[Elecraft] K3 - 6m Internal Birdie

Wayne Burdick n6kr at elecraft.com
Wed Apr 8 13:46:04 EDT 2015


Tom Lizak, K1TL, wrote:

> I have been having a problem with a 6m internal birdie around 50.096 ... I did the "birdie removal tool" thingee but seems to put a small "void" in that area when I did it.


Hi Tom,

The SIG RMV method can, in some cases, introduce a small dip in audio response at that specific frequency. The way the K3 owner's manual describes it is: "You may hear a slight tuning artifact as you tune in and out of the affected VFO segment." (See CONFIG:SIG RMV description.)

This is an unavoidable side effect of how SIG RMV works. SIG RMV allows you to apply a small shift to both the first and second local oscillators, which can shift the birdie completely out of the passband while shifting the crystal filter passband only a small amount (and keeping the VFO accurate). But if the target signal happens to be close to the edge of the crystal filter passband, even a small shift could attenuate that point in the tuning range by a few dB. Generally, this is preferable to hearing the birdie.

How successful SIG RMV is depends entirely on whether the birdie results from the first LO's fundamental, or from a harmonic. If it's due to a harmonic, which is usually the case, SIG RVM can be very effective. For example, shifting the LO 100 Hz would shift the birdie 1000 Hz in the case of a product involving the 10th harmonic of the LO.

If the birdie you're removing is due to the fundamental of the LO, you may be forced to move it farther, creating a greater chance of an audible tuning artifact.

Unfortunately there's no way to completely eliminate birdies in a high-dynamic-range superhet receiver. When we designed the new synth, we spent a lot of time tuning through the ham bands, slowly, at multiple bandwidths. We selected divider values (etc.) for the new synth such that ham-band birdies were "virtually" (but not entirely) eliminated. 6 meters is especially difficult because both the target band and its image band are both 4 MHz wide. Nonetheless, I think we did a pretty good job of minimizing birdies here.

There are other things you can do about birdies if a small residual artifact is an issue. In CW mode you might try using CW reverse instead of CW normal. Or you can use a different crystal filter with a different offset. In SSB mode you can turn on the notch filter. But before taking any of these more drastic steps, I would also check the orientation of the coax cables associated with the synthesizer(s). Sometimes just moving a cable a bit can greatly attenuate one problematic birdie.

Let me know if you have any further questions about this.

73,
Wayne
N6KR



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