MFJ-852 Instruction Manual
AC Line Noise Meter
7
THEORY OF OPERATION
The MFJ-852 is a single-frequency broadband AM receiver operating in the 135
MHz region. RFI signals are picked up by a built-in ½ wave dipole antenna that
is decoupled from the receiver PC board by a choke-style balun. Image rejection
is provided by bandpass filter L1/L2, and preamplifier Q1 boosts signals by
approximately 20 dB prior to mixer stage U1. U1 also contains the receiver's
local oscillator, which operates at approximately 124.3 MHz. The 10.7 MHz IF
filter FL1 sets the message channel bandwidth at around 100 kHz, and IF pre-
amplifier Q1 boosts signals prior to IF amplifier/AGC gate U2. U2 provides up
to 50 dB gain on weak signals, and delivers a 70 dB measurement range for
signal input levels exceeding 2 uV in strength. A diode AM detector
simultaneously demodulates noise signals and generates a DC level for AGC
control. This composite energy is amplified by op-amp U3a. The AF
component is routed to the receiver's headphone monitoring circuit, and the DC
component is routed to U3b where it is amplified and re-biased to drive the AGC
input of U2. The AGC line also drives the receiver's semi-log signal-strength
indicator meter.
BPF
Q1
Q2
U1
U2
Det
U3a
M
U3b
Local
Osc
RF Amp
Mxr/Osc
IF
IF/AGC
Det Amp
ALC
RFI Level
Phones
Antenna
L3
IN CASE OF DIFFICULTY
Your meter has few internal adjustments and is a trouble-free design. However,
it has a sensitive electro-mechanical meter that may sustain damage if dropped.
Also, the MFJ-852 is not "weatherproof", and should not be exposed to heavy
rain, snow, or other harsh environmental conditions. If you experience a
problem, look through the checklist below to determine if it's something simple
you can fix yourself.
Fails to Power Up:
Check battery condition.
