Why can this RF amplifier produce more output power in SSB than CW? - Page 1
Forum section: electronics / rf-microwave

author: Plasmateur
author_rank: Regular Contributor
timestamp: October 23, 2019, 04:34:02 pm
content: Hereis the amplifier I'm talking about. In fact alot of the amplifiers I see are able to supply more output power in SSB than CW. I'm not sure why this is.

author: fourfathom
author_rank: Super Contributor
timestamp: October 23, 2019, 04:45:04 pm
content: Probably thermal limitations. Voice modulation on SSB has a high peak/average ratio, while FM is constant envelope. A 600W FM signal probably has the same average power as a 1000W PEP (Peak Envelope Power) voice SSB signal. If you are using SSB to transmit a digital signal with a high average power (many digital modes use FSK) then the lower power rating probably applies even though the mode is SSB.

author: Bud
author_rank: Super Contributor
timestamp: October 23, 2019, 04:49:03 pm
content: I am thinking along same lines. They may give you different specifications for CW and SSB. PEP will look better from marketing perspective than average power. Make sure you use the same metric for both modes.

author: radiolistener
author_rank: Super Contributor
timestamp: October 23, 2019, 05:32:04 pm
content: you can take some 50 ohm 10W resistor and connect it to 600W output.If you apply short pulse 600W, the resistor can survive and will not be damaged.But if you apply 600W for several minutes. It will be burned out.The same thing with amplifier. It can survive from 1000W PEP.But if you apply continuous CW, FM or AM, it will be burned out.

author: m3vuv
author_rank: Super Contributor
timestamp: October 23, 2019, 08:50:08 pm
content: I think the words your looking for are "duty cycle" !.

author: CJay
author_rank: Super Contributor
timestamp: October 24, 2019, 11:23:24 am
content: Them's Italian watts, I.E. at least 25% bigger than the watts you get in any other country.The amplifier uses 4 SD1407 transistors, I encourage you to go and find the datasheet for those devices and then see what you think of their claims.

author: ebclr
author_rank: Super Contributor
timestamp: October 24, 2019, 05:58:59 pm
content: SSB have only 1/2 the spectrum, CW and Fw have the full spectrum, all the power can go to only 1/2 spectrum, not to the upper and lower band, This is quite obvious

author: Plasmateur
author_rank: Regular Contributor
timestamp: October 24, 2019, 06:49:41 pm
content: Quote from: CJay on October 24, 2019, 11:23:24 am...I'm going to guess and say...25% less than what they claim?

author: Plasmateur
author_rank: Regular Contributor
timestamp: October 24, 2019, 06:57:37 pm
content: Quote from: ebclr on October 24, 2019, 05:58:59 pmThe answer isn't obvious from your explanation though. It just leaves me thinking...why should power output limitations care about spectrum?But the previous explanation of...Quote from: fourfathomIllustrates what's happening with that power over time.

author: fourfathom
author_rank: Super Contributor
timestamp: October 24, 2019, 09:05:34 pm
content: There are (at least) two factors when looking at the power output capabilities of an amplifier.The first will be the peak output power, which is determined by the peak voltage and current that can be delivered instantaneously without clipping or excessive distortion. This might be specified as Peak Envelope Power. Voice over SSB has a high peak to average power ratio. Voice over AM has a lower peak to average ratio because of the constant carrier, so for a given power in the modulation sidebands the average power is higher than it is for SSB.The second will be thermal limits, and this is driven by the average (or RMS) power, and time, since it takes a while for the transistors and heatsinks to reach their high-temperature-limit thermal equilibrium. FM and constant-envelope data modes will have a very high average power, and so an amplifier with a typical heatsink will only support a lower average power.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: October 24, 2019, 11:30:46 pm
content: Spectral density of the signal and duty cycle of the amplifier are the important concepts here.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 25, 2019, 12:06:02 am
content: Quote from: ebclr on October 24, 2019, 05:58:59 pmActually, it is quitewrong.CW is a pure RF signal with no modulation, so ideally, it has zero bandwidth.SSB is a form of amplitude modulation, hence sidebands are produced, one of which is filtered out, along with the original carrier.The resulting signal will have an RF bandwidth closely corresponding to that of the modulating signal.FM is much more complicated as far as bandwidth is concerned, which is more concerned with the system frequency devation.An FM transmitter may be deviated to its maximum capabilities by very low audio frequencies, just as easily as high ones.Peak Envelope Power (PEP) is what the average power would be if the power available at the positive peak of modulation was extended for all time.PEP meters often use a "sample & hold" circuit to save the highest voltage levels, which are then displayed as Power readings in a given impedance (usually 50 Ohms.)A classic case of this is with an analog VSB TV transmitter, where, when the Tx power meter reads 10kW, the average power as measured using a water cooled test load, is around 5.9kW.The PEP of an analog TV transmitter is sometimes referred to as "sync tip power" to reflect this

author: CJay
author_rank: Super Contributor
timestamp: October 25, 2019, 06:00:13 am
content: Quote from: Plasmateur on October 24, 2019, 06:49:41 pmIt's actually about 17% less, my badThe SD1407 is a 28V 125W transistor so when they claim 4 of them are good for 600W CW at 24V, you can do the maths.Italian CB 'burner' manufacturers are rather prone to exaggerating using the absolute maximum ratings of parts and then adding a dollop of extra on top, RM are at least nice enough to offer up 'technical' manuals for their products which show up their claims to be 'fanciful'

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 25, 2019, 08:47:41 am
content: Worth noting that some amplifiers will have the oppposite happening if not limited thermally but by linearity. CW signals like FM and PM modulations have less need for linearity, since the envelope contains no information. As a result, you can operate an amplifier well into compression, where you will not only get more output power, but also higher power efficiency, and so sometimes you can actually get more power out of the same thermally limited package because the efficiency is far higher.In your case, the actual output power will be the same in both cases. Your peak power might be higher, but your average, which is still what defines the thermal load, will be the same.

author: w2aew
author_rank: Super Contributor
timestamp: October 25, 2019, 05:09:55 pm
content: You might find this older video of mine useful when thinking about peak and average power for different modes: