Receiver Will Acquire Signal, But Audio Breaks Up - Wegener DR95 Instruction Manual

7. Receiver Will Acquire Signal, but Audio Breaks Up

a. Verify the receiver status using the 'RC' terminal request. The top of the screen should
read "Status of Acquired Carrier."
b. In versions of the receiver firmware preceding 1.4, it is possible for the squelch threshold
error-count to be set lower than the unsquelch threshold. If this is the case, please
correct. (See receiver manual Appendix B, paragraph B-1, 'ERRORCOUNT'
command.)
c. Check the receiver's front-panel LED's. If the "Status" LED stays red, there may be a
problem maintaining sync in the audio decoder. In the 'RC' screen, next to "MPEG
Decoder," see if the status is "Locked" or "Unlocked." Also, under the 'RC H' screen,
look at the number of "Alarms, No MPEG" being reported. Note whether the MPEG
decoder is, or is not staying synchronized.
d. If there are chirps, pops, and squeals in the audio, there is either a poor input SNR ( and
the squelch threshold has been set too high), co-channel RF interference, or there is
severe phase noise on one or more L.O.'s in the signal chain. To rule out the first two,
refer to section 8, "Receiver Acquires Signal But Signal-to-Noise Ratio is Poor."
e. Look at the "Error Count" on the terminal's 'RC' screen. If it is lower than about 400,
and the MPEG decoder is occasionally losing sync, the main suspect is L.O. phase noise
somewhere in the signal chain. The most likely suspect is the antenna LNB. Be sure its
L.O. meets the requirements listed in table 2-2 of the DR95-1X/DR96-1X manual.
Occasionally, someone establishes uplinks using low-cost video upconverters. These
also can cause the same problem. The transmit upcoverters must have L.O.'s whose
phase noise performances are much better than the LNB requirements in table 2-2 of the
manual.
f. If the signal-to-noise ratio of the RF link is very high, the questions raised in step e may
be answered by monitoring the I/Q constellation at test points inside the receiver. On the
motherboard, monitor TP15 and TP16 on an oscilloscope (both channels AC-coupled, 50
mV/DIV) set for X-Y display. This "bowtie" display should closely hug one axis. If it is
"swinging" up close to the other axis, phase noise is the problem. (Using an rms
voltmeter, the ratio of the channels should exceed 5:1 in noise-free laboratory loop-
backs.)
g. If the audio runs without impairment, but suddenly cuts off briefly with the cycle
repeating endlessly, check the clock frequency supplied to the DE96 MPEG Audio
Encoder at the transmit end. If the DM96 is sourcing the clock, this is most easily done
by connecting a precision frequency counter to TP7 on the 2662=XX upper card (XX
defined in procedure 1, step i). The frequency should be within 50 ppm of the nominal
data-rate frequency. If it is not, contact Wegener Communications Service Department
for instructions.
h. Lastly, substitute another receiver and note the behavior; if it continues, suspect the
audio encoder at the transmit end. If not the encoder, return failed receiver to Wegener
Communications Service Department
DR95/DR96-001 4-10