[Elecraft] Comparing the K2 to other rigs: background noise

[Wayne Burdick]

Christoph Rheker wrote:

> ... My TS870 sounds in most cases like
> there is some kind of life on the bands. For example, during
> evenings on 15/20m there is always some kind of background noise.
> When I switch to my K2, there is no such thing! 

Chris, the K2 is quieter than the TS870 because it has a single I.F. and less
overall gain. Because of this, really low-level band noise won't activate the
K2's AGC. (The TS870 is probably "in the AGC" with normal band noise, even on 10
meters, because of all of the excess gain.)

But I want to make sure I understand what you mean by "no such thing" in
reference to band noise. When you connect a resonant antenna to your K2 on any
band, including the higest bands, you should hear an increase in background
noise. This should be true even when there's no unusual atmospheric noise on the
band. Of course the effect will be much more dramatic if you're using a wide
filter bandwidth.

I'll give you an example, using a test I just did on two different K2s:

1. Switch to one of the highest bands (15 m up), USB (FL1 = OP1 = about 2.1 kHz bandwidth).

2. Disconnect the antenna.

3. Connect an AC voltmeter (DMM set to lowest AC volts scale) across the
headphone jack (use a plug only, no headphones)

4. Turn AF GAIN and RF GAIN all the way up, and turn the preamp ON.

5. The AC voltmeter should indicate around 20-30 mV RMS or higher. This is the
baseline (no antenna) noise, generated within the K2 itself. It will be on the
low end of this range if you have a KAF2 audio filter installed, or if you use
the CW filter rather than the filter on the KSB2. The narrower the filter
setting, the lower the voltmeter reading will be.

6. Connect a resonant antenna, preferably the lowest-noise antenna available for
the selected band.
 
7. The AC RMS voltage should go up by a factor of two (6 dB) or more. (If it
goes up a lot more than this, it could be because there's a signal on the
frequency you're tuned to, or because there's a lot of atmospheric noise.)


I did this test on two different K2s. On the one with a KAF2 installed, the
noise on 10 meters went from 20 mV to 50 mV RMS when the antenna was connected.
On another K2 with no KAF2, and using the CW filter set for 2.20, the noise went
from 35 mV to 70 mV RMS.

If you do this test and discover, for example, that both the antenna off and
antenna on conditions show much lower AC voltages at the headphone jack, you
could have a problem with your K2. Try turning AGC off (hold PRE/ATTN and AGC
together) and see if the results differ significantly. If it turns out that the
AGC threshold is set too low, you can cure this by reducing the value of R1 on
the Control board. If the signal amplitudes are too low even with the AGC
threshold set correctly, it could be due to excess loss in the band-pass or
crystal filters. 

If you do the same test on the TS870 you'll no doubt see larger antenna off/on
figures due to the much greater gain in the receive path. But this is to be
expected; the K2 design aims for both good sensitivity and low current drain, so
low-current devices are used at every stage, and there are far fewer stages.


> One problem with I don't like is the S-Meter activation issue. With my
> antennas I often get S7 S-Meter readings without any "real" signal
> present. Yes, I have done the RFC11 mod. No, I have not done the mod
> to the front-end band pass filters. Another item on my list.

The narrower filters should help quite a bit.

73,
Wayne
N6KR