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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.