I was wondering if anyone knows the history of what appears to have been the standard audio modulation frequency for RF signal generators.

Why 400Hz? How was it that this modulation frequency was adopted by signal generator manufacturers?

Just curious!

Gary W. Prutchick

:Gary: This was an outgrowth of the telephone industry and was chosen because 400 Hz is sort of the geometric center of the human voice fundamental and is easy for the human ear to hear. Loudspeaker and microphone manufacturers also used to specify their Z at 400 Hz.
:I was wondering if anyone knows the history of what appears to have been the standard audio modulation frequency for RF signal generators.
:
:Why 400Hz? How was it that this modulation frequency was adopted by signal generator manufacturers?
:
:Just curious!
:
:Gary W. Prutchick
:

::Gary: This was an outgrowth of the telephone industry and was chosen because 400 Hz is sort of the geometric center of the human voice fundamental and is easy for the human ear to hear. Loudspeaker and microphone manufacturers also used to specify their Z at 400 Hz.
::I was wondering if anyone knows the history of what appears to have been the standard audio modulation frequency for RF signal generators.
::
::Why 400Hz? How was it that this modulation frequency was adopted by signal generator manufacturers?
::
::Just curious!
::

::Gary W. Prutchick

Once upon a time, Ma Bell had a signalling system where 1000 Hz., turned on and off 20 times a secomd. was semt on lines that went through audio amplifiers that were not capable of passing the normal 20 Hz. at 105 Volts. A relay, seeing the 1KHz., operated at 20 Hz. and produced the ringing Voltage of 105V/20Hz. Testing with 1KHz. confused those circuits, so 400 was used. The industry followed Ma Bell, for a while, but 1K and 400 are about the same thing for testing purposes.
Lewis

::
:

400 Hz is favored for aircraft because of weight. 400-Hz magnetic devices, such as motors, alternators, or transformers, are quite a bit smaller and lighter than those rated at 50- or 60-Hz.

The reason for this is that for a given VA rating, less iron is needed in a magnetic core to avoid saturation.

:400 Hz is favored for aircraft because of weight. 400-Hz magnetic devices, such as motors, alternators, or transformers, are quite a bit smaller and lighter than those rated at 50- or 60-Hz.
:
:The reason for this is that for a given VA rating, less iron is needed in a magnetic core to avoid saturation.

After 35 years of working on airliners and avionic accessories, may I state taht 115 Volts @ 400 Hz HURTS!
:

:400 Hz is favored for aircraft because of weight. 400-Hz magnetic devices, such as motors, alternators, or transformers, are quite a bit smaller and lighter than those rated at 50- or 60-Hz.
:
:The reason for this is that for a given VA rating, less iron is needed in a magnetic core to avoid saturation.

After 35 years of working on airliners and avionic accessories, may I state taht 115 Volts @ 400 Hz HURTS!
:

Lewis, does 400 Hz hurt more than 60 Hz? You'd be getting zapped 800 times per sec instead of 120 - if that would make any difference.

Nuclear submarines, many moons ago, used 60 Hz for general power distribution, but 400 Hz for anything involving the missiles.

:
:After 35 years of working on airliners and avionic accessories, may I state taht 115 Volts @ 400 Hz HURTS!
::
:

:400 Hz is favored for aircraft because of weight. 400-Hz magnetic devices, such as motors, alternators, or transformers, are quite a bit smaller and lighter than those rated at 50- or 60-Hz.
:
:The reason for this is that for a given VA rating, less iron is needed in a magnetic core to avoid saturation.

Right! You can have a transformer stepping 115 V. down to 28 for a landing or taxi light that may be 1 or 2.5 kW, and if you have to change a burned out transformer (doesn't happen very often) you can hold the thing in your hand like it was a power transformer for a five tube radio (well, maybe a little larger). It looks strange to see some 18 ga. wiring going to a tiny transformer and 10 ga. wires going to the landing light.
Motors, too are very small compared to their 60 hZ counterparts, the three phase electric hydraulic motors look very small after you see the Amperage ratings of the circuit breakers. Still stings like the devil to get across,
Lewis

Everyone,

Thanks for all of the information and insight.

Gary

:
:I was wondering if anyone knows the history of what appears to have been the standard audio modulation frequency for RF signal generators.
:
:Why 400Hz? How was it that this modulation frequency was adopted by signal generator manufacturers?
:
:Just curious!
:
:Gary W. Prutchick
: