Prev: Algorithm and voltage monitoring
Next: Back on topic: Penne and Pork Meatballs Arrabiata
From: john1987 on 26 Jul 2010 19:44
Hi,


I need to introduce a 90 degree phase shift using a RC low pass
filter. The input single frequency sinusodial signal is 200KHz with
2.5volts peak to peak.

I know that at F3db will introduce 45 degree shift and 10F3db will
introduce 90 degree shift. I need the 90 shift to detect the peak of
the circuit. I know that the formula is

f = 1 / ( 2*pi* R* C)

10f = 1 / ( 2*pi* R* C)

Usually, R <= Rload / 10. but the problem is that the load is input
impedance of an op amp. Can anyone advice me that how to choose the
value of R?
Thanks
John
From: Tim Wescott on 26 Jul 2010 20:06
On 07/26/2010 04:44 PM, john1987 wrote:
> Hi,
>
>
> I need to introduce a 90 degree phase shift using a RC low pass
> filter. The input single frequency sinusodial signal is 200KHz with
> 2.5volts peak to peak.
>
> I know that at F3db will introduce 45 degree shift and 10F3db will
> introduce 90 degree shift. I need the 90 shift to detect the peak of
> the circuit. I know that the formula is
>
> f = 1 / ( 2*pi* R* C)
>
> 10f = 1 / ( 2*pi* R* C)
>
> Usually, R<= Rload / 10. but the problem is that the load is input
> impedance of an op amp. Can anyone advice me that how to choose the
> value of R?

10x the 3dB frequency won't give you 90 degrees; it'll give you 84
degrees, and a ton of attenuation. You'll never get to 90 degrees with
a single stage RC, and the further you go up in frequency the worse the
attenuation will get.

Cascade an RC and you'll get 90 degrees at f = 1/(2*pi*R*C), with a
signal strength that's only been attenuated 3:1. Put that into the +
end of a voltage buffer or positive-gain op-amp and you'll be fine.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
From: john1987 on 26 Jul 2010 20:09
Hi,

Did you mean Cascade two RC circuits together one after another?
please explain !


Thanks

John

From: Tim Wescott on 26 Jul 2010 20:16
On 07/26/2010 05:09 PM, john1987 wrote:
> Hi,
>
> Did you mean Cascade two RC circuits together one after another?
> please explain !
>

Vin ___ ___ Vout
o-----|___|---o---|___|----o------o
| |
| |
--- ---
--- ---
| |
| |
=== ===
GND GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
From: Jim Thompson on 26 Jul 2010 20:20
On Mon, 26 Jul 2010 17:06:13 -0700, Tim Wescott <tim(a)seemywebsite.com>
wrote:

>On 07/26/2010 04:44 PM, john1987 wrote:
>> Hi,
>>
>>
>> I need to introduce a 90 degree phase shift using a RC low pass
>> filter. The input single frequency sinusodial signal is 200KHz with
>> 2.5volts peak to peak.
>>
>> I know that at F3db will introduce 45 degree shift and 10F3db will
>> introduce 90 degree shift. I need the 90 shift to detect the peak of
>> the circuit. I know that the formula is
>>
>> f = 1 / ( 2*pi* R* C)
>>
>> 10f = 1 / ( 2*pi* R* C)
>>
>> Usually, R<= Rload / 10. but the problem is that the load is input
>> impedance of an op amp. Can anyone advice me that how to choose the
>> value of R?
>
>10x the 3dB frequency won't give you 90 degrees; it'll give you 84
>degrees, and a ton of attenuation. You'll never get to 90 degrees with
>a single stage RC, and the further you go up in frequency the worse the
>attenuation will get.
>
>Cascade an RC and you'll get 90 degrees at f = 1/(2*pi*R*C), with a
>signal strength that's only been attenuated 3:1. Put that into the +
>end of a voltage buffer or positive-gain op-amp and you'll be fine.

Or use an all-pass. Then it's easy to get 90�

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Spice is like a sports car...
Only as good as the person behind the wheel.
 |  Next  |  Last
Pages: 1 2 3 4 5
Prev: Algorithm and voltage monitoring
Next: Back on topic: Penne and Pork Meatballs Arrabiata