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 _______________________________________________ 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.