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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