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 ' _______________________________________________ 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.