[Elecraft] Re: Help with Birdie (not "tweet tweet!")

[Mike Harris]

G'day folks,

I think we're going off on the wrong route here.  It's nothing to do
with bad joints, wobbly components, instability and all.  Bashing a
frequency determining components around the VCO is bound to change the
frequency, that isn't necessarily the issue.  Squeezing the case
causes the BFO to move.

Birdies are a consequence of the necessary mixing required to convert
the RF to audio.

Oscillators don't just produce the desired fundamental frequency but
also harmonics.  Mixers produce sum and difference products of the
signals offered to them including sum and difference products of the
oscillator harmonics.  It is common for some combination of these
products to produce inband signals.  It depends upon many things,
mixing scheme, oscillator spectral purity, filtering, screening and
on...

They are low level in my K2.  I can hear them but they don't get in
the way and can provide handy test signals.

You have to be careful when switching the circuit configuration and
noting any change in signal level.  Changing from a matched ant-1 port
to a possibly miss-matched ant-2 port for example will have an effect
on the level because it changes the load on the receiver front end.

If for example the 2nd harmonic of the VCO was involved then the
tuning rate of that harmonic is twice the rate of the fundamental,
this is why the tuning rate gives you a clue as to how high up the
harmonic ladder one has to consider for the mixing products.  The
direction of tuning, sometimes when you tune HF the birdie goes LF,
sometimes HF depending upon whether it is a caused by a sum or
difference product, another clue.  The fact that these birdies have
accelerated tuning rates confirms that they are mixing products caused
by variable oscillators and not just fixed signals e.g. the 1 MHz
harmonics generated by the CPU which some of us use for setting the
4MHz reference oscillator.  These of course behave like regular
external signals when you tune across them.

As I've mentioned before it can get very messy and make your brain
hurt.  As long as the birdie is near the noise level it's no big deal.

Regards,

Mike VP8NO