[Elecraft] 10M birdie, crystal osc. frequency, and alternatives for (-) DC generation

[wayne burdick]

When we designed the crystal oscillators in question (KIO2, KPA100), we 
did extensive testing using dozens of crystal frequencies to find the 
optimal ones. The frequencies presently in use are as good as anything 
else that might be tried, I believe, unless you have a specific issue 
that can be dealt with by moving an oscillator a short distance.

If someone wants to experiment with this, they should keep in mind that 
spurious signal search can start on paper, but then must be done on the 
K2 itself, in both RX and TX modes, across all bands. In TX mode this 
is cumbersome: you have to move the VFO a kHz or so, press 
TUNE+DISPLAY, check the spectrum analyzer, cancel TUNE, then repeat. 
(You can't just spin the VFO knob during TX, unless I write some new 
firmware  :)

You have to also be prepared to accept a compromise solution (as we 
did), because the mixing process involves harmonics up through high 
VHF, making it nearly impossible to eliminate every last ham-band 
birdie of any amplitude. To achieve that would require extensive 
internal shielding, which would have made the K2 too expensive to be 
profitable (for us) and affordable (for you).

Regarding custom crystals: Expect to pay about $10-20 for very small 
quantities, depending on how quickly you want them. Cal Crystal is a 
good choice. If a very promising alternative frequency is found, 
crystals can be bought in large quantities for under $1.

I should mention that there's another, perhaps cleaner way to generate 
negative DC supply voltages: use a high-amplitude AF sine wave 
oscillator rather than an RF oscillator. This takes a lot more parts, 
assuming you want a very pure sinewave that doesn't droop or become 
distorted when loaded. I'd suggest a twin-T RC oscillator or a 
carefully trimmed op-amp Wein bridge oscillator, followed by a 
high-input-impedance buffer and a power amp running from 12 V. You then 
have to add a detector similar to the ones used in the KIO2 and KPA100, 
but of course the capacitors will be much larger due to the lower drive 
frequency. This would certainly not fit on the existing KIO3 PCB; it 
would have to be redesign using all SMD parts.

Note that RS232 ICs with on-chip charge-pumps use square waves, not 
sine waves. They will create loud, broadband noise across the lower HF 
bands unless completely shielded or implemented using very small SMD 
components, ideally on a 4-layer board, with extensive I/O and power 
supply decoupling.

73,
Wayne
N6KR


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