[Elecraft] KX3: nearby noise post-mortem

[Ralf Wilhelm]

The problem with all receivers using a local oscillator (direct conversion or superhet) is that they emit a small part of the local oscillator power via the mixer to the antenna port (maybe a typical number of L.O. to R.F. Port Isolation of a mixer is 40 dB or something like that). In a superhet, the local oscillator's frequency is far away from the rx frequency and the front end filtering keeps the oscillator signal from leaving the RX. 
In a direct conversion RX, the oscillator is on the same frequency as the RX. So basically, you have a very little transmitter that is always exactly on your rx frequency. In principle, this signal can be reflected by some structure and get back into the receiver (which would cause a d.c. offset) or in your (and my) case, it can be "absorbed" by the wiring, modulated by the 60/120 Hz in one of the power supplies and than be re-emitted by the wiring before being received via the antenna. In this case, you receive a carrier plus 60 Hz sidebands, but the carrier is at zero beat so you only hear the sidebands. 

The additional isolation (the oscillator signal "tries" to pass the amp in the "wrong direction") of the preamp makes the emitted fraction of the local oscillator power maybe 20 dB weaker so that is buried in the atmospheric noise.

73, Ralf, DL6OAP


Am 10.02.2014 um 17:49 schrieb Nicklas Johnson <nick at n6ol.us>:

> Interesting, and if I may ask a potentially-dumb question, what do we really mean when we use the word "isolation" in this context, and how does it provide the benefit of keeping the buzz out of the detector (or to ask the question in the reverse, how does the buzz get in, in the absence of additional isolation)?
> 
>    Nick
> 
> On 10 February 2014 08:40, Ralf Wilhelm <ralf at super-deutschland.net> wrote:
> Hi Nick,
> 
> Turning on the preamp increases the isolation between the local oscillator and the antenna port. I also wasn't sure if my "buzz" went away when I turned on the preamp (I couldn't hear it any more) but I used the spectral display of the MixW-Software and the microphone of my laptop to check and I found it didn't go completely away (was still visible in the waterfall) but got much weaker...
> 
> -- 
> N6OL
> Saying something doesn't make it true.  Belief in something doesn't make it real. And if you have to lie to support a position, that position is not worth supporting.