Anyone have ideas regarding how to implement a voltage-variable phase shifter useable at 210MHz? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Do you remember the MXR Phase 90? It was a guitar effects pedal that used a chain of op amp all-pass filters:
as a phase shift network and a JFET as a voltage variable resistance in parallel with the resistor to ground to vary the phase.
Could one build something similar just for a higher frequency, maybe? 500 MHz GBW op amps are not terribly expensive these days; use PHEMTs as the variable resistance elements?
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Buffer the RF input, and delay it (only takes a few inches of transmission line) to generate a 90 degree phase-shift. Then, wiggle two transistor bases with both those signals. Current-modulate the differential emitter current with your voltage input. Sum the collector currents, of course.
Do you want to buy it as a part, or build it into one of your IC designs? How much phase shift do you need?
There are tons of analog IQ modulator chips around, some with a built-in 90 degree phase shifter. They can do full the 360 degrees if you apply sin/cos multiplier inputs.
Small phase shifts can be done with a varicap RC or LC network. Or an RC or LC shifter and a multiplier or two.
Extreme: an inductor-varicap variable-delay transmission line. The HP8133A had several, insanely long ones.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
I've used a variation of the normal op amp all-pass filter for this, except with a high frequency diff amp:
*---------------* | | 0-----*-----RRRR-------*----| | | | | | | R | Diff amp | | Gain of 0.5 R | | | R | *-----Out | | | | | | | | | GND | | | | | *-----RRRR--*----*----| | | | | | | | *---------------* | L | L | L | L L | L | L | L --- Varactor | ^ | | | *---RRRR----0 V_phi | | | | CCC | CCC big C | | GND GNDYou pick the voltage divider to have a gain of 0.5; the series L resonates the varactor just past the low-voltage end of V_phi; and the shunt inductor resonates the series combination just past the high end of V_phi. You can get about 150 degrees in one stage, with linearity of +- a few degrees. Good medicine.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
It would be helpful if you indicated whether your input signal was a constant sinusoid, square wave or some arbitrary signal (in which case it would also be helpful if you indicated the signal bandwidth, as this makes a difference). It would also be useful to know whether the phase shift should be proportional to the voltage, or the time delay through the circuit should be proportional to the voltage, or whatever the control function should be. Also the linearity of the control input would be an important spec. as well as any requirement on the phase noise introduced (e.g. if there is noise added to the phase control signal).
That's what the customer is currently using except for a capacitance multiplier made with an AD834(?) ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Crikey! Why didn't I think of that... marvelous! Thanks for the pointer!
...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
For small phase shifts, you could bias a jfet or something, with a d-g feedback cap, and vary Vgs (ie, transconductance) to change the phase shift. Dirt cheap. An inductor somewhere might be interesting.
People used to make FM modulators like that, with toobs. Pre-emphasis made phase shift into frequency shift. Reactance tube modulator, something like that.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers
My go-to toob book, "Electron Tube Circuits", by Samuel Seeley, second edition 1958
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My go to toob book, "Electron Tube Circuits", by Samuel Seeley, second edition 1958, has a section on reactance tube modulators:
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Sorry, something weird going on with my Android newsgroup reader. Anyway, I was going to say that the book has a section on reactance tube modulators:
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feedback cap, and vary Vgs (ie,
shift. Dirt cheap. An inductor somewhere
Followed by some gross amount of frequency multiplication to get the modulation index up where it needs to be for wideband FM. IIRC it was the noise and instability from multiplication that made people abandon that approach back in the day.
Cheers
Phil Hobbs
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