MFJ MFJ-224 User Manual

MFJ-224 Instruction Manual 2-Meter FM Analyzer

Introduction

Congratulations on purchasing the MFJ-224 2-Meter FM Analyzer. With this versatile handheld instrument, you can tune in any signal between 143.5 and 148.5 MHz to monitor field strength in dBm and FM-deviation in kHz. Accurate tuning is made simple, thanks to the built-in discriminator-meter function. Battery status is also displayed. In addition to metered functions, the MFJ-224 lets you visually analyze modulation waveforms and measure instantaneous-peak deviation by plugging into an oscilloscope. Finally, a headphone monitor circuit helps you tune in and identify signals easily. Before exploring the MFJ-224's many uses, please take a few minutes to read this manual. A special orientation section is provided to help you get started.

Technical Specifications

Frequency Coverage.................... 143.5 - 148.5 MHz

Selectivity.................................... -6 dB @ 20 kHz

Receiver Type.............................. Dual Conversion, 10.7 MHz 1st IF, 455 kHz 2nd IF

Oscilloscope Output .................... 1 V p-p for 1-kHz tone at 5-kHz deviation

Phone Jack Output....................... Lo-Z, preset volume level

RSSI Range ................................. -100 dBm to -40 dBm (60-dB range)

Deviation Range......................... 0-7 kHz on Meter, 0-20 kHz on scope (1-kHz tone).

Discriminator Meter Range......... +/- 3 kHz, Zero-centered

Operating Voltage ....................... 6.5 - 9.0 Volts DC

Power Source............................... 9-Volt rectangular alkaline battery

Frequency

MFJ-224

Signal

Discriminator

Deviation

Battery

Power

Meter

Monitor

Controls and Jacks

1. Power Off/On Switch

2. Power "On" LED

3. Oscilloscope Output (RCA)

4. Headphone Jack (3.5mm)

5. Headphone Monitor "on" switch

6. Meter Function Switch

7. Tuning Knob--Frequency

8. Meter, 3" Precision Movement

9. Antenna Connector (SO-238)

MFJ-224 Instruction Manual 2-Meter FM Analyzer

What Your Meter Can Do

The MFJ-224 performs countless jobs around the shop or radio shack. Here are some uses we think you'll appreciate:

•••• Evaluate Antenna Performance: Measure key yagi specifications such as

dipole

(dBd),

beamwidth

(-3dB points),

front-to-back ratio

sidelobe suppression

, and

gain over a

. You can compare antennas to see which works best, and check the real-world performance of experimental antennas against NEC-based computer predictions.

•••• Detect Feedline Faults: Document end-to-end cable loss in dB. Find out if kinks,

corrosion, and moisture have deteriorated performance--or discover if a new feedline meets factory specs. See how much signal is actually lost between the antenna and radio.

•••• Map Repeater Field-strength: Plot repeater or packet-node field strength in dBm or

microvolts throughout the coverage area. Find out where the signal is going--and where it isn't. Evaluate the impact of site changes with accuracy.

•••• Site your Antennas: Take the MFJ-224 to the mountain, on the roof, or up the tower

to position antennas for best performance (a few feet either way can make a big difference over difficult paths). Aim yagis with absolute pin-point accuracy.

•••• Measure Preamp Gain: Tune preamps for best gain and noise figure using your

meter, a scope, and a weak signal source. Measures

exact gain

in dB.

•••• Fox Hunt: Track down hidden transmitters--or nail jammers fast! High resolution 60-

dB RSSI display is amateur radio's most accurate S-meter.

•••• Check and Set Deviation: Measure transmitter deviation anywhere in the band. Use

the built-in meter display with a test-tone, or plug into a scope for accurate instantaneous-peak readings on speech, packet, DTMF, and CTCSS tones. Help your fellow hams--the MFJ-224 can measure the deviation of any signal you can hear off-air!

•••• Analyze Audio Quality: Use the oscilloscope output to visually evaluate the quality of

speech or tones by viewing the audio waveform. See if tones are clipped or distorted, if the speech limiter is working improperly, etc. Solve FM audio problems fast!

•••• Scan the Band: Tune in and identify signals using monaural or stereo Lo-Z

headphones. Check speech quality of your radios or use as a second receiver to monitor activity.

•••• Tune Transmitters and Filters: Use with RF-sniffer probe to tune low-power

transmitter stages. Tune high-Q filters and networks for best response, lowest loss.

MFJ-224 Instruction Manual 2-Meter FM Analyzer

How Your Meter Works

A Motorola FM-receiver IC with logarithmic RSSI metering circuitry measures signal strength over a 60-dB dynamic range. This allows input levels from -100 dBm to -40 dBm to be displayed on a linear meter scale with 1-dB resolution. In addition to providing accurate RSSI, the IC features a built-in tunable oscillator--plus outputs to drive the FM-deviation detector, headphone monitor circuit, and a discriminator tuning meter. The block diagram below illustrates how the MFJ-224 is organized:

Op Amp

Audio

Monitor

RSSI

Disc

Dev

Detect

Bat Status

ScopePhones

Pwr

Meter

Display

Ant

1st Mxr

1st LO

Receiver IC

Vcc

Reg

Osc

Tune

Detailed operating instructions will help you understand how each feature works--and how to get the most from your meter..

