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