Yes, those are the correct resistor values for a 63 dB attenuator, but
as a practical matter, you may find that you have trouble getting that
much attenuation in a single stage due to the stray capacitance of the
series resistor. It might be better to break the attenuator up into two
sections of about half the attenuation.
As far as resistor choice, all resistors have some stray inductance and
capacitance. In my experience, "non-inductive" resistors like carbon
film or metal film tend to have best broad-band performance with values
in the vicinity of 100-200 ohms (at least for through hole resistors).
Higher values are limited by stray capacitance; lower values are limited
by the stray inductance.
So if you need a low resistance, two or more equal-value resistors in
parallel may work better. For a high resistance, several in series may
give better results.
Alan N1AL
On 12/24/2017 09:11 PM, Tom Norris wrote:
I have completed the K3S kit assembly but had questions about the receive
sensitivity. On the advice of Gary, AB7MY at Elecraft Support (Thank you) I
have went through the filter configuration again via the K3 Utility and
improved things to the point that I now hear AM broadcast stations but few
hams down in the mud (band condx? and poor single #14 insulated long wire
antenna 6' in air-my son is to help me get OCFDP up when we can (have 6
element tri-band yagi that needs rebuilt on saw horses :-)). The only
signal generator I have is an antenna analyzer with a -10dbm output. After
some research to use the analyzer to supply a low level signal to the K3S I
have come up with these values for a Pi attenuator circuit and would like
to have someone check my results and give advice:
-10dbm analyzer fixed output or 71 millivolt rms attenuated to 50
microvolts rms would be, I believe, a 63db attenuation. For a Pi attenuator
the resistor values would be two R1=50 ohm resistors (signal to gnd), for
the one R2=35.3 kohm resistor (in series with the signal between the two R2
resistors). Probably just use two 47 ohm and one 33 kohm. I don't intend to
do ANY calibrations with this device, just a way to see how the K3S is
functioning and that my thinking is in the ball park.
Thank you and 73,
Tom NB5Q
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