[time-nuts] OT: Levelled sine wave generator

WB6BNQ wb6bnq at cox.net
Mon Dec 15 06:40:27 EST 2008


Bruce Griffiths wrote:

> Magnus Danielson wrote:
> > Bruce Griffiths skrev:
> >
> >> David C. Partridge wrote:
> >>
> >>> Sort of related, but only just - however the signal to noise ratio here is
> >>> so good that I feel impelled to ask.
> >>>
> >>> For 'scope calibration I'm considering building a levelled sine wave
> >>> generator.
> >>>
> >>> Ideally the specs I'm looking for are:
> >>>
> >>>  o Close to DC (10kHz or 100kHz would be fine) up to at least 1GHz.
> >>>     more would be better but not critical
> >>>
> >>>  o Output levels from 0.5Vp-p(-2dBm) to at least 4Vp-p(+16dBm) into 50R
> >>>      (up to >6Vp-p(say +20dBm) would be better)
> >>>
> >>>  o Output flatness levelled within 2% of desired output level (+/- 0.086dB)
> >>>     across the entire frequency range at the final connector to the DUT
> >>>     This will almost certainly mean an external levelling head.
> >>>
> >>>  o Modulation - not critical, FM or AM might be useful.
> >>>
> >>>  o A logarithmic sweep capability might be nice, but isn't necessary.
> >>>
> >>>  o Frequency display - nice to have but output to external counter is OK.
> >>>
> >>> Generating the basic signal is probably just a case of using something like
> >>> an HP VTO-8200, mixing it with 2GHz (Mini-Circuits RMS30?), low pass filter,
> >>> an AGC stage (see below) and then amplify probably using an MMIC like the
> >>> Mini-Circuits ERA-2SM followed by an additional stage to get the extra few
> >>> dB.   For more accurate frequency control some sort of synthesiser locked to
> >>> a reference might be in order (I had to get a time-nuts hook in here
> >>> somehow).
> >>>
> >>> The question is what should go in the sensor head?
> >>>
> >>> Logically I need to sample a proportion of the signal delivered to the
> >>> output connector, compare the output of the sensor against a DC reference
> >>> level telling it the desired output level, and feed back a voltage to a
> >>> wideband AGC stage (any suggestions for this?) in the main unit.   I also
> >>> need to be able to detect that output is not levelled.
> >>>
> >>> Or should I just forget the whole idea and go talk to R&S with a large
> >>> cheque in hand?
> >>>
> >>> Cheers
> >>> Dave
> >>>
> >>>
> >> The phase noise wont be particularly low especially for low output
> >> frequencies.
> >>
> >
> > I think this is acceptable for the intended application.
> >
> >
> >> A diode double balanced mixer with dc current applied to the IF port is
> >> useful as a wideband current controlled AGC device.
> >>
> >
> > Indeed. BTW, I used that trick when using my network analyzer for
> > sweeping the Caesium-beam. I unplugged the 12,6 MHz and inserted my
> > output port which was split to also go into a mixer. I picked up the
> > detector voltage and feed it into the IF port and the resulting
> > modulated sine was sent to the receiver port. Worked like a charm and
> > provided me with a nice user-interface.
> >
> >
> >> NB mixer IF response must extend to dc.
> >>
> >
> > It should not be too hard to find a suitable mixer that matches that
> > description thought.
> >
> > As for sensor-head, an old technique is to use a pair of diodes
> > thermically connected, where one is fed a 100 kHz square wave and the
> > other is used as a detector. The reference diode is bias adjusted and
> > the reading from the other is compensated with the same amount and thus
> > allowing for a linearization. I don't know if there is any modern ways
> > which is more suitable. I think it could be a bit of a challenge for
> > detecting it all the way down. I'm sure Bruce can elaborate some on that.
> >
> > One possible issue would be that input impedance could mess things up.
> > So maybe one should consider making quadrature readings in the head such
> > that with some processing the propper level can be given considering the
> > impedance mismatch. For the intended repeatability this might be
> > something to consider.
> >
> > Cheers,
> > Magnus
> >
> >
>
> Since the oscillator is intended for oscilloscope scope calibration the
> leveling detector can operate with a fixed input level and an attenuator
> can be used to set the output level.
> A resistive splitter will have a wider operating frequency range than
> most alternatives.
>
> Diode detectors using a pair of matched diodes can be very stable.
> NIST once used an elaborate coaxial dual diode differential RF detector
> arrangement complete with temperature stabilisation.
>
> Bruce

What about some of the "log" detector made by Analog Devices ?

Bill....WB6BNQ

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