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RF Logic Limited.
Unit 18, The Enterprise Centre,
Coxbridge Business Park, FARNHAM,
Surrey GU10 5EH
Tel +44 (0)1252 268340
Email: sales@rflogic.co.uk
web: www.rflogic.co.uk
Model RF257 Automatic Modulation Meter
The model RF257 modulation meter has been designed to simplify the task of modulation
measurement. The model RF257 always locks to the highest level signal available, ignoring
spurious signals and harmonics. AM and FM measurements can be made over the full
frequency range of 1.5MHz to 2.0GHz. The unit operates usefully with reduced sensitivity to
at least 4GHz.
FM measurement of peak positive, peak negative or mean deviation, with 5 deviation
ranges from 1kHz to 100kHz full scale. AM measurement of peak, trough or mean in
percentage modulation with 5 ranges from 1% to 100% full scale. The audio measurement
bandwidth is selectable and the demodulated audio is available at the front panel. The IF is
available on a BNC connector on the rear panel.
The unit is small and lightweight, making it ideal for the bench or field work, especially with
the internal battery option installed.
Automatic Modulation Meter
Model RF257
Service Manual
Electronic Product Design
Page 1
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Summary of Contents for RF Logic RF257
- Page 1 Service Manual Model RF257 Automatic Modulation Meter The model RF257 modulation meter has been designed to simplify the task of modulation measurement. The model RF257 always locks to the highest level signal available, ignoring spurious signals and harmonics. AM and FM measurements can be made over the full frequency range of 1.5MHz to 2.0GHz.
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Page 2: Table Of Contents
Operating Instructions Power Requirements. AC MAINS OPERATION Input Signal. FM Measurement. AM Measurement. Internal Battery Operation (Option 03). Demodulated Audio Output. IF Output. RF257 Specification RF Input FM Measurement AM Measurement Audio Filters Front Panel Rear Panel Power Requirements Environmental... - Page 3 Bessel Zero Method Frequency Difference Method Additional Checks Battery Tests (Option 03) Battery Monitor Parts Lists RF257-03 Complete Unit 900.308 Chassis Module 900.310 Front Panel Module 900.132 Front Panel PCB Assembly 900.303 AF Board Assembly RF115v03 RF Board Assembly Circuit Diagrams...
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Page 4: Operating Instructions
'ON'. The instrument is immediately ready for use; no warm-up time is required. At power on, the RF257 defaults to the FM 100kHz range with the mean detector and the 3.5kHz filter selected. -
Page 5: Fm Measurement
FM Measurement. Select the FM mode with the 'MODE FM' pushbutton. Select the appropriate 'RANGE' with the < > pushbuttons. Five ranges are available with full scale deviations of 1, 3, 10, 30 and 100kHz. Select the required 'AF FILTER' with the < > pushbuttons. Five filter functions are available; three bandpass filters with nominal upper cut-off frequencies of 60kHz, 15kHz and 3.5kHz;... -
Page 6: Rf257 Specification
RF257 Specification RF Input Frequency Range 1.5MHz to 2.0GHz and a useful response, with reduced sensitivity, to at least 4GHz. Impedance 50 Ω nominal. Level 2mV to 1V rms Full specification for noise, accuracy etc applies over the input range 10mV to 1.0V Max Input 0.5W continuous. -
Page 7: Audio Filters
Audio Filters 60kHz Filter 250Hz - 60kHz ± 0.5 dB 12Hz - 72kHz ± 3 dB typically. HF roll off at 80 dB/decade. 15kHz Filter 250Hz - 15kHz ± 0.5 dB 12Hz - 19.5kHz ± 3 dB typically. HF roll off at 60 dB/decade. 3.5kHz Filter 250Hz - 3.5kHz ±... -
Page 8: Internal Battery (Option -03)
Internal Battery (Option -03) Discharge Time >8 hours. Typically, 10 hours for a fully charged battery. Recharge Time 14 hours. Battery Test Pressing the Bat Chk push button displays the battery condition on the display. A reading of between 8 and 10 is required for normal operation. Fuse 1A slow blow on rear panel. -
Page 9: Lock Indication
when the two inputs are in quadrature, i.e. when the phase shifting network is at resonance (420kHz). When the IF deviates from 420kHz, the phase sensitive detector is driven off balance, in a direction determined by the state of the phase switch. This causes the integrator voltage to rise or fall, as appropriate to adjust the L.O. -
Page 10: Af Circuit
to a voltage by VR3 with T15 and is fed to the AF Board; VR3 sets the AM output amplitude. The other output is applied to C22, which is backed off by the a.g.c. reference current source (T17 to T19); VR6 sets the a.g.c. threshold. The potential on C22 is applied to the a.g.c. - Page 11 is then further amplified (U5c) to provide a 0dBm AF output from 600ohms (R33, R34 in parallel). The output of U5c is then attenuated by R35 and R36 to be an equal in amplitude but inverted, version of the output of U5b. Potentiometer RV4 sets the amplitude to be identical.
