Re: Frequency Standard Crystal filters


usuallyqrt@...
 

A word of caution in using 198KHz R4 as a frequency standard. As its never mentioned either everyone
knows, or maybe not. The 198KHz signal contains a 25Hz +/- 45deg phase mod (or thereabouts as the
BBC failed to supply me with accurate info on enquiry). You will not see the phase mod if you simply put
the 198KHz into an internally triggered scope as the scope will just follow the signal. You must externally
lock your scope and by using both beams and maybe even configuring as an xy lisajoe display you can
study the magnitude of the phase mod which is quite gross. A long time constant will remove the phase
wobble, it appears to get quite asymetric at times but the mean frequency over a period must of course
remain correct, refer to the long time constant approach used in the G4JNT 60KHz system. A way around
the need for the 198KHz crystal is to divide your PLL reference frequency to 200KHz and mix with the
198KHz producing 2KHz. A 2KHz RC tuned amplifier works well as a filter. An FM chip works well as a
limiting amplifier removing the mod on the incoming 198KHz. Add further capacitance to 455KHz IF coils
for easy 198KHz tuned circuits. Further divide the reference crystal to 2KHz and apply both to an XOR
gate followed by a very long time constant as above. This approach gives a very good answer comparing to
an HP Z3801A GPS reference. Since the stability will suffer at night with the ionospheric effects (although
its still perfectly adequate at night on 10G) I never bothered to publish this arrangement as it was overtaken
by the availablity of OEM GPS modules which of course overcome the nightime effect and also possible
mains sourced interference which effects should not be underestimated on reception. Lastly capacitance
effects to the antenna if your using a ferrite rod should not be under estimated. Walking anywhere near a
ferrite rod antenna will pull the phase of the signal. It will need screening, not with a shorted turn!
73s john.

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