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Power control of Class D Rf AMP

by dnyberg2
Tags: class, control, power
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Averagesupernova
#19
Dec3-12, 07:00 PM
P: 2,537
Quote Quote by dnyberg2 View Post
It is more like ask but the coding scheme keeps the duty cycle at 50 percent minimum all the time
I don't understand what you are saying here. What is ASK?
dnyberg2
#20
Dec3-12, 07:04 PM
P: 69
Amplitude Shift Keying.
dnyberg2
#21
Dec3-12, 07:05 PM
P: 69
Because the data keys the RF square wave on and off at the data rate it acts more like ASKing
Averagesupernova
#22
Dec3-12, 07:13 PM
P: 2,537
I still don't believe that the RF is turned completely off. I suspect it is AM. I have never heard of anything referred to as Amplitude Shift Keying. Can you see on a scope what this signal looks like after its filtered?
dnyberg2
#23
Dec3-12, 07:18 PM
P: 69
Looks like modulated RF with a peak at plus and minus 1 MHz around the carrier on a specan...
Averagesupernova
#24
Dec3-12, 07:27 PM
P: 2,537
A CW signal would look similar on a spec an but it would be alot noisier than AM. That is why I don't think it is CW. I don't think a class C amplifier will work for you. When you look at it on an OSCILLOSCOPE, not a spectrum analyzer, does the RF go to zero at any point or does the amplitude just get lower?
dnyberg2
#25
Dec3-12, 07:34 PM
P: 69
Let me run that test in the morning. I'll get back to you. And thanks for your patience and help!
Averagesupernova
#26
Dec3-12, 07:41 PM
P: 2,537
Not sure if you have fully understood what CW (continuous wave) is. It SOUNDS like a carrier on all the time with no change. It is not. It is a carrier that is switched on and off like morse code. Generally not fast. Every time the carrier is turned off or on it generates extra noise in the spectrum around the carrier. Anyway, just thought I'd throw that in. I'll be waiting for your results.
dnyberg2
#27
Dec4-12, 06:14 AM
P: 69
Averagesupernova, As a one time ham, I know CW intimately. Attached are some screenshots from my RF source. The output is turned way down and I used a 100 MHz digital scope to capture these with the 50 ohm termination turned on. There is some proprietary info in here so the scales have been omitted on purpose, but trust me, this is ~50 MHz.
Attached Thumbnails
ScreenHunter_03 Dec. 04 04.07.gif   ScreenHunter_04 Dec. 04 04.08.gif   ScreenHunter_05 Dec. 04 04.08.gif  
Averagesupernova
#28
Dec4-12, 10:08 AM
P: 2,537
As a one time ham I am surprised you needed to ask this question. Looks to me like class C will work for you.
dnyberg2
#29
Dec4-12, 10:22 AM
P: 69
Hey! I was a kid! ;-) Now, trouble is, correct me if I'm wrong, but doesn't class C have bad efficiency? Maybe a class D with the switcher for the supply is better?
Averagesupernova
#30
Dec4-12, 07:05 PM
P: 2,537
Sigh................
No, class C is the most efficient of RF amplifiers. Class D is NOT used at radio frequencies. I am not sure you understand what a class D amp does. A class D amplifier turns the output either full on or fully off so very little power is ever dissipated in the transistor. Suppose we have a switching frequency of 100 Khz and the duty cycle is varied and the output is low-pass filtered. If the duty cycle is varied at 100 Hz, a signal of 100 Hz comes out of the low pass filter. Class D can be used in switch mode power supplies, but you still need the RF amplifier in your case. The RF amp and the class D switcher are two seperate parts.
dnyberg2
#31
Dec4-12, 07:33 PM
P: 69
Okay, so why do the architects of the system I'm suffering with use the term class D?
There is a fixed 50 MHz source that is being turned on and off (OOK) at the data rate. I have seen the schematic on a white board. Its two or four fets in series on a die whose power supply is controlled by the programmable switcher I told you about... I've heard the term Class D power osc flung around as well...
Averagesupernova
#32
Dec4-12, 08:22 PM
P: 2,537
Probably because they started with a square wave generator which technically would be class D and filtered the hell out of it. Class C is closely related to class D but C is made to run at a resonant frequency. There is a parallel tank circuit in place of the collector resistor. I have no idea why they would run all the way out to the last amplifier stage with a square wave. Good engineering practice would be to filter as soon as possible in the signal chain. I guess I wouldn't be too surprised if what you have is actually class C but for whatever reason is mis-named.
Greg-ulate
#33
Dec4-12, 10:41 PM
P: 72
So you have a On-Off-Keyed 50MHz signal switched on and off at 1MHz and you want a adjustable gain amplifier that is as efficient as possible to step up the power?

A fixed gain amplifier wont work for you because you have found it necessary to vary the output of your current amplifier by adjusting its power supply voltage, correct?

I'm looking at the diagram for the Class C amplifier on wikipedia. Would varying the value of the collector resistor change the gain of that circuit?


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