K3 DSP
From Zerobeat
K3's DSP operates at a very high level.
DSP functions are augmented by an external pre-DSP hardware AGC, external hardware roofing filters, and external hardware noise blanking. The engineeers at Elecraft have programmed the firmware to optimize the assistance of these external hardware devices within the DSP. The function of the pre-DSP hardware AGC is to protect the A to D converter when very strong signals, that would otherwise exceed the ADC dynamic range, are present in the crystal filter passband.
The automatic and manual NOTCH filters are within the DSP.
Rob Sherwood found that under some conditions the DSP AGC on transceivers similar to the K3 seriously exaggerate transient noise on weak signalsand can make QRN much worse to listen through. He presented his findings at the Dayton Hamvention 2007, which incidentally also featured Elecraft's introduction of the K3 radio. Rob suggested that he would be interested in seeing how K3 managed in this scenario. Wayne Burdick's response to the challenge was to offer a setup menu action intended to minimize this effect. The basic action was simulated and optimized at the Elecraft factory and may possibly be subject to further refinements when data from production units in the field is available.
It will be interesting to notice under worst case conditions with lots of noise pulses and the K3 pulse setup option disabled, if hams really still prefer to use their old analog tube radios in contests as the Dayton presentation seemed to imply.
Update February 2008 - After Rob Sherwoods test methods were applied, K3 was found not to AGC overshoot in any significant manner as compared to other transceivers in it's class. The new option (AGC SLP) that was installed in the K3 setup must be set to "NOR".
Transceivers that are affected by the overshoot condition can use the noise blanker in some cases to improve the overshoot condition, however the side effects of operating with the noise blanker on all of the time can be undesirable.
K3 Noise Reduction
NR explained by Lyle Johnson
I think there is a fundamental misunderstanding of what NR is and how it works, at least in the context of the K3.
1) NR is better named SE (signal enhancer).
2) It is a "short" FIR filter whose coefficients are continuously being recalculated.
3) It defaults to suppressing everything.
4) When it senses that there is something that correlates (i.e., has a pattern that doesn't seem to be entirely random), it attempts to build a filter around those frequencies that seem less random.
5) Because the FIR filter that is being implemented is short, the filter being built is less selective than the normal DSP filters in the radio.
6) NR is rarely useful if the bandwidth is narrow. If you set your CW width to 400 Hz or less, for example, there is no point in running NR *unless* you want to use it as a sort-of "smart squelch."
7) The narrower you set your WIDTH, the more that noise appears like a signal, and the worse the NR will perform. And the less noise there is anyway, assuming there is a signal present. NR cannot compete with a narrow filter, and was not designed to.
8) I find NR most useful during CW operation with the 2.8 kHz roofing filter NORM'ed so the Rx bandwidth is wide. Assuming band activity is low, the Rx is quiet. If a CW station comes on within the Rx passband, a filter will be built around the station and I can hear it.
9) Similarly, I find NR in SSB is mostly useful as a sort of squelch when tuning around, or monitoring a frequency you are expecting a call on (probably a net or a sked). I use NR1-1 or NR1-2 for this. Mild suppression, not too much impact on fidelity, and lets me hear weak signals, too.
I hope this is helpful in better understanding how NR is implemented in the K3.
