[time-nuts] OT: Levelled sine wave generator

[Bruce Griffiths]

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