Ramsey Electronics AF1 User Manual

E - Z CW

AUDIO FILTER

Ramsey Electronics Model No. AF1

Here’s a quick and easy way to eliminate interfering CW
signals! This filter uses a digital bandpass filter to knock out
those unwanted signals. The adjustable center frequency and
switchable bandwidth add some versatility to this hard working
kit!

• Utilizes “state of the art” switched capacitor bandpass filter IC’s !

• Four selectable bandwidths - 750 Hz, 500 Hz, 250 Hz, and 100 Hz -

all with digital accuracy.

• Adjustable filter center frequency for convenient listening.

• Overload protected input accepts headphone or speaker level

audio.

• “Smart” power input allows for AC or DC operation.

• Audio bypass when unit is switched off - no need to disconnect

unit when not in use.

• Speaker and headphone level outputs with adjustable volume. No

need for an external audio amp.

AF1 • 1

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KIT NAME KIT INSTRUCTION MANUAL

Ramsey Electronics publication No. MAF1 Revision 1.1

First printing: October, 1994

COPYRIGHT

14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.



1994 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York

AF1 • 2

Ramsey Publication No. MAF1

KIT ASSEMBLY

AND INSTRUCTION MANUAL FOR

AF1 CW

AUDIO FILTER

TABLE OF CONTENTS

Introduction to the AF1 .................. 4

How it works .................................. 5

Parts list ........................................ 6

AF1 Assembly instructions ............ 8

Schematic diagram ..................... 10

Parts Layout diagram .................. 11

Hook-up configurations ................ 13

Troubleshooting ........................... 14

Ramsey kit warranty .................... 15

Price $5.00

AF1 • 3

RAMSEY ELECTRONICS, INC.

590 Fishers Station Drive

Victor, New York 14564

Phone [phone]

Fax [phone]

www.ramseykits.com

INTRODUCTION

Radio men know that the CW signal is the most reliable mode of
transmission when operating on the amateur radio bands. There are,
however, some inherent problems that exist in CW reception.

When listening to a carrier wave, or CW signal, our receiver is simply letting
us hear the difference between the local oscillator inside the radio and the
desired receive frequency. While this is the simplest form of reception (no
demodulation of the signal is required) it is also prone to receiving multiple
signals at one time making the desired carrier quite difficult to copy.

In an attempt to limit the number of carriers heard through the receiver one of
the easiest solutions is to filter the received audio to only allow a single CW
tone to be heard. That's where the Ramsey AF1 filter comes in to help.

AMPLITUDE

0

BANDPASS
FILTER

250

500

750

1000

FREQUENCY (Hz)

1250

DESIRED SIGNAL

1750

1500

UNDESIRED
SIGNAL

The audio filter board creates an audio frequency bandpass filter to remove
the undesired tones. It also has a selectable bandwidth (the width of the
bandpass filter) of 100, 250, 500, or 750 Hz. The center frequency of this
filter is also tunable from 400 - 1000 Hz, so you can tune the radio to
frequency that you would like to hear, not to whatever frequency the filter
wants to “listen to”.

When designing the new Ramsey SX series of HF frequency transceivers,
our engineers developed a superior audio filter using the latest in switched
capacitance filter technology integrated circuits. In fact, the filter performed
so well that our AF1 kit is a "spin-off" from the HF radio project.

AF1 • 4

THEORY OF OPERATION

Let’s have a look at what makes our audio filter so special. Have a look at the
schematic diagram and follow along.

First, having a look at the power supply section of the AF1 notice that diodes
D3, 6, 7,and 8 form a full wave bridge rectifier. We used this configuration so
that an AC or DC power source may be used for the unit (and the polarity
connection for a DC supply can be either way). This “raw” voltage is smoothed
by several large capacitors and routed to the voltage regulator IC to provide a
crystal clear source of DC voltage for the filter IC’s. You’ll also notice a few
bypass capacitors on the supply in case any of those nasty RF signals try to
get in and mess up the power supply.

At the heart of our kit is a pair of the MF8 switched capacitor bandpass filter
IC’s. Without these little marvels of technology this kit would not even exist!
Included on each chip is a pair of bandpass filters which can be cascaded to
provide sharper filter characteristics, a “Q LOGIC” binary input to set the filter
bandwidth, an adjustable internal oscillator to provide the center frequency for
our filter, and an extra op-amp to boot! Even the usually stingy data book
devotes 16 pages to the design possibilities for this hard working IC. The
overall bandwidth is controlled using switches S2 and S3 to provide a binary
input at the B, C, and D inputs of the MF8 IC. Resistors R12 and R13, along
with capacitor C8 provide the RC timing component necessary for the CMOS
oscillator.

The rest of the circuit is pretty straight forward - with whatever features could
be built in. The power switch doubles as an audio bypass, or feed through
when in the “OFF” position. Input protection diodes D1 and 2 limit the audio
input voltage to protect the valuable filter IC’s. Headphone jack J2 is set up to
accommodate either stereo or mono headsets, and headphone use will switch
the external speaker output off when the jack is inserted.

The headset / speaker output is from the LM380 audio amp IC. This chip
provides about 3 watts of noise free audio with a minimum of external
components.

AF1 • 5