Maker Pro
Maker Pro

allpass filter

  • Thread starter Alessandro Esposito
  • Start date
A

Alessandro Esposito

Jan 1, 1970
0
HI need to shift the phase of a signal between 0 to 180°. I usually work
with sinuswave between 10KHk and 100MHz.
I found out it is possbile to build a so called allpass filter to make this:

R1
+----\/\/\-+
| -\ |
R1 | | \ |
+-\/\/\+-|- \ |
Vin ---+ | >--+--- Vout
+-\/\/\+-|+ /
C | | /
| -/
\
/ R2
\
|
GROUND

in which R2 is variable and control the phase delay.

I have to question:

1) I do not have formulas to setup the values of C and Ri... is, for
example, R1 the input impedence of the circuit?
2) Is it possible to use this circuit up to 100MHz? Which opamp could work?

A less important aspect:
I would like to modify this phase via a PC: Do you think is is reliable the
use of a DAC to control the voltage on a FET base to use it instead (or
with) R2?

Thanks
Alessandro
 
A

Alessandro Esposito

Jan 1, 1970
0
R1
+----\/\/\-+
| -\ |
R1 | | \ |
+-\/\/\+-|- \ |
Vin ---+ | >--+--- Vout
+--||--+-|+ /
C | | /
| -/
\
/ R2
\
|
GROUND
 
K

Ken Smith

Jan 1, 1970
0
-=-=-=-=-=-

R1
+----\/\/\-+
| -\ |
R1 | | \ |
+-\/\/\+-|- \ |
Vin ---+ | >--+--- Vout
+--||--+-|+ /
C | | /
| -/
\
/ R2
\
|
GROUND

With an ideal op-amp the phase shift of this circuit is 90 degrees at
1/(2* PI * R2 * C). This is the simplest form of the "all pass" filter.
The phase varies quite slowly with frequency. It takes about 2 decades
of frequency to go through 180 degrees.

The form you drew, inverts the input at DC. If you ignore this inversion,
the phase can be calculated with only a little math. The transfer
function of the circuit is:

(1-X)/(1+X)

Where:
X = j * 2 * PI * F / (R2 * C)

Where:
j is the square root of -1 (AKA i in math class)
PI is the usual 3.14159....
F is the frequency in Hz
 
R

Rene Tschaggelar

Jan 1, 1970
0
Alessandro said:
HI need to shift the phase of a signal between 0 to 180°. I usually work
with sinuswave between 10KHk and 100MHz.
I found out it is possbile to build a so called allpass filter to make this:

R1
+----\/\/\-+
| -\ |
R1 | | \ |
+-\/\/\+-|- \ |
Vin ---+ | >--+--- Vout
+-\/\/\+-|+ /
C | | /
| -/
\
/ R2
\
|
GROUND

in which R2 is variable and control the phase delay.

I have to question:

1) I do not have formulas to setup the values of C and Ri... is, for
example, R1 the input impedence of the circuit?
2) Is it possible to use this circuit up to 100MHz? Which opamp could work?

A less important aspect:
I would like to modify this phase via a PC: Do you think is is reliable the
use of a DAC to control the voltage on a FET base to use it instead (or
with) R2?


A normal multiplying DAC does not handle 100MHz for the reference input.
The fastest one I found was 10MHz.
And yes, there are OpAmps doing 100MHz. Analog Devices have some.


Rene
 
A

Alessandro Esposito

Jan 1, 1970
0
No.... the DAC should change the phase lag in a slower manner than 80MHz....
At the moment I spend 60ms to change the phase between to phase locked
signal generators controlled by PC... if with a simpler circuit and only one
signal generator I can approch the same speed (or go down) is ok :
Thanks
 
Top