TUNING OF THE DISCRIMINATOR:
The discriminator is automatically fine tuned to the sub carrier frequency. However, if
the sub carrier frequency has been changed more than 1 MHz, it may be
advantageous to optimize the THD (Total Harmonic Distortion) by adjusting C115 &
C215. Connect the audio generator, set to 1000Hz, to the external input of the FM
generator. Connect the RF output of the FM generator, set the desired subcarrier
frequency, to the RF input of the modulation meter. Connect the demodulated audio
output of the modulation meter to the audio input of the distortion analyzer.
Connect the output of the distortion analyzer to the vertical input of the
oscilloscope. Connect the auxiliary output of the signal generator to the external
trigger input of the oscilloscope. Adjust the 1KHz level to cause a subcarrier
deviation of +/- 50 KHz as measured on the 100 KHz scale with a 3 KHz band-pass
selected.
Measure the THD of the audio oscillator FM generator combination. It should
be less than 0.5%, if not, do not attempt to reduce the FMR622S demodulator
distortion, as the adjustment may add distortion in an attempt to cancel that of the
test equipment. If the THD of the test equipment measures better than 0.5%,
connect the RF output of the FM generator, set to 30mV, to the sub carrier input of
the FMR622S. Terminate the subcarrier output with 75 Ohms.
Connect the input to the distortion analyzer to the wiper of VR131 (Left),
(VR231), if right channel. The audio level should measure within 3 dB of 0 dBm (1.5
to 3 Vp-p. Adjust C115 for minimum THD, it should be better than 0.7%
TUNING
THE
DE-EMPHASIS:
The de-emphasis tracking adjustments must be done on both channels if either
is adjusted, to assure that the two are identical. These adjustments can be
satisfactorily accomplished only with sufficient instrumentation, please attempt them
only if you must.
The FMR622S utilizes a sliding de-emphasis circuit, which complements that
at the modulator. At 100% modulation the circuit frequency response is essentially
flat. The circuit responds to program level by increasing the de-emphasis as the
program level drops. There are two tracking adjustments in each channel of the
FMR622, the first is to set the low frequency through level to a standard, the second
is to set the high frequency through level to the same level at 100% modulation.
The standard through levels must be identical on each channel for optimum
stereo separation and frequency response, these levels will change if the sub carrier
frequency is changed, necessitating retuning the de-emphasis as follows.
FMR622Sisb
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