[Elecraft] Dramatic improvement in K3 RX sensitivity below 500 kHz -- looking for testers

[Guy Olinger K2AV]

Outstanding!  It is such fun to watch you guys at work.

73, Guy

On Wednesday, April 29, 2015, Wayne Burdick <n6kr at elecraft.com> wrote:

> Hi all,
>
> In honor of the FCC's proposed "2200 meter" allocation (137 kHz), we
> decided to see if we could improve the K3's receive sensitivity in the
> 100-500 kHz range. This extended coverage is made possible by the new synth
> (KSYN3A).
>
> After a lot of digging, we discovered the cause of the wide-band
> low-frequency noise. We also found a way to shift the passband of the KBPF3
> lower.
>
> With a couple of simple modifications, the MDS (minimum discernible
> signal) can be improved by about 25 dB at the low end (100 kHz). Here are
> some test results (preamp off, 400-Hz bandwidth):
>
>   137 kHz  -114 dBm   FCC's proposed 2200 meter band
>   160 kHz  -117 dBm   low end of the "Low-Fer" band
>   250 kHz  -126 dBm   test frequency in the long-wave beacon band
>   472 kHz  -133 dBm   630 meter experimental band
>
> This represents an improvement throughout the range, but especially at the
> low end. I believe -114 dBm MDS should suffice at 137 kHz, given the high
> band noise in this range.
>
> Another indication of how well the mods work: With no antenna connoted,
> the K3's S-meter now just barely flickers (S1) at 100 kHz. Without the
> mods, the no-antenna S-meter reading could be as high as S7 due to the
> noise source we've now identified (a voltage regulator). (By the way, this
> regulator noise had no impact on frequencies about about 500 kHz.)
>
> Here's a summary of the changes required:
>
>   RF board:              Add one leaded capacitor
>   KRX3 (if applicable):  Add one leaded capacitor
>   KBPF3:                 Remove two inductors; parallel three new
> capacitors
>
> If you're interested in this low frequency range, and you'd like to try
> the mods, please email me directly.
>
> Note: To tune this range with the K3, you'll need the new synth (KSYN3A),
> a KBPF3 module (on the main and/or sub receiver), and a KXV3 module. A KXV3
> is required because the antenna for the 100-500 kHz range should be
> connected to RX ANT IN or XVTR IN. The ANT1/ANT2 path goes through the T/R
> switch, which has a high-pass filter to protect the K3's fast PIN-diode T/R
> circuitry.
>
> 73,
> Wayne
> N6KR
>
>
>

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