Tone mode on ADF ?

[discus2]

Quick question about ADF... there are 4 modes, OFF, ANT, ADF and TONE. What exactly does the TONE mode do? All I can find is that it receives 1000Hz keyed continuous wave (CW) signal.

Thanks for answering

[Lightning6]

The Tone Mode gives you the Morse ID of the NDB you are tuned into, the indicator will then point towards that NDB.

[411A]

The Tone Mode gives you the Morse ID of the NDB you are tuned into, the indicator will then point towards that NDB

Ahhh, no, at least not on the ADF receivers that I have used.
TONE is another term for BFO (beat frequency oscillator) and selecting this position, gives the pilot the ability to select the center frequency of the selected station, whilst manually tuning the desired frequency.
Not normally used on digital frequency selections on those receivers so equipped, as they have no means of manual tuning.

[ampclamp]

Depends too whether the beacon has a modulated carrier ie a tone superimposed onto the RF carrier signal or, as above, a beat frequency oscillator, BFO, is required to enable the pilot to hear a tone where no tone is on the carrier.

As I understand it...
One has the audio tone ident on the transmitted signal, the other the carrier is just a carrier keyed (no audio) with the ident and the tone is added at the receiver by beating 2 frequencies together making it audible.

[Lightning6]

Ahhh, no, at least not on the ADF receivers that I have used.
TONE is another term for BFO (beat frequency oscillator) and selecting this position, gives the pilot the ability to select the center frequency of the selected station, whilst manually tuning the desired frequency.
Not normally used on digital frequency selections on those receivers so equipped, as they have no means of manual tuning.

Having not used ADF for many years, I'm a bit out of touch.

From what I can gather, the digital ADF'S automatically find the center frequency once you've selected the NDB frequency in 'Tone' mode. I stand to be corrected though.

[Lightning6]

The point I was trying to make, in simple terms, is that the 'Tone' function was purely to ID the NDB....Who uses it these days anyway? GPS apparently rules. I know I'm sounding like an old git now, but I thought VOR was something else yonks ago. :) , ADF...Handy if you want to listen to the radio.

[SIUYA]

411A's correct!

The tone selector turns on the beat frequency oscillator (BFO) in the ADF receiver.

Some ADF stations turn their transmitters on and off to transmit the station's morse code station identifier, and to process that station's signal, the Tone Selector on the ADF control panel receiver must be selected (to BFO mode) on the rceiver that you're using to select the ADF station.

[NSEU]

TONE or BFO is required for non-amplitude modulated ground station signals.

A normal ADF signal has a carrier (radio) wave at a specific (single )frequency. This carrier wave is amplitude modulated by an audio ident. I.e. the morse audio ident is superimposed on the ground station's carrier wave and, as a result, the carrier wave/signal gets weaker or stronger in beat with the morse ident ... and your ADF receiver audio detector converts this weakness/strength into speaker audio.

However, some ADF signals are not amplitude modulated, so, although the aircraft receiver will DF, there will be no audio ident. However, these non-amplitude modulated carrier radio signals generated by the ground station are switched on and off very quickly in beat(in time) with the station's morse ident. Even though the carrier wave is being switched on and off (pulsed), there are enough radio waves for the aircraft to work with to DF. However, the receiver has no audio signal to detect. This is where TONE or BFO comes into play. A "local oscillator" (tone generator) signal in the aircraft receiver mixes with the incoming signal to give the pilot something he can hear (and, depending on the aircraft, this morse ident is identified and the station ID is displayed on the navigation displays).

Rgds.
NSEU

[fireflybob]

Tone mode (BFO On as 411A correctly said) is necessary to ident NDBs which have A1 emission as opposed to A2 which dont.

The essential difference is the A1 beacons make up the ident by keying the transmitter on and off (with CW, thats carrier wave only). To render the ident audible you have to have the BFO on. (An interesting footnote is that when the beacon is coding you lose the bearing since there is a break in transmissions, hence A1 beacons are not approved for letdowns). A1 beacons have longer range and are intended as an aid for enroute navigation. Also the ident is available less often than with an A2 beacon once again due to loss of bearing with the ident.

A2 beacons have a continuous transmission with the ident played on it (I think with amplitude modulation).

The last A1 beacon I can recall in the UK was at Lichfield. The French still have quite a few A1 beacons.

