Re: NC30, plus info on mixing schemes

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From: Wayne Burdick (burdick@interval.com)
Date: Fri Oct 13 1995 - 13:31:29 EDT


Hi Walt,

>Did you ever get the NC30 parts list done? Never saw it?

I built an NC30A but haven't had time to turn it into an addendum to the
manual. Meanwhile, though, Ed Burke (call?) has done the conversion too,
and I'm hoping to get him to write it up. Please call him and affirm this
request if you're interested:

  503-685-3414, 503-635-7423

Bob Dyer knows about this and I'm sure would appreciate someone duplicating
Ed's mods.

>What do you think about changing the VFO to 5.2 MHz and leaving the crystal
>filter/BFO/TX mixer as is [to put the NC40A on 30 meters]...

Nice try. :) This is actually a much less desireable approach, because
the mixer products will only be a few dB down from the desired output on
transmit, and the images will be bad on receive. This is because the I.F.
is 4.9MHz--not very far away from a 5.2MHz VFO. If you mix a VFO at 5.2
with a 4.9MHz TX oscillator signal to get 10.1, you'll have a huge spur at
5.2*3 - 4.9 = 10.7, not to mention 4.9*3 - 5.2 = 9.5, and many others. On
receive, this close proximity of signal frequencies will result in big
image responses, especially on a band like 10MHz where there are many AM
broadcast stations.

The 8MHz I.F. is a better way to go because it keeps the VFO frequency low
and has fewer image responses. But note that even this combination isn't
perfect: there will be a spur at 8*3 - 2.1*7 = 9.3. This one is
mitigated somewhat by the low-pass filter on the VFO, but it will still be
there--hopefully >40dB down. I haven't put it on the analyzer yet.

Actually, the scheme used in the Sierra is much better, but of course more
complicated: Pre-mix the VFO with a high crystal oscillator frequency,
then *subtract* to get the I.F. On 30 meters, for example, the Sierra does
this:

       (18.000{xtal osc} - 2.985{VFO @ 10.1})
    - 10.1{R.F.}
    = 4.915.

I used a high crystal frequency (18MHz), subtractive mixing, odd VFO and
I.F. frequencies, and a computer program for estimating mixer products.
The result is an exceptionally clean mixing scheme--there are almost no
birdies on receive in the Sierra on any band. (The reason the odd VFO and
I.F. frequencies help will be apparent if you try plugging in integer-value
I.F.s into the above equations--consider the harmonics of these I.F.s when
the VFO is set at a band edge! There are other rigs on the market that use
integer-value I.F.s and as a result have ear-splitting "band-edge markers"
as a feature.)

Hope this makes sense. Choosing signal frequencies is something of an art!

73,
Wayne


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