On Tue, 13 Jan 2015 18:28:39 -0700, Jim Thompson
wrote:

On Tue, 13 Jan 2015 19:18:37 -0500, Phil Hobbs
wrote:

On 1/13/2015 3:17 PM, Jim Thompson wrote:
Anyone have ideas regarding how to implement a voltage-variable phase
shifter useable at 210MHz?

...Jim Thompson


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 GND


You 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

That's what the customer is currently using except for a capacitance
multiplier made with an AD834(?)

...Jim Thompson

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

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com