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Page 12: Control Lines
Control Lines Tables follow to show decoding for FSD Ranges R1, 2, 3, AF Filters F1, 2, 3, Detectors D1, 2. RANGE CONTROL LINES U3d Gain U5a Gain 1kHz 3kHz x3.3 10kHz 30kHz x3.3 100kHz FILTER CONTROL LINES Filter 60kHz Bandpass 15kHz Bandpass 3.5kHz Bandpass Psophometric Filter... -
Page 13: Calibration
Calibration Test Equipment Required Digital Voltmeter (DVM); ≤ 10M ohms input impedance. 30MHz Oscilloscope with x10 probe. 10MHz Frequency Counter. Signal Generator, 1 - 2GHz (AM/FM/CW) Signal Generator, 1 - 100MHz, AM/FM Variable modulation frequency, 10Hz - 100kHz. FM 0 - 100kHz peak. AM 0 - 100% AF Level Meter and distortion analyser. -
Page 14: Fm 60Khz Bandpass
Connect the RF board to P4. Connect signal generator (Item 5) to the RF257 RF I/P. Adjust the FM modulation to 100kHz at 800Hz AF. Attenuate the audio input until the DVM reads 0.300V. -
Page 15: Fm 3.5Khz Bandpass
FM 3.5kHz Bandpass Select 3.5kHz filter. Swing the AF from 20Hz to 4kHz and check that the reading remains within ± 0.5dB over the range 25Hz to 3.5kHz. The reading must fall by more than 0.5dB at 4kHz. Psophometric Filter Select psophometric filter and set the AF to 800Hz. -
Page 16: Rf Board Test
RF Board Test Discriminator Connect the signal generator, set to 1.5MHz at 0dBm level, to the INPUT socket. Connect the oscilloscope (item 2) using the x10 probe to TP5 (RF Board). Set the oscilloscope to 5V/cm DC. and increase the signal generator frequency to obtain the first lowest DC level on the oscilloscope (2.3MHz approx.). -
Page 17: Am Calibration
Connect a calibrated signal generator to the INPUT socket and set it to give 100kHz deviation at a 1kHz rate. Adjust RV4 (RF Board) for 100kHz on meter. *If an accurately calibrated standard signal generator is not available, the following procedures should be used. -
Page 18: Additional Checks
Set the PSU voltage to 11.0V. Press the Batt Chk switch and adjust RV9 to give a reading of 8 on the meter. Switch off the RF257 and reconnect the battery wires to the battery. Make sure the red wire goes to the positive of the battery. -
Page 19: Parts Lists
M3 x 10 Flush Head Stud 610.111 M3 B.Z.P. Nut 2902 M3 x 8 Hex Spacer 610.119 M3 Plain Washer 1091 257 RF Logic Graphic Label 620.122 Spire Clip for No6 UNC screws 2406 M3 x 6 Pan Head Pozi Screws Page 19... -
Page 20: 900.132 Front Panel Pcb Assembly
900.132 Front Panel PCB Assembly Part No Details Part No Details 2899 257 Front Panel bare PCB 420.102 L.E.D. HE Red 3mm 660.112 Wire 7/0.2mm PVC 1.2mm Black 420.102 L.E.D. HE Red 3mm 660.114 Wire 7/0.2mm PVC 1.2mm Red 130.127 680R Metal Film 250mW 1209 3.2mm Fork Terminal Crimp... - Page 21 Part No Details Part No Details 240.107 100nF X7R MCL 50V 410.102 1N4148 GP Diode 240.107 100nF X7R MCL 50V 410.102 1N4148 GP Diode 240.107 100nF X7R MCL 50V 410.102 1N4148 GP Diode 5031 10nF Capacitor 50V 2% PPS Film 410.102 1N4148 GP Diode 1063...
