #ubitx #ubitx #bitx40

Jerry Gaffke
 

It's an Rds(on) thing, the 2n7002 is spec'd for a maximum on resistance of 7.5 ohms with Vgs at 5v.
Don found he could cherry pick 2n7002's for better attenuation, they likely had a lower Rds(on).
Assuming a 50 ohm series resistance at the source and a 50 ohm load, a 2v source voltage
gives 1v at the load.  With 7.5 ohms in parallel with the load, I get
    7.5 ohms in parallel with 50 ohms = 6.52 ohms, and a load voltage of (2v/(50+6.52))*6.52=0.23v
Would need down around 0.1v at the load to get Don's 20dB attenuation, but that 7.5 Rds(on)
is a worst case over process and temperature.  So we're in the ballpark.

Putting two 2n7002 in parallel will cut Rds(on) in half, giving you an extra 6dB of attenuation.
It also doubles the capacitance seen at the drain, this will hurt our minimum possible attenuation,
especially at 30mhz.

My bet is on the BAP64Q.

Jerry


On Tue, Mar 27, 2018 at 08:21 am, Tim Gorman wrote:
 Jerry,

I forgot to reply to this.

Would this really help? I'm going to put together Jerry's agc circuit.
It wouldn't take much to drop one 7002 on top of the other.

Is this a voltage thing instead of a current thing? Two parallel
resistors pull more current but don't change the voltage drop across
each. If the attenuation works by dropping the RF voltage seen by the
receiver than adding a second 7002 probably won't change anything.

tim ab0wr

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