[time-nuts] OT: Levelled sine wave generator

[Bruce Griffiths]

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
>
>   
Detector reference:

http://ts.nist.gov/MeasurementServices/Calibrations/upload/78-871.PDF


Bruce