phase shifter circuit needed

[Patrick]

I'm looking to build to a phase shifter circuit that can supply about 60
degrees of phase shift at 70 MHz (bandwidth not important). The phase shift
needs to be adjustable in real time, ie, with a variable R or C.

The catch is that the input and output of the network needs to remain
matched to 50 ohms.

Can this even be done with purely passive components?

(Specifically, what I have is a transmission line that needs to provide a
180-degree shift from the start of the line to the end. The problem is we
have no way to precisely measure the length of the TL. So we need a way to
adjust the "length" of the line in real time. Now, I'm aware of the
trombone method, wherein a piece of TL is slid in and out to provide a
variable lenght. However, this is a mechanical solution and not suited for
the application; ergo I need a way simulate this behaviour)

Thanks very kindly

 

[Ban]

Patrick wrote:

I'm looking to build to a phase shifter circuit that can supply about
60 degrees of phase shift at 70 MHz (bandwidth not important). The
phase shift needs to be adjustable in real time, ie, with a variable
R or C.

The catch is that the input and output of the network needs to remain
matched to 50 ohms.

Can this even be done with purely passive components?

(Specifically, what I have is a transmission line that needs to
provide a 180-degree shift from the start of the line to the end.
The problem is we have no way to precisely measure the length of the
TL. So we need a way to adjust the "length" of the line in real
time. Now, I'm aware of the trombone method, wherein a piece of TL
is slid in and out to provide a variable lenght. However, this is a
mechanical solution and not suited for the application; ergo I need a
way simulate this behaviour)

Thanks very kindly

It depends a lot on your signal, if it needs to be broadband, you can use an
allpass filter which can be made variable.
This allpass will delay from 0.1 to 1ns, if you need more range you can
cascade more of them. You will have to tweak the values a bit, that you get
an even gain for all frequencies, because of the finite gain of that opamp.

0...10p
___ ||/
.-|___|--o----||-.
| 56 | /|| |
| | |\ |
| '-|+\ | ___
o---o | >--o--|___|-o
| .-|-/ | 47
| | |/ |
| ___ | ___ |
'-|___|--o-|___|-'
470 470
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
You can not easily make a passive version, at least not with a single
adjustable element. It will be also better to use a true differential
amplifier with a balanced filter in this range, but then U can maybe make a
dual adjustable capacitor with some tuning diodes

--
ciao Ban
Bordighera, Italy

 

[Jamie]

Patrick wrote:

I'm looking to build to a phase shifter circuit that can supply about 60
degrees of phase shift at 70 MHz (bandwidth not important). The phase shift
needs to be adjustable in real time, ie, with a variable R or C.

The catch is that the input and output of the network needs to remain
matched to 50 ohms.

Can this even be done with purely passive components?

(Specifically, what I have is a transmission line that needs to provide a
180-degree shift from the start of the line to the end. The problem is we
have no way to precisely measure the length of the TL. So we need a way to
adjust the "length" of the line in real time. Now, I'm aware of the
trombone method, wherein a piece of TL is slid in and out to provide a
variable lenght. However, this is a mechanical solution and not suited for
the application; ergo I need a way simulate this behaviour)

Thanks very kindly


a cap in series to electrically extend one of the lines ?

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5