[Elecraft] New beta K3 firmware!

[Joe Subich, W4TV]

On 9/21/2012 6:40 PM, Wayne Burdick wrote:
 > The 6 kHz filter band-limits the transmit noise originating from the
 > DAC and the transmit mixer to a bandwidth appropriate for AM and ESSB
 > modes. The 15 kHz filter would allow a noise bandwidth over twice as
 > wide.

In an attempt to quantify this wideband transmit noise, I set up the
K3 on the test bench with a dummy load, directional coupler, some
attenuators, another receiver and the SDR-IQ.  The directional coupler
provided a 1 dBm signal with 100W from the K3.

1) I measured the "in band" noise (noise in the transmit bandwidth)
    -80 dB relative to 100W output.  That is, when the sample was
    applied directly to the test receiver, the noise pedestal *within*
    the normal SSB or ESSB bandwidth is ~S8.  "Out of band" (opposite
    sideband) noise or noise more than 3 to 4 KHz above carrier is at
    least -100 to -110 dB below PEP (20 to 30 dB below the in band
    noise).

    Note:  all tests were performed with the microphone unplugged but
    the mic gain at normal levels and compression set to approximately
    15 dB compression (normal operating conditions).

2) In SSB (with the 2.8 KHz roofing filter) the in band noise is
    roughly 3 KHz wide based on tuning a receiver until the noise
    drops below MDS (-147 dBm measured) or looking at the spectrum
    using the SDR-IQ.  The out of band noise extends about 1 KHz
    below Fc (measurements were made on 20 meters USB) and is at
    least 20 dB below the in band noise level.

3) In ESSB/4.0 KHz (with the FM roofing filter) the in band noise
    measured 4.5 KHz wide with the receiver and/or SDR-IQ.  The LSB
    noise is 20 to 30 dB below the in band noise level and extends
    about 4 KHz below Fc.

In either case, the "out of band" noise is at least -100 dB relative
to PEP and is likely to produce detectable QRM only to the closest
of neighbors with high power operation and even then only within
5 KHz on the opposite sideband.

For grins, I looked at the in band/out of band (opposite sideband)
noise of a conventional up conversion (Icom) transceiver that I had
on the test bench.  The in band noise pedestal was also around -80
dB/PEP with out of band (lower sideband) noise about 20 to 30 dB
below the in band level extending 3 KHz below Fc.

In summary, while using a narrow filter (2.8 KHz for normal SSB,
6 KHz for AM/ESSB) will reduce the "out of band" products somewhat,
using the FM filter results in a signal that is comparable with
"conventional" transceivers.  Based on experience with conventional
up conversion transceivers, the effects of the opposite sideband noise
are generally *less* than those from AGC pumping or transmitted phase
noise.  Anyone who would complain about transmit noise when the K3
is using an FM filter for ESSB/AM would have the same complaint with
a conventional transceiver.

One further note, "opposite sideband noise" was slightly lower in
DATA A (and AFSK A) than in USB - perhaps due to changes in filter
offset.  However, the -80 dB/PEP in-band noise pedestal was still
present for the full 3 KHz bandwidth.

73,

    ... Joe, W4TV


On 9/21/2012 6:40 PM, Wayne Burdick wrote:
> The 6 kHz filter band-limits the transmit noise originating from the
> DAC and the transmit mixer to a bandwidth appropriate for AM and ESSB
> modes. The 15 kHz filter would allow a noise bandwidth over twice as
> wide.
>
> This in-band noise "pedestal" could be 20-30 dB above the normal
> transmit noise floor if you were using a lot of mic gain and/or
> compression. If you were using high power in these modes, your signal
> would now be much more likely to bring up the receive noise floor at
> nearby stations.
>
> Do you really want to create a 15-kHz wide swath of broadband noise
> when using ESSB or AM?
>
> I don't. That's why I haven't modified the code to allow use of the FM
> filter for this purpose. I suppose it could be YAMU (yet another menu
> entry), forcing you to do at least a minimum amount of soul-searching
> before "going broadband" :)
>
> 73,
> Wayne
> N6KR
>
>