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pmather
Joined: 17 Sep 2009
Posts: 33
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 Posted: Sun Jun 17, 2012 6:02 am Post subject: OBS resolver signals |
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Can anyone explain or point me at any documentation that explains the OBS
resolver signals for setting the course on a NAV set, e.g. a King 165.
What I know is the the NAV set produces a rounded off square wave at either
30HZ or 300HZ. The OBS knob then returns two signals representing the sine
and the cosine of the position. So for example sine of 90 degrees is 1 and
cos is zero, sine of 270 degrees is -1 and cos is still 0
What I don't know is whether the returning signals go negative (the older
CDIs use transformer technology so could easily produce negative voltage) or
whether they are referenced to the mid point of the excitation signal and
whether the excitation signal itself goes negative or it just varies between
0 and the reference voltage.
Any help or pointers to docs appreciated - google has failed me.
Thanks
Peter
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dlj04(at)josephson.com
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| Posted: Tue Jun 19, 2012 11:52 am Post subject: OBS resolver signals |
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On 6/17/12 11:56 PM, AeroElectric-List Digest Server wrote:
| Quote: | Can anyone explain or point me at any documentation that explains the OBS
resolver signals for setting the course on a NAV set, e.g. a King 165.
What I know is the the NAV set produces a rounded off square wave at either
30HZ or 300HZ. The OBS knob then returns two signals representing the sine
and the cosine of the position. So for example sine of 90 degrees is 1 and
cos is zero, sine of 270 degrees is -1 and cos is still 0
What I don't know is whether the returning signals go negative (the older
CDIs use transformer technology so could easily produce negative voltage) or
whether they are referenced to the mid point of the excitation signal and
whether the excitation signal itself goes negative or it just varies between
0 and the reference voltage.
|
If you want to interface with a KX165 you might look at the schematic
of that unit, but the nav radios I'm familiar with (just checked a
couple of older King and Radair schematics) are all capacitor-coupled so
the signal should stay positive with respect to the audio reference
ground. Some CDIs may have a transformer input to prevent ground loops
-- the return side of the transformer would be run back to the nav radio
audio reference ground rather than through the airframe.
A VOR station transmits a reference subcarrier at 9.96 kHz, frequency
modulated with a 30 Hz tone, and amplitude modulation, also of a 30 Hz
tone. The phase angle between the 30 Hz frequency modulation of the
subcarrier, and the 30 Hz amplitude modulation of the main carrier,
corresponds to the angle between north and the direction from the
station to you. The audio input from the nav receiver contains both the
30 Hz AM tone and the 9960 Hz subcarrier with its FM reference
modulation. The CDI or VOR converter demodulates the 9960 Hz carrier so
you now have two 30 Hz tones. The resolver introduces a known phase
shift in the reference; when you turn the OBS knob, you change the
amount of phase shift. The VOR needle is driven by a phase comparator
that measures the phase difference between the 30 Hz variable signal and
the demodulated and phase-shifted 30 Hz reference signal. When the
needle centers, the phase shift is zero because the resolver has
introduced a phase shift equal to the phase difference between the two
30 Hz signals.
Localizer is a different signal entirely but sent on the same wire;
there are two tones, 90 and 150 Hz, and the needle reflects the ratio
between the amplitude of the two tones. When you are on the centerline
of the localizer, you're receiving equal signals from the two sets of
antennas on either side of the centerline, one of which is transmitting
with 90 Hz modulation and the other with 150 Hz.
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