- Page 22 Part No Details Part No Details 2677 27k 1% Resistor 0603 R100 150.143 7K5 1% Metal Film 250mW 130.134 6K8 Metal Film 250mW R101 130.141 8K2 Metal Film 250mW 130.145 180K Metal Film 250mW R102 130.141 8K2 Metal Film 250mW 130.154 390K Metal Film 250mW 190.118...
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Page 23: Rf115V03 Rf Board Assembly
RF115v03 RF Board Assembly Part No Details Part No Details 4692v03 257 Mod Meter RF Board V03 2012 1n0 X7R Capacitor 50V 0603 2003 10n X7R Capacitor 50V 0603 2062 100p NP0 Capacitor 50V 0603 210.124 1p8F Ceramic Plate 2% 100V 2062 100p NP0 Capacitor 50V 0603 4668... - Page 24 Part No Details Part No Details 4661 BAS16 Signal Diode 85V 215mA 2022 100R 1% Resistor 0603 4653 BBY40 VHF Varicap Diode 2077 330R 1% Resistor 0603 4653 BBY40 VHF Varicap Diode 2083 3k3 1% Resistor 0603 4661 BAS16 Signal Diode 85V 215mA 4469 390R 1% Resistor 0603 4661...
- Page 25 Part No Details Part No Details 2084 4k7 1% Resistor 0603 170.111 2k2 500mW Open Ceramic Pot 2080 47k 1% Resistor 0603 SK A 1104 82 MCX PCB Receptacle Gold 2080 47k 1% Resistor 0603 4672 BF545B N JFET RF 2026 100k 1% Resistor 0603 4671...
- Page 26 Part No Details Part No Details 4655 BC847 NPN Transistor 45V 100mA 4657 MMBT2369A NPN Transistor 15V 4655 BC847 NPN Transistor 45V 100mA 4658 BFS17 NPN Transistor 15V 25mA 4655 BC847 NPN Transistor 45V 100mA 620.101 PIN Solder Terminal 4655 BC847 NPN Transistor 45V 100mA 620.101 PIN Solder Terminal...
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Page 27: Circuit Diagrams
Circuit Diagrams AF Board Circuit 1 of 3 Page 27... -
Page 28: Af Board Circuit 2 Of 3
AF Board Circuit 2 of 3 Page 28... -
Page 29: Af Board Circuit 3 Of 3
AF Board Circuit 3 of 3 Page 29... -
Page 30: Rf Board Circuit 1 Of 4
RF Board Circuit 1 of 4 Page 30... -
Page 31: Rf Board Circuit 2 Of 4
RF Board Circuit 2 of 4 Page 31... -
Page 32: Rf Board Circuit 3 Of 4
RF Board Circuit 3 of 4 Page 32... -
Page 33: Rf Board Circuit 4 Of 4
RF Board Circuit 4 of 4 Page 33... -
Page 34: Front Panel Board Circuit
Front Panel Board Circuit Page 34... -
Page 35: Pcb Legends
PCB Legends AF Board Legend Page 35... -
Page 36: Rf Board Legend
RF Board Legend Top Side Bottom Side Page 36... -
Page 37: Front Panel Board Legend
Front Panel Board Legend Front Rear Page 37...
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