[time-nuts] OT: Levelled sine wave generator

WB6BNQ wb6bnq at cox.net
Mon Dec 15 12:02:43 UTC 2008


Bruce Griffiths wrote:

> WB6BNQ wrote:
> > 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
> >
> >
> Their operating frequency range isnt large enough.
> If the frequency response extends to dc the upper limit is less than 1GHz.
> When the frequency response extends to 1GHz or more the lower limit isnt
> low enough.
> The Analog Devices tru-power rms detectors are similarly afflicted.
>
> Bruce
>

Good point !  Perhaps a combination could be dreamed up where two such detectors could
be used with some kind of cross-over scheme derived from the frequency generator.
This way optimization could be performed with two or more such detectors as I think
the cost per detector may not be that much.  A thought anyway.

Bill....WB6BNQ

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