[Elecraft] K2 AF noise level?

[Ron D'Eau Claire]

Curt, K3EY asked:

What am I hearing on my K2/100 when on exactly 7.000 plus or minus a few
KC's I hear a steady tone when switched to the dummy load? At first I
thought it was coming from my computer monitor but it's not. When I switch
back to the antenna system the tone is gone or at least so attenuated I
can't hear it.

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That's a known "birdie" generated internally within the K2 receiver. One
disadvantage of a superhet receiver is that *all* superhets have birdies and
spurious responses produced by the multiple oscillators and mixing schemes
involved in the design. The trick is to work out a design using frequencies
that avoids birdies in the normal tuning range. With nine bands covering the
HF spectrum, that's a pretty formidable task. One of the advantages of the
single-conversion design used in the K2 is that is produces a minimum of
such signals, but there are a few and that one at 7 MHz is one of them. 

Offhand, I'm not sure which combination of oscillators mixing together is
producing it, but it is perfectly normal. Indeed, that's the signal the
manual recommends you use to adjust L34 in the Alignment and Test procedure,
Part II. 

As you noticed, it's a very weak birdie that is normally audible only if
there's no antenna connected to the receiver. Normal off-air background
noise at 7 MHz is much stronger than the birdie, so it's not audible in
normal operation. 

Ron AC7AC