[Elecraft] K3 RF gain cal questions
I have performed the RF gain calibration (and also re-calibrated the s-meter) on my K3 using the Elecraft XG-2 generator hooked up to the receive antenna input of the KXV3 module. I used this input for two reasons: One reason is there is no chance of RF coming out of this input. Secondly, I have a KD9SV receive antenna switch box (Dxpedition model, with preamps for 80/160m) which allows me to have the XG-2 conveniently available at all times, simply by leaving it hooked up to one of the receive antenna jacks. Of course, I left the KD9SV preamps off for these procedures. First question: Is there any problem using the Rx input jack for the RF gain cal, instead of the ANT1 input as directed by the latest version of the K3 utility ? Second question: 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? Third question: What exactly is calibrated during an RF gain cal? I understand in a general sense that the overall RF/IF system gain is determined by many things, including scaling constants in the DSP. Thanks 73, Chuck Guenther NI0C K3 s/n 1061 __ 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
Re: [Elecraft] K3 RF gain cal questions
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
Re: [Elecraft] K3 RF gain cal questions
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
