On Wed, May 18, 2011 at 5:29 PM, Ron D'Eau Claire <r...@cobi.biz> wrote: > Don't forget that the K3 *is* a largely analog radio. The signal path only > goes digital at the second I.F. > > I believe the K3 (and K2) handle manual RF gain control just like any analog > radio does - they insert an adjustable d-c bias gain control voltage that > replaces voltage originally created by the received signal.
Well, not so much analog as you think. ALL of the smart stuff is done digital, including the frequency synthesizers. NONE of the gain devices are DIRECTLY controlled from a knob or a switch. You can easily verify this with some time in the schematics, which are out in the open on the Elecraft web site. The RF and AF gain pots all have a standard voltage on top and the wipe in the pot is fed to an analog to digital converter (ADC). The ADC's now digital advice from those pots is passed to the CPU. Firmware decides what voltage should be sent to the variable gain IF stage and can completely ignore the RF and AF pot's numerical advice if it wants to. All the audio output devices are FIXED GAIN. The conversion from the number stream representing the audio is fixed. The audio gain pot's numerical advice is interpreted by firmware and the output of the speakers was a flat multiplication of the number stream fed to the speaker's digital to analog converter chip (DAC). There is no RF or AF pot voltage lead off the front panel boards. It's buried in multiplexing done by the FPU and buried in the number soup passed to the main CPU. The PRE and ATT button states have to be interpreted by the CPU to set an internal state for each of those. The CPU then sends a state "message" to a demultiplexer which puts high or low on an output pin to appropriate lines which throw diode switches which either include or exclude the preamp and attenuator altogether from the signal path. This procedure is why such things can be changed from commands coming in on the CAT connection. There is a defensive hardware (analog) AGC which will keep the output of the 15 kHz IF from overranging the last IF's and RX signal ADC with very loud signals. This is the only conventional analog gain device, and if PRE and ATT are being used properly, it will infrequently be used to throttle the variable IF gain. It has nothing to do with the menu-driven firmware AGC processing. Wayne's description of this is that the main AGC simply assumes that any defensive AGC assertions on the signal are coming in on the antenna. And the result is as if magically no signal coming in ever overranges anything. A K3 has close to as little analog stuff as is possible, second only to a Flex which goes digital one step closer to the antenna. But the Elecraft method saved having a PC with expensive top end audio devices. Pretty sound trade-off IMHO. Yes it does have some analog circuitry in it, but it sure ain't your daddy's analog radio. 73, Guy. ______________________________________________________________ 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
