[Elecraft] KX3 AM-breakthrough

[Ralf Wilhelm]

Sorry but I do not agree:

If the "mixing process" involves the local oscillator (cross-modulating the local oscillator's "carrier" in the AC mains), the KX3 detects the AM wherever it is set and would not show discrete signals. If the modulated local oscillator's signal is re-radiated by the AC wiring, a AM signal is exactly on your rx frequency, whatever the rx frequency would be.

Discrete signals would only occur if the mixing process does not involve the local oscillator but only different AM stations. 

The two Deutsche Welle signals I mentioned would not be local stations ("Ortssender") for me, but especially the 3995 one used to be very strong in the evenings. I order to have the "Ortssenderproblem", one only needs a strong signal, however it doesn't have to be a local one.

What I call "true AM breakthrough" (rectification of a strong signal somewhere in the mixer), is a potential problem of any d.c. receiver. This rectification occurs in any receiver, but if you are not using "zero-if" you don't notice it (the if filtering will not pass the audio frequencies to the following stages). This is the reason why the 8kHz shift "solves" the problem.

When I had my "hum problem", I started having the KX3 connected to a FP757HD analog power supply and changed to running it from the internal batteries (which didn't solve the problem). 

I really was concerned about the AM breakthrough problem, and the discussion on the internet was the reason why I ordered my KX3 without the roofing filter. Testing for AM breakthrough with the strong (swiss?) broadcast signal on 3965 was one of the first things I tried when I obtained my KX3. The interesting result was that a true 0 dBm (measured on the calibrated Perseus s-meter the following evening) signal did not produce AM breakthrough but only 2nd order IMD. 

I think a more systematic approach (calibrated signal generator with AM modulation) is needed to figure out if the KX3 itself produces the breakthrough or if it is produced somewhere in the "environment". Maybe someone with this kind of lab equipment could perform a quick test and find out at what signals the KX3 starts to directly rectify AM signals.

 Maybe someone experiencing AM breakthrough can check if he detects AM on the signal emitted by the local oscillator of the KX3.

If the AM breakthrough is a real (direct rectification) one, the 90 degree phaseshift between the two quadrature "channels" would probably not occur (in contrast to "true" signals). This could probably be used for suppressing it by software (?)

Vy 73

Ralf, DL6OAP






Am 29.07.2013 um 22:01 schrieb Klaus Dittrich <kladit at arcor.de>:

> Thank you all for taking the time to answer to this issue.
> 
> I will respond to some of the arguments and tips ..
> 
> Klaus, I already have current baluns at both ends of the coax cable.
> 
> 
> The KX3 is driven by an analog old PS30 power supply, not a switching
> power supply.
> 
> I already mentioned the AM stations are far ones.
> The last "Ortssender" around, Stuttgart Mühlacker, has been put out of service.
> 
> Emory, same here, only when the broadcast bands open, good conditions,
> the effect takes place. Your double size G5RV delivers similar strong rf-levels as my antenna then. It is not one AM-station, depending
> on ionospheric conditions it may be and often is every time another one.
> 
> Uwe, "very annoying", that's the point.
> 
> Ralf, I think, because the AM-breakthrough signals are hearable
> at every frequency they are not mixing products from a nearby
> electronic device or a diode build by corrosion. This would cause
> a lot of but discrete signals.
> 
> Matt and Don, if this is a problem with _any_ zero
> frequency IF (direct conversion) receiver then the KX3's
> concept would be a design error. ( I hope it is not.)
> 
> If I switch to RX SHIFT I will have a receiver with the
> most advertised KX3 feature, narrow spaced dynamic range,
> dropping to values of rigs of the last century, like IC-735
> or Lowe HF-235 or Drake SW8.
> (see http://www.sherweng.com/table.html)
> 
> Additional I would have thrown the money for the roofing filters
> out of the window, as we say here in Germany.
> 
> I hope there is a possiblity of rejection by software, but I
> have no expertise in SDR at all.
> 
> No one of Elecraft has answered up to now, so I stay curious.
> 
> 
> -- 
> 73 Klaus DF1TL
> 
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