Wayne,
Thanks for the very prompt and very thorough answers to all of my question!
73,
Chuck NI0C
Wayne Burdick wrote:
Chuck Guenther wrote:
Is there any problem using the Rx input jack for the RF gain cal
No. Works equally well.
Suppose that I do an RF gain cal, then change the filter
gain (in the Crystal filter configuration part of the K3 utility) of the
roofing filter I used during the gain cal? Should I repeat the RF
gain cal?
Yes, if you want it to be as accurate as possible. But typical
per-filter gain settings are in the low dB range (or should be), so a
small change won't noticeably affect S-meter accuracy.
What exactly is calibrated during an RF gain cal?
The K3 uses a low-noise JFET IF amp. Gain control of this stage --
what we refer to as hardware AGC -- is achieved in two ways: by
varying the current through the preceding PIN-diode attenuator, and by
varying the JFET source voltage (its gate is at ground, so this varies
the transconductance). A very wide range of IF gain control is
achieved in this manner. But the gain curve varies a bit from one
radio the next, because JFET pinch-off voltage varies (the voltage
differential between gate and source at which drain-source current
ceases to flow).
The RF gain calibration procedure injects a DC voltage at the hardware
AGC control node, sweeping it slowly over a 0 to 3 V range using a
D-to-A converter. The DSP measures the resulting signal level (from
your XG2), building a table of control voltages vs. 1 dB attenuation
steps. This table can later be used by the DSP to determine the
strength of incoming signals that activate hardware AGC; it simply
monitors the AGC detector voltage at the control node.
S-meter readings are a composite of the detector-voltage-to-dB table
value and the DSP's internal representation of signals below the
hardware AGC activation level (also in dB), so the S-meter is now
calibrated. The RF GAIN pot, when rotated counterclockwise, simply
adds a DC voltage at the control node (via the DAC mentioned above),
so it is now calibrated as well, given that the DSP knows exactly what
voltage to inject to achieve the desired attenuation.
Wayne
N6KR
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