Remember also to switch the tone OFF after you have idented especially with modern EFIS which displays the ident (except for A1 beacons) otherwise the EFIS ident will not show for the next A2 beacon you use.

Hope this makes sense - have just got back from clubbing!

[Jumbo Driver]

fireflybob is essentially correct - despite his evening clubbing!

Types of emission from radio beacons can be classified as follows:

A0 = unmodulated carrier wave (CW).
A1 = unmodulated CW with ident transmitted by ON/OFF keying of the CW itself in the pattern of morse code to produce the ident.
A2 = CW with ident carried by a superimposed modulation (usually AM).
A3 = CW with more complex superimposed modulation (e.g. speech)

NDBs are generally classified as A0 (no ident), A0A1 (CW ident) or A0A2 (modulated ident). RTF is A3.

To tune A0, or tune and identify A0A1 beacons, a BFO within the receiver is applied to the received signal to produce a beat (or difference) frequency which is heard as an audible tone, the pitch of which can usually be adjusted by further turning the BFO (or TONE) knob. In the case of A0A1 transmissions, this tone is interrupted when the CW transmission (A0) is periodically coded with the A1 ident.

As fireflybob correctly says, during A0A1 ident the transmission itself is interrupted and the needle will therefore tend to swing away and back until the transmission is again continuous (A0). Because of this, ident coding on an A0A1 beacon will normally be much less frequent than on an A0A2 NDB to minimise tracking errors.

The BFO (TONE) facility is also used on an older continuous tuning (rather than digital) ADF receiver to seek an A0A2 station, after which the BFO should be switched OFF so that the A2 component may be heard.


JD
:)

[A37575]

In the South Pacific Area certain airports I think still require the BFO position on the ADF in order to receive the call-sign. These airports include Tontouta airport at Noumea, New Caledonia, Wallis Island, Port Vila, Vanuatu and other islands being French protectorates. Possibly Tahiti as well. Mind you that was certainly the case 20 years ago when I operated into those airfields but may have changed since then.

[Checkboard]

fireflybob is correct in everything, except the comment that 411A was correct! :}

[fireflybob]

Checkboard - I thought I'd just be a bit charitable towards him!

[411A]

Read my statement again, Checkboard.
What I stated, within the limitations outlined, is absolutely correct.

[Loose rivets]

A well answered thread, but I'll just natter on cos of terminal boredom.


It's funny to read these explanations of old radio transmission methods.


The first mention of a Difference Frequency came rather late, and I feel that it's important.

Just to paint the picture.

A good receiver, should not allow the audio output to be 'influenced' when an un-modulated carrier is received. It is though. The Automatic Gain Control will haul the set's...erm, Gain, back - cutting back on background noise. Markedly so. In this way one could tune in without hearing the actual station, by just tuning out the background noise.

An understanding of AM rejection might help to get an in depth feel for the concepts.

A police pal would never quite believe that my FM receiver's good AM Rejection, meant that he couldn't monitor his frequency while skiving off for coffee at my house. A carrier wave that was being interfered with in Amplitude should not have, and did not, allow the output to be affected.

He thought my set was crap.

Okay, so what's this difference frequency?

Say the receiver is tuned to 200 kHz. 200,000 Hz. If we turn on a 'local oscillator that is designed to always be 400 Hz high or low of the received signal, say 200,400 Hz, and then beat it with the incoming frequency, one of the results will be 400 Hz. We then filter that one out for our audio circuits.

These beat frequencies are everywhere. For example. A concert grand piano's tonal quality is all about the mixing of multiple string resonances and even the vibrations from the case. Steinway even tune the 'scrag ends' of the strings...you can't hear them, but you can hear the result of beating them with the multiplicity of other tones the strings produce.

There was some talk above about single frequencies. Rather theoretical. Unfortunately, modulating a signal, even with AMrequires a certain Band Width. I can recall saying to my electronics lecturer in 1957, that I didn't understand why AM needed to consume more than a spot frequency. He was, for half a century, a very respected teacher, but he found it difficult to explain, and even now, in discussions about when a photon becomes 'discrete' in the electromagnetic spectrum - it's hard to translate to a quantum model.

When I took my senior (then separate) exams for the ATPL , I worked quite hard on most stuff. However, I skipped over the Radio Aids, cos of years in telecoms. I passed everything...except radio aids.