Quote:
Originally Posted by John L Stewart
Hi Alejandro- Looks to me the PS voltages are incorrectlty labeled on the 807 modulator at that link, the 300 & 800 should be reversed.
But here are more examples of Class B vacuum tube modulators. These were referred to as Zero Bias Amplifiers.
Cheers, John
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In an earlier post I mentioned that the audio pass band of interest was 30 to about 3000 Hz. In commercial narrow band FM (NBFM) systems voice is usually 300-3400 Hz. It is very similar to the telephone C-Msg passband. In Europe it is similar to the Psophometer passband.
In voice comm the users are interested in what they call 'talk power', the ability to communicate even in difficult envirnments. So the circuitry often includes speech clippers & audio BW limiting in order to maintain a high modualtuion index. Not hifi at all. Hifi would occupy too much RF BW.
Below 300 Hz is reserved for signalling & control. One widely used sysem was CTCSS. There are about 50 standard tones used depending on the manufactuer & system. Refer to link-
http://en.wikipedia.org/wiki/Continu...Squelch_System
More recently comm systems are converting to the digital APCO25 which can occupy 12.5 or 25 KHz. It is backwards compatible to older sysems still in use.
More advanced systems are the NXDN by Kenwood & ICOM. Another is the HPD by Motorola. Both can run in 6.25 KHz BW but are not compatible with each other. The modulation schemes are quite different.
See the Aeroflex 3920 test set here-
http://www.aeroflex.com/ats/products...o_Test_Set~171
System testing is getting complicated, I sold many of these.
Cheers, John