The 2n7002 has an Rds(on) of 7.5 ohms.
This is in a 50 ohm environment, so reducing the voltage by roughly a factor of 10 seems about right, giving -20dB.
Perhaps other FET's have a lower on resistance, but either capacitance or price would go up.
PIN diodes have an inherent low frequency limit determined by the charge carrier lifetime.
This will make a pin diode attenuator at the receiver front end not so good a choice
if interested in anything below 80m.?
A better position for the attenuator might be in either the 45mhz or 12mhz IF.
This would also prevent the attenuator from degrading the receiver noise figure, which should be
pretty well established by that first amp at 45mhz.
If you don't like losing 2dB, could get the audio pre-amp to run a little hotter.? With AGC now in front
of that pre-amp, the pre-amp's dynamic range is no longer an issue.
Jerry
Assuming we can afford the loss of a couple dB,?
The pi network PIN diode attenuators try to maintain 50 ohms,
so they might work well with the BiDi amps of the uBitx.??
resistance of 7.2 ohms
toggle quoted message
Show quoted text
On Mon, Mar 26, 2018 at 03:26 pm, Tim Gorman wrote:
Don,
I forgot you gave the circuit in the first mesage. Sorry.
The mosfet should be able to be driven from zero attenuation, Ids=0,
to almost a direct short or infinite attenuation, Vds=0, correct?
Is the problem the range of Vgs needed to go from Ids=0 to Vds=0? That
this only provides a 20db dynamic range?
Is it not possible for the mosfet to reach Vds=0?
tim ab0wr
