On Sun, 15 Feb 2004 23:48:47 +0000, Paul Burridge
wrote:

What's the maximum multiplication factor it's practical and sensible
to attempt to achieve in one single stage of multiplication? (Say from
a 7Mhz square wave source with 5nS rise/fall times.)

While you might be able to generate odd harmonics of a 1 kHz square
wave up to several hundred megahertz, there are two practical
problems.

First you would need some method to separate the wanted harmonic from
the unwanted.

For low multiplication factors in HF/VHF a series of bandpass LC
filters would be needed to attenuate the unwanted harmonics. For
higher frequencies some helical or cavity resonators may be needed.

One old method to separate nearby harmonics is to use a wave analyser.
The wanted harmonics is mixed down with a VFO to some fixed
intermediate frequency in which a fixed crystal filter is inserted
(bandwidth 0,5-50 kHz depending on application). The filtered and
amplified signal is then mixed back to the original frequency by the
same VFO. The absolute stability of the VFO does not matter very much,
since any drift is cancelled in the up-conversion. However, the
stability must be sufficient to keep the desired harmonics within the
IF filter bandwidth. This kind of tricks was once used to multiply
some high precision frequency standard to some odd (say 61th
harmonic).

The other problem with high multiplication factors is that the
amplitude of the higher harmonics is quite low, thus needing quite a
lot of amplification after filtering. However, the level of the
original harmonics was low compared also to the wide band thermal
(white) noise, thus, after amplification, the wide band thermal noise
level is also high, reducing the final signal to noise ratio and in
reception, cause reciprocal mixing programs.

Thus, it is better to use several multiplier stages with low
multiplication factors, since it easier to filter out the desired
harmonics after each multiplier. The gain distribution is also better,
thus the noise floor does not become uncomfortably close to the wanted
signal.

However, if some strange multiplication factor (such as the 17th) is
needed (in which case a series of multipliers can not be used), these
days it would be easier to use a PLL with a fixed digital divider.
Keep the VCO tuning range as small as possible, thus reducing the
MHz/V sensitivity and noise through the tuning line and use a large
loop bandwidth to clean the areas around the generated signal.

Paul OH3LWR