[time-nuts] OT: Levelled sine wave generator
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Dec 15 11:31:33 UTC 2008
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
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