Battery Installation

Before using your meter for the first time, you must install a fresh 9-volt battery. The MFJ-224 draws about 20 mA when in use, and can operate for many hours between battery changes. However, we suggest you check

battery status

each time you turn the unit on. The MFJ-224 is voltage regulated, and battery voltage may drop as low as 7volts before operation becomes erratic. When purchasing a battery, be sure to select a premium-quality alkaline type--such as Duracell MN1604, Eveready 522, Ray-O-Vac A1604, or Radio Shack 23-553.

Begin installation by removing the mounting screws from both side panels of the meter case. Gently separate the front and back sections, taking care not to pull on the antenna lead or battery wires. If replacing a spent alkaline battery, dispose of it in a prescribed manner. To install the new battery, snap on the 9-volt connector and press the case

firmly

into the retainer clip (make sure it can't pop loose). Now, re-assemble the case.

IMPORTANT NOTE: To protect your unit from damage due to battery leakage, remove the battery when storing for prolonged periods.

MFJ-224 Instruction Manual 2-Meter FM Analyzer

Orientation--Hooking Up Your Meter

To become familiar with the various features and functions of your MFJ-224, connect it up as shown below (if you lack an oscilloscope, omit that portion of the orientation).

1. Headphones: You'll need a pair of low-impedance headphones outfitted with a 3.5 mm (1/8") plug. Without phones, you will be unable to identify signals and tune them in properly. The headphone circuit is configured to work with either stereo or monaural plugs and wiring, but a pair of good-quality stereo phones work best. Headphone audio level is pre-set to a low-but-comfortable listening level.

2. Antenna: The MFJ-224 accepts any 50Ω source outfitted with a UHF connector. Avoid directly-connecting RF levels exceeding -20 dBm (.03 volts) to the meter. To sample stronger sources, use a pick-up probe or resistive attenuator. For the purpose of this orientation, connect a 2-Meter station antenna to monitor off-air signals.

IMPORTANT NOTE: Your MFJ-224 is a sensitive instrument. To avoid damage, never connect a transmitter or powerful RF oscillator directly to the antenna jack.

3. Oscilloscope: Connecting a scope to the MFJ-224 enhances its value as a deviation meter and audio analyzer. Since only audio frequencies are monitored, any generalpurpose scope will have sufficient frequency response. However, a well-calibrated unit will yield more accurate deviation readings. Connect your meter to the scope's verticalamplifier input using shielded cable. To begin, set the sweep for .5 mS and vertical gain for 0.2 volts per division. A 1-kHz test-tone modulating a FM transmitter to 5-kHz of deviation normally produces a sine-wave output of about 1.0 volts p-p.

2-Meter Antenna

Oscilloscope

0.2V

Vert

Antenna

Headphones

MFJ-224

Scope Phones

You are now ready to explore the basic features and functions of the MFJ-224.

MFJ-224 Instruction Manual 2-Meter FM Analyzer

RSSI

MFJ-224 M

Orientation--Learning To Use Your Meter

1. Power On: To power-up your unit, depress the

power

switch

The green pilot LED

should illuminate.

2. Battery Status: To check battery condition, turn the meter selector switch to and note the indication. The meter should swing into the

good

zone of the scale.

battery

3. Headphone Monitor Switch: To monitor signals, plug in headphones and depress

monitor

the

switch. You should hear a strong background hiss. This is FM-receiver

noise.

4. Tuning: The

Frequency

(variable frequency oscillator) control tunes the meter's FM receiver. Use the vernier-reduction tuning knob to scan across the band for active repeaters in your area. These will provide sample signals for you to analyze.

- dBm

Dev - kHz

-100

-90

eter Face

RSSI - dBm

-70

-80

Dev - kHz

Discr

-60

-50

-40

Bat

Field-strength scale displays level of

incoming signal.

Deviation scale displays FM-carrier swing during

modulation of signal.

Battery

Displays battery voltage, indicates if battery is okay.

Discriminator

Tuning aid--meter centers when

signal is tuned in correctly.

5. Discriminator Meter Function: Turn the meter selector to

discriminator

. This function is a tuning aid to help you zero-in signals before making measurements. Accurate tuning is important because readings taken while the receiver is mistune-tuned may be incorrect. The discriminator meter normally hovers around center-scale with no station present--then deflects sharply as you tune across a signal. Practice tuning stations

center-scale meter reading

for a

. The indicator is

very

sensitive, and tuning "dead on"

may take some practice!

6. Signal Meter Function: The

Signal

RSSI (Recovered Signal Strength Indicator)

, or function, displays incoming signal strength. The meter scale is calibrated in dBm, a unit of RF power (0 dBm = 1 mW @ 50-Ω). The dBm unit is especially convenient because it may be used for making signal-level comparisons directly in dB. Note that it is normal

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