Why Doppler VOR subcarrier frequency is 9960 Hz?

Answer 1

The 9960 Hz subcarrier is the same for conventional VOR (CVOR) or Doppler VOR (DVOR).

A conventional VOR has three Amplitude Modulated (AM) signals encoded on a VHF carrier:

1) a 30 Hz variable (VAR), which is modulated by the antenna, not the transmitter;

2) a 9960 subcarrier, which is in turn frequency modulated (FM) with a 30 Hz reference (REF) signal;

3) and a voice / identifier channel, which includes 1020 Hz "Morse Code" identifiers and aural voice signals.

The VAR signal is created by spinning a slightly directional antenna at 1800 rpm (30 revolutions per second). The phase of this 30 Hz signal is variable, dependent on the location of the receiver.

To determine VOR bearing, another 30 Hz signal is required. The REF signal is perceived as being the same phase regardless of position of the receiver relative to the VOR.

Here is the problem: the VAR 30 Hz signal is already modulated on the carrier. If the REF 30 Hz signal is modulated onto the carrier without processing, a receiver would find two 30 Hz signals (just one signal if REF and VAR signals are in phase). How would the receiver know which signal is the REF and which is the VAR?

To get around this problem, the VOR takes a 9960 Hz carrier and frequency modulates this carrier with the REF 30 Hz signal. The modulation index is 15, meaning the 9960 carrier has a deviation of 450 Hz (30 Hz times 15). In other words, the subcarrier varies between 9510 Hz to 10410 Hz (9960 +&- 450 Hz). This frequency excursion occurs 30 times per second (30 Hz). The subcarrier signal spectrum does not overlap with the spectra of the VAR or aural signal; therefor it can be amplitude modulated on the RF carrier.

The reason for frequency modulation of the REF signal on the 9960 carrier, as opposed to amplitude modulating the REF signal, is that the AM detector in a VOR receiver would still output two 30 Hz ambiguous signals and a 9960 signal, all summed together.

The VOR instrumentation processor takes the detected VOR signal (called VOR composite video signal (COMP)), and processes the signal as follows:

1) COMP is processed through a low pass filter that preserves 30 Hz to get the VAR signal;

2) COMP is processed through a high pass filter to reject the VAR and aural signals, then an amplitude limiter, and then though an FM detector to get the REF signal. The FM detector could be a discriminator (used in the bad old days), or a phase lock loop (used in modern equipment);

3) COMP is not processed by VOR instrumentation; however it may be filtered to please the listener, i.e. range filter (1020 Hz bandpass), voice filter (200 to 3000 Hz bandpass).

Why 9960 Hz and not something lower? The baseband spectrum of the whole VOR signal consists of a reasonably narrow 30 Hz signal (VAR), the aural / ident signal (approximately 200 Hz to 3000 Hz), and the 9960 subcarrier (9510 to 10410 Hz). No overlap can be allowed. Also, the receiver has to separate the signals; therefore a significant gap between the signals is provided.

A Doppler VOR varies from a conventional VOR in that the VAR channel is 30 Hz frequency modulated instead of 30 Hz amplitude modulated. The spectrum of the CVOR REF signal is a narrow signal at the RF carrier frequency (fc) (between 108 to 117.95 MHz), and two side bands, one at fc + 30 Hz and the other at fc - 30 Hz. The DVOR REF signal has the same spectrum components as the CVOR REF signal, with the addition of sidebands at (plus and minus) 60 Hz, 90 Hz, 120 Hz ... and on (diminishing in amplitude). The VOR receiver does not react to the sidebands at 60 Hz and above because the VAR signal is separated by a low pass filter; therefore it does not matter to the receiver whether the signal comes from a CVOR or DVOR.