## Filter notes. | |

- Introduction
- Transforming cascaded 1.order filters to 2.order filter and vice versa
- Unity gain Sallen-Key bandpass filter with identical capacitor values
- Amplitude response of RIAA network
- Amplitude response of 1.order high-pass filter
- Amplitude response of 1.order high-pass filter with zero
- Amplitude response of 1.order low-pass filter
- Amplitude response of 1.order low-pass filter with zero
- Amplitude response of pink noise filter
- IEC 60268-1 filter
- Peaking frequency and amplitude for 2.order LP and HP filters
- References
- Copyright and disclaimer

This is some notes on different filters.

They are in no particular order.

Some of the calculations can be downloaded as a spread-sheet here.

f1 and f2 are the 2 1.order filters.

Fig.1: Sallen Key bandpass filter [1].

Select f, Q, C and R4.

Fig.2: RIAA filter frequency response.

t1 is a low frequency zero.

t2 is an optional IEC time constant [ 7950 µs ]

t3 is a RIAA time constant [ 3180 µs ]

t4 is a RIAA time constant [ 318 µs ]

t5 is a RIAA time constant [ 75 µs ]

t6 is a high frequency zero

K is the low frequency gain of the filter. The 1 kHz gain is 20 dB lower.

f is the frequency [ Hz ]

If you do not want t1, set it to a very high value ( or remove it from the equation ).

If you do not want t2, set t1 and t2 to a very high value ( or remove them from the equation ).

If you do not want t6, set it to zero.

f0 is the filter cut-off frequency [ Hz ]

K is a gain factor [ V/V ]

f0 is the filter cut-off frequency [ Hz ]

fz is the filter zero frequency { Hz ]

K is a gain factor [ V/V ]

f0 is the filter cut-off frequency [ Hz ]

K is a gain factor [ V/V ]

f0 is the filter cut-off frequency [ Hz ]

fz is the filter zero frequency { Hz ]

K is a gain factor [ V/V ]

f0 is the filter cut-off frequency [ Hz ]

f0 must be much lower than any frequency of interest.

K is a gain factor [ V/V ]

It is normally used in series with a pink-noise filter for shaping a white-noise signal.

Fig.3: IEC 60268-1 filter.

R31 and C31 is a 700 kHz roll-off for the input. They are not part of the IEC 60268 filter.

The input buffer is required as the filter must be driven from a low impedance.

C32, C33, R32, R33 is a 2.order Sallen-Key high-pass filter.

C34, C35, C36, R34, R35, R36 is a 3.order Sallen-Key low-pass filter.

As the roll-off frequencies for the HP and LP filter are far apart, they can share the same OP-AMP ( with some slight component value adjustment ).

Fig.4: IEC 60268-1 filter 500 runs Monte Carlo analysis.

The red traces are the limits from the specification and the lime traces is the simulation result.

The upper plots use the left axis and the lower plots the right.

This simulation was done with OPA134 OP-AMPs.

The upper roll-off is very close to the limits, so for some applications I might replace C34..C36 with 1% types.

Avpeak is the peak gain [ V/V ]

flp is the peak frequency for a low-pass filter [ Hz ]

fhp is the peak frequency for a high-pass filter [ Hz ]

[1] | R. P. Sallen and E. L. Key, “A practical method of designing RC active filters" You may be lucky to find a copy of this in your local technical library. |

[2] | UL Standard for Safety for Audio, Video and Similar Electronic Apparatus - Safety Requirements, UL 60065. |

Copyright Notice.

This web-page, including but not limited to all text, drawings and photos, is the intellectual property of Poul Petersen, and is Copyright ©.

Reproduction or re-publication by any means whatsoever is strictly prohibited under International Copyright laws.

The author grants the reader the right to use this information for personal use only.

Any commercial use is prohibited without express written authorization from Poul Petersen.

The information is provided on an "as-is" basis and is believed to be correct, however any use of the information is your own responsibility.

This web-site may contain links to web-sites outside Poul Petersen domain ( www.poulpetersen.dk ).

Poul Petersen has no control over and assumes no responsibility for the content of any web-site outside Poul Petersen own domain.

Poul Petersen does not use cookies to "enhance your experience" on this website.

Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.

E-mail: pp@poulpetersen.dk

Copyright © Poul Petersen 2019. Last update: 20191029. | Valid HTML! |