Specifications

41D2355 4-WAY SPLITTER UHF
PUB13-01 Rev 0 Mar 4, 2013 9
Finally, Q505 serves as a low impedance video source. FL502 is a color subcarrier trap. Q503 serves as a buffer
and Q501 serves as another buffer. Following is filter FL501 which removes the aural carrier from the back porch.
Subsequent downstream sampling circuits monitor the blanking level and therefore require a clean back porch. The
inverted and amplified video signal is fed to sync separator U509-2. When sync is detected, the sync output at pin 1
delivers a positive-going composite sync pulse, which turns on Q511 whose collector then goes LOW. If no sync is
detected by U509, its pin 1 remains LOW and Q511remains off.
After U509-1 pulse has finished, Q511 turns off and its collector output goes HIGH. This LOW to HIGH transition
activates blanking multivibrator U507 and an active low pulse is fed to Q510, turning it on.
Q502, C504, and U502A form a sample-and-hold circuit that samples the signal originating from the emitter of Q501
and which is buffered by Q518. Sampling occurs during the back porch, and holds during the subsequent horizontal
line. This DC sample is amplified in U502A .
Because a single supply op-amp is used at U502, the output seen on TP502 will contain a small DC offset which must
be minimized because low level signals are near ground/earth potential. With no RF input, this offset voltage is
adjusted by potentiometer R504 as near as possible to ground/earth. The setting can be seen when using a DC
coupled scope. Adjust R504 to move the DC level toward ground/earth potential; stop turning the potentiometer
immediately the DC ceases moving. A residual voltage offset of 20mV can be expected for the LM358 family of
op-amps.
Outputs from unity gain op-amps U501A, U501B, U502B drive the forward (reflected) power metering A-D circuits and
provide telemetry and AGC (VSWR) signals through other A-D converters. Bench test calibration consists of adjusting
R518 with rated, properly modulated input while observing the voltage at TP501, which should read 4.0 volts DC for full
scale calibration.
5.1.2 J18 Channel – Aural Forward
The sample of the signal detected by CR501 from J18 port is buffered by Q503. The wideband visual/aural signal
drives an aural bandpass ceramic filter FL503. The filter is chosen for the broadcast standard in use. C512 couples
the filtered aural CW signal to amplifier Q507. Following is a buffer Q506 and RF amplifier U504.The aural signal
from U504 output is then fed to peak detector consisting of CR502, C515, R545. Q508 buffers signal before
applying to sample and hold circuit consisting of Q509, C516 and R552. Sampling aural signal during the back
porch provides level independent of video content.
Because a single supply op-amp is used at U503B, the output seen on TP503 will contain a small DC offset which
must be minimized because low level signals are near ground/earth potential. With no RF input, this offset voltage is
adjusted by potentiometer R534 as near as possible to ground/earth. The setting can be seen when using a DC
coupled scope. Adjust R534 to move the DC level toward ground/earth potential; stop turning the potentiometer
immediately the DC ceases moving. A residual voltage offset of 20mV can be expected for the LM358 family of
op-amps.
U503B provides DC amplification of detected signal. Following is another amplifier U505A which gain is set by
potentiometer R559, so DC level at TP504 is in the range of 7VDC. This provides enough safety range for the signal
not to saturate or not being high enough for 4VDC calibration at TP505. The sample of aural output is fed to
differential amplifier U503A to compensate for aural carrier influence on visual metering in internally diplexed
systems. R542 is used to minimize this influence.
5.1.3 J19 Channel – Reflected
The RF sample is applied to input J19 and is terminated by R561IIR603. CR504 and Q516 form an envelope
detector. CR504 is forward biased slightly by R594 and R604 to overcome CR504 conduction threshold voltage,