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I decided to add a few
words to explain a little about the HP8640B that I'm overhauling.
The chassis carries two pieces
of test equipment, a signal generator and a frequency counter
combined together to give you an accurate readout of output frequency
which is very useful and far superior to similar equipments where
a separate counter or complicated reference procedures are needed.
The frequency counter can also be used independently. The basic
8640B covers the range 500KHz to 512MHz (plus a little extra
for the benefit of overlapping ranges) but can be extended (in
the factory) to additionally cover the range 512 to 1024MHz.
Something which I found at first slightly puzzling, is that the
range switch has this extra setting, and although this range
can be selected, and the tuning knob shows the higher frequencies
on the display, the RF output stays the same as the previous
setting of 256-512MHz. I suppose one could use the 2nd harmonic
which is present at a lower level.
Many signal generators use loads
of mixers, but not this.. which instead relies on digital circuitry
to progressively divide the oscillator frequency to match each
range. Because these digital outputs are square waves a lot of
filtering is necessary to reduce harmonics so, coupled to the
range switch are matching filters which are designed to reduce
the harmonics and to end up with a respectable sine wave for
each range. Clearly this method of operation must employ an oscillator
to match the highest range, so HP elected to use a cavity-tuned
oscillator covering 256 to 512MHz. The cavity is tuned by a mechanical
arrangement covering something like eight turns of the tuning
knob and each range has a decent overlap so in fact the cavity
oscillator tunes something like 230MHz to 550MHz.
The cavity oscillator output
passes to a digital divider chain consisting of a set of nine
divide by two stages in series providing x2, x4, x8, x16, x32,
x64, x128, x256, x512. The lowest range uses all nine stages
and covers about 449KHz to 1074KHz which represents 230/550MHz
divided by 512. The next higher range has divide by 256 giving
898KHz to 2.148MHz and so on. The width of each range does result
in a major problem in that the filter for a given range needs
to perform two contradictory actions. The filter needs to pass
say 1MHz but attenuate the second harmonic of 2MHz, but that
same filter, when the oscillator is at the high end of the band,
needs to pass 2MHz with next to no attenuation. That, alas is
impossible unless the filter shape changes with input frequency,
which I imagine is theoretically possible, but mighty difficult
to implement, so HP just use two filters. One filter known as
the Low Filter operates in the lower half of the selected range
and attenuates the second harmonic and higher and the High Filter
does the same, but starting midway through the range. Coupled
to the oscillator tuning control is a potentiometer which signals
to the filter assembly that relays should select either the Low
or High filter. Every range has a low and high filter but lower
ranges additionally add another (the next higher range filter)
to help attenuate higher harmonics. This feature is necessary
because, as the number of dividers increase the pulse train will
have squarer edges and less symmetry. For example the tops of
the pulses may be wider or narrower than bottoms resulting in
a distorted sinewave (stronger even harmonics) plus the squarer
shape of the pulse train will result in more and stronger odd
harmonics, so to improve the lower frequency performance more
than one filter is employed in series.
What about stability? Ordinarily
a design would be based on an oscillator having relatively low
frequency because its easier to keep stable but the HP VHF oscillator
is inherently very stable at the expense of a rather large mechanical
layout. The 8640B does however incorporate a clever technique
whereby the master oscillator can be locked to a 5MHz crystal
or, if desired, an external reference. |