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MFJ-224 Instruction Manual
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
MFJ-224
Discriminator
2
1
Power
3
9
8
-
+
Frequency
7
Meter
Signal
6
.
Deviation
Battery
5
Monitor
4
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)
2-Meter FM Analyzer
1
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Summary of Contents for MFJ MFJ-224
- Page 1 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.
- Page 2 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 • • • •...
- Page 3 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 7- volts 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 A- 1604, or Radio Shack 23-553.
- Page 4 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.
- Page 5 MFJ-224 Instruction Manual 2-Meter FM Analyzer 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 battery and note the indication.
- Page 6 MFJ-224 Instruction Manual 2-Meter FM Analyzer for the meter to read below -100 dBm with no signal present. Check the repeater signals in your area and note the level differences between them in dB!
- Page 7 MFJ-224 Instruction Manual 2-Meter FM Analyzer 7. Deviation Function: Use the deviation function to check transmitter modulation. The 0-7 kHz scale on the meter is accurate only when a continuous audio or packet calibration tone is applied to the transmitter under test. However, you can get a pretty good idea who's over-deviating and who's not by watching meter movement on normal speech.
- Page 8 MFJ-224 Instruction Manual 2-Meter FM Analyzer 3. Comparing Antenna Performance: The Signal display is useful for comparing two or more antennas for best performance. In most cases, a local repeater may be used as a signal source. These results can be extremely accurate, but only if steps are taken to prevent error.
- Page 9 MFJ-224 Instruction Manual 2-Meter FM Analyzer Sidelobe Suppression: Sidelobes are "peaks" in antenna response that occur off the side of a directional antenna. A well-designed yagi will have a clean well-defined pattern with only a few weak sidelobes. However, a poorly-designed yagi may have several strong peaks off to the side.
- Page 10 When adjusting FM transceivers, study all relevant service documents carefully and follow the manufacturer's instructions closely. The MFJ-224 is a sensitive instrument and you must take care not to overload it during transmitter deviation tests. To limit ambient RF levels in the test area, terminate the transmitter under test with a shielded 50Ω...
- Page 11 3 (Packet Calibration Tone) 5 (1-kHz tone or Peak Speech) It should be noted that this chart provides p-p voltage levels generated by a typical MFJ- 224 scope-monitor circuit, and minor variations may occur from unit to unit. If you monitor deviation often, it may prove helpful to make up a clear-plastic overlay for your CRT screen calibrated for direct readings in kHz.
- Page 12 In this case, use the discriminator meter to locate the exact center of the signal. 3. The MFJ-224 receiver is, by design, approximately 20 dB less sensitive than a typical VHF-FM transceiver (this is done to accommodate bench measurements involving strong transmitter signals).
- Page 13 2-Meter FM Analyzer Field Service Guide The MFJ-224 has several serviceable calibration adjustments. If you lack the necessary equipment or expertise to service sensitive VHF communications equipment, please do not attempt these adjustments! Instead, contact a qualified 2-way service technician in your area for assistance or return your unit to the factory for service.
- Page 14 MFJ-224 Instruction Manual 2-Meter FM Analyzer Schematic...
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