[SOLVED] Analog Electronics Communication basics

[Eshal]

Hi

I have electronic communication paper on 24th October. So I want to clear some questions which bear in my mind related to exam.

Q) Derive the total power for SSSC (J3E).

I don't know where to start derivation for this. Although I know formula for total power is
Total power=Pc + Pusb + plsb
where,
Pc=carrier power,
Pusb=upper sideband power,
Plsb= lower sideband power,

for SSSC Pc=0
But first I will have to derive the equation for total power. So where to start that equation?
Should I start with total voltage's equation? i.e.
Total voltage=Ec+Eusb+Elsb
where,
Ec=carrier voltage,
Eusb=upper sideband voltage
Elsb=lower sideband voltage

Q I saw heading in the book named "Commercial AM Broadcast Band". Under this heading I read a line, quoted below,
"10KHz was allocated to each of the channels in the AM broadcast band and the band of frequencies from 535 to1605 KHz was set aside for AM broadcasting only."

Can you explain this, what is being said here?

Regards,
cute princess..

 

[chuckey]

The upper modulation frequency in a AM transmitter is limited by a filter, so the filter passes 4.5 KHZ and reduces 5 KHZ by 40 db or so. When you modulate your AM transmitter, the minimum frequency of the lower sideband is Fc -4.5KHZ and the maximum frequency of the upper sideband is Fc+4.5KHZ. The channel width required to get both theses side bands is 9 KHZ, so they put the next transmitter on Fc+ 10KHZ, to allow for a little mistuning of the receiver.
In your AM band extending upwards from 535 KHZ, the first transmitter frequency will be 535 +5 = 540 KHZ, the next one 550 KHZ. . .
SSSC = SSB, single sideband , no carrier. When there is no speech Pssb = 0, as you speak the power increases, because there is no carrier to cut, there is no limit on the power being radiated. So I do not know what sort of equation you are after. With instruments you can't tell if somme one is shouting into a 100mW transmitter, or whispering into a 1MW transmitter. The important thing is the PEP, Peak Envelope Power, because this will determine the maximum power you can get out off your amplifier before it starts to distort the RF envelope and produce ips (intermodulation products). These cause audio distortion to the receiver and interference on adjacent channels. So there are specification for the allowable amount if ips.
Frank

 

[Eshal]

Hi, thanks for reply. Nice wordings.
You use the word PEP (Peak Envelop Power). Can you tell me what is this? Define this term please.

 

[chuckey]

Its the maximum power that a transmitter can reach over a short term like on a speech peak. the peak voltage is measured across its output and the equivalent power is calculated. Class B linear amps take more DC power as the RF power increases, because of inefficiencies, a substantial part of the DC power ends up being dissipated by the power amplifier as heat, so a continuous high RF power means the power amplifier (and its power supply) over heat.
Frank

 

[Eshal]

OK great explanation.

Can you differentiate among RF amplifier, power amplifier, voltage amplifier and a driver amplifier?

 

[chuckey]

Power amplifier, this is the last stage before the output. It must be able to handle the rated output power + a little bit more to account for losses. If the power amplifier is handling a large amount of power it is often helpful if the previous stage has also a more moderate power handling capacity. For instance a TV transmitter might have 10 power amplifier modules each of 1KW to generate 20 KW. Feeding the input of all 10 amplifiers might be a driver of 500W with its output split 10 ways. The input to the driver might be the system RF level of 10 mW.
Voltage amplifier, does exactly the raises the voltage, but at a low power. i.e. input = .1V across 10K, output 10V across 10K.
RF amplifier, this can tuned to a working frequency in which case, it can be class A,B or C. If it is untuned then it must be a linear amp, i.e. class A or class B in push pull.
Frank

 

[BabaYaga]

@chuckey actually gave clear explanation of all amplifiers but i guess he forgot about driver amplifier, let me explain this:
some times there is an impedance miss-match between the source and the next stage preceeding it, in those cases and impedance matching amplifier like a basic Common Collector configuration based transistor amplifier is used which is also called as a driver amplifier

 

[Eshal]

Oh great.. Thanks I was reading chuckey's post again and again so that I could gain knowledge about driver amplifier but you helped me. Thank you.
OK I want to ask some more thing, obviously my question is related to communication but not according to thread title. May I ask it here?

Regards,
Cute Princess.

 

[BabaYaga]

Well the forum rules says 'No', but if you are asking me then please go ahead and i will try to answer it.

- - - Updated - - -

The explanation that i gave for driver amplifier is just the one which was not discussed here by others, but a driver amplifier can be defined in several other ways, one of the discussions on this forum which talks about driver amplifier.

https://www.edaboard.com/threads/27091/

 

[Eshal]

Can you explain what is Compressor circuit?
And what is attack and release time? How attack and release time affect the output of the circuit.

There is not much explanation about compressor circuit in the book. Can you explain this?

 

[FvM]

There's a verbose explanation of audio compressor operation, including attack and release time at Wikipedia:
https://en.wikipedia.org/wiki/Dynamic_range_compression

 

[Eshal]

Thanks dear. But I don't want to go in deep with compressor circuit. Can't you be specific with me?

 

[betwixt]

As simply as I can describe it:

A compressor is a circuit that tries to leave low level audio untouched but as it gets louder it progressively turns the gain down. Because it reduces the peaks, the average can be increased without the maximum level being exceeded. In SSB and SSSC systems, the power envelope is proportional to the modulation level so compressing the modulation signal first allows you to get more average power out of the transmitter.

Brian.

 

[Eshal]

OK nice description. It is enough for me. Thank you guys.
Regards,
Cute Princess. :-D

 

[BabaYaga]

@Eshal,
If you are interested in audio based circuits and design then i would suggest you to go to the below forum
https://www.diyaudio.com/index.php
This forum is dedicated completely to audio, and so you might actually get what you need, its a really good forum for audio.

 

[Eshal]

OK thank you friend. Well are you there at that forum?