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ZXF36L01

Variable Q filter

Zetex => Diodes 

ZXF36L01

FILTERING HIGHER FREQUENCIES USING

THE MIXER

Frequencies above 150 kHz cannot be filtered directly;

the mixer enables the notch pass filter to function up to

700kHz.

The signal to be filtered is mixed with another

frequency (local oscillator), chosen so that the

difference (intermediate) frequency equals the filter’s

centre frequency, fo. The local oscillator signal

waveform can be of any shape (sine, square, etc.) but

must be approximately 50% duty cycle.

Example

Input frequency = 300 kHz, Local Oscillator (LO)

frequency = 250 kHz,

Output (IF) Frequency = 50 kHz.

If the bandwidth of the 50 kHz filter were 1 kHz, the filter’s Q

factor would be:

MIXER CONFIGURATION WITH NOTCH PASS FILTER

(with attenuating skirts)

The mixer can only be used with this filter configuration, as

the other types have 0dB stop bands. The mixer output

‘MXO’ becomes the input of the filter.

As the gain of the notch filter changes with Q, the output of

the mixer must be attenuated by some factor (VRAtten). This

will prevent the filter from being overdriven and allows the

user to set the required output level.

Note: As the local oscillator input, LO has a low input

impedance (60 Ω), it will often be necessary to increase it

for driving circuitry. As the input voltage required is low

(around 5 mV pk-pk min.), a series resistor ‘RMixer’ can be

inserted. A value of 1 kΩ per 100mV (pk) oscillator signal

input will be suitable.

50/1 = 50.

The bandwidth of the filter is still 1 kHz when 300 kHz is

applied to the mixer’s input, but now the Q factor is:

300/1 = 300.

The mixer provides a Q factor improvement equal to the

ratio of the input frequency and the intermediate frequency.

The effective centre frequency can also be externally

controlled by changing the LO frequency. This allows

frequency tuning, trimming or sweeping while employing

fixed resistors and capacitors for the filter.

As the LO signal can be a square wave, this allows ‘fo’ to be

controlled using a microcontroller or microprocessor.

1

VRAtten

C

R

R

C

Ri

Rf

VSS

FI1

C1

RC1

R2

BI

MXO

RC2

GP1

GP2

GP3

VSS

5V

VDD

FI2

FO

MXI

LO

BI

BG

N/C

N/C

N/C

PD

VDD

24

100nF

100nF

R Mixer

Output Signal

Input Signal

Oscillator Input (LO)

100nF

5V

100nF

ISSUE 3 - JANUARY 2002

9

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