On Wed, 21 May 2008, Alan Peake wrote:

Hello all,
I am trying to muliply a 72MHz crystal oscillator to 1296 MHz. My first
approach was to just make a series of X2 or X3 transistor mulipliers to get
the require X18 multiplication fact. However, I have found two pieces of
equipment in the junk box which just use diode multipliers. One is an old
Electrophone UHF CB radio which multiplies the crystal oscillator by 17 (I
think, from memory)then uses helical resonators to filter the desired
harmonic. The same approach is used in an old King aircraft transponder where
the 138 MHz crystal is multiplied to 960 MHz with just a diode and uses the
first two stages of an interdigital mixer to get rid of unwanted harmonics.
So, the question is, which is the better approach? I just want a reasonably
clean signal source to test a 1296 MHz down-converter.
The diode approach seems simpler but is it likely to contain more spurious
signals than a transistor multiplier chain?
Alan
VK2ADB


I don't know, and one thing to remember is that what was done years ago
may no longer be the solution because other things have come along.

A single stage of multiplication is of course simplest. But, if you
do it in one step, the signal may be so weak that you need stages of
amplification at the ultimate frequency. Once upon a time, frequency
limits may have made that unfeasible.

Also, if you have one stage that basically generates harmonics, and
then you expect to pick off the desired frequency, that filtering
may need to be much better than multiple stages. If you start
with a low enough crystal frequency, the next harmonic may be too close
and some of it will get through the filter on the ultimate frequency.
If you have a string of multipliers, each does filtering so the next
stage only has to deal with filtering out a relatively high frequency.

Note that your two examples aren't comparable. The first example you
say multiplies by 17, while the second only multiplies by 6. Even in
the old days, it wasn't uncommon to see a jump like 6, but something
like 17 was less common.

The real trick seems to be to start with as high a frequency as possible.
Then the multiplication needed is limited, and it's far easier to filter
out harmonics from a higher frequency crystal than a lower one.

Michael VE2BVW