MEMO

By Gary Goodman from HP document collection. Dated 24/3/85

The required count is calculated from Φ2 and the audio frequency required.

Count (decimal)

{ Φ₂ / ( 2 x F ) } - 2

for 1000Hz and a 1MHz Φ₂ :

Count = { 1,000,000 / ( 2 X 1000 ) } - 2 = 498

498d → $01F2

Run as XEQALPHATIALPHA

Prompt F for required audio tone frequency.

Enter freq

R/S

Result is the 2-byte value for T1CL & T1CH

e.g.

XEQALPHATIALPHA

1000 

R/S

Result:

$001F2

The Counter should be set to

T1CH = $01
T1CL = $F2

Entered as follows (make sure you're in DECS mode in Ladybug while entering the program - otherwise the 1000000 won't be interpreted correctly). When viewing/editing the program later it will look different!

LBL ¬TI
DECS
WSIZE 20
¬ F
PROMPT
STI 01
1000000
ENTERI
LDI 01
2
x
/
2
-
HEXS
RTN

Butterworth HPF/LPF Filter designer

LBL ¬BFIL
SF 00
¬MHZ ?
PROMPT
1 E6
*
STO 11
¬N
PROMPT
x<0?
CF 00
STO 10
XEQ 06
LBL 01
RCL 12
INT
XEQ 00
FS? 00
1/x
STO IND 12
ISG 12
GTO 01
¬COEFFICIENTS
FIX 3
STOP
ENG 3
XEQ 06
LBL 02
RCL 12
INT
RCL IND 12
RCL 11
2
*
PI
*
/
STO IND 12
ISG 12
GTO 02
XEQ 06
LBL 03
RCL 12
INT
2
/
FRC
FS? 00
GTO 09
GTO 10
LBL 09
¬HPF
x=0?
GTO 05
GTO 04
RTN
LBL 10
¬LPF
x=0?
GTO 04
GTO 05
RTN
LBL 04
RCL IND 12
50
*
STO IND 12
ISG 12
GTO 03
RTN
LBL 05
RCL IND 12
50
/
STO IND 12
ISG 12
GTO 03
RTN
LBL 06
RCL 10
ABS
1000
/
1
+
STO 12
RTN
LBL 00
2
*
1
-
180
*
RCL 10
ABS
2
*
/
SIN
2
*
RTN
END

XEQALPHABFILALPHA

MHZ ? 

5 R/S

N

Order/poles = N

Positive for HPF

Negative for LPF

5 pole LPF = -5

5CHSR/S

COEFFICIENTS
0.618

(at this point you can RCL 01 etc. up to RCL 05 to see the calculated Butterworth Coefficients)

• R01 : 0.618
• R02 : 1.618
• R03 : 2.000
• R04 : 1.618
• R05 : 0.618

R/S

LPF
393.5 -12

The calculated component values are now in Reg 01 - Reg 05

• R01 : 393.5pf
• R02 : 2.575uH
• R03 : 1.273nF
• R04 : 2.575uH
• R05 : 393.5pF

XEQALPHABFILALPHA

MHZ ? 

1 R/S

N

Order/poles = N

Positive for HPF

Negative for LPF

7 pole HPF = 7

7R/S

COEFFICIENTS
2.247

(at this point you can RCL 01 etc. up to RCL 07 to see the calculated Butterworth Coefficients)

• R01 : 2.247
• R02 : 0.802
• R03 : 0.555
• R04 : 0.500
• R05 : 0.555
• R06 : 0.802
• R07 : 2.247

R/S

HPF
17.88 -06

The calculated component values are now in Reg 01 - Reg 07

• R01 : 17.88uH
• R02 : 2.553nF
• R03 : 4.416uH
• R04 : 1.592nF
• R05 : 4.416uH
• R06 : 2.553nF
• R07 : 17.88uH

Page created Thu May 26 18:35:00 2022 by John Pumford-Green

Page last updated: 06/03/25 06:49 GMT

— John Pumford-Green 24/04/23 16:07