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 > > > > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
