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 Post subject: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 6:34 pm 
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I'm not really a HAM hobbyist so don't tend to come across any receiver that uses S.S.B. What I do know is it filters out the carrier wave and thereby allows the sidebands to be transmitted with more efficiency.
The other night I read the section in my 1960 AARL handbook that related to S.S.B. I thought the explanation was a bit brief but the book does refer you to some 1950's magazine articles that explain the system in more detail. Where they will be now is anybody's guess.
Going by the diagram provided by AARL, I can only deduce the bridge diode system shown works roughly as follows. RF is often fed to the diode bridge in parallel whereas the AF input is in series and uses push-pull (to bias the diodes). The book states the RF fed to the diodes can't emerge at the output but doesn't explain this in any detail, except to say the diode forward resistances are all equal. So, looking at the diagram, I am assuming there are voltage drops that are equal and cancel each other (opposite polarity) when the RF carrier is fed to the circuit. I assume this because the diodes allow and block positive and negative cycles of the RF. The book is kind of silent here, though.
Finally it states the AF biases the diodes which then allows RF to pass through to the output.
The most helpful diagram in AARL was one showing a tube circuit for the diode system.
I find it sort of hard to digest an AF frequency being transmitted without a carrier. I will grant it's clearly a clever idea. So much power is lost in the carrier.
Does anyone often use S.S.B. here and would you say the range is far superior to normal plate modulation?


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 6:57 pm 
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10.7Megahertz wrote:
Does anyone often use S.S.B. here and would you say the range is far superior to normal plate modulation?
That's ARRL (American Radio Relay League), not AARL.

Sideband is the norm for modern HF operators, both amateur and commercial.
It generally gives longer range for a given power level, but that can vary.

You can find some AM operation on the ham bands, but only a small percentage.

- Leigh

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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 7:40 pm 
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Google is your friend here, I've found several PDFs online in the past and there are several schemes for SSB generation. Two that come to mind online are w9oka-ssb and Single Sideband for the radio amateur ARRL 1970.
Back in the 50s it appears much was written to explain SSB to hams.

Another great reference is a book on the subject by Don Stoner - it discussed SSB schemes relative to the equipment available back then- a great reference for us boatanchor enthusiasts.

I have not yet been on AM; however with my mediocre 100 watt 50-year-old Swan I've had SSB contacts on 20 meters from my NJ shack thousands of miles to as far west as Flagstaff and east to Finland. I know that doesn't answer your question re: Range - there are many other factors involved, atmospheric, antenna config, etc. and you'd have to hold those equal to compare. Although I'm a relatively new ham, most of us think about watts and other characteristics rather than miles.

The very best way for you to find out is to take the test and get on the air :)

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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 9:01 pm 
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There were many schemes in the past for producing SSB because the simple methods we use now were inhibited by the cost of suitable crystal filters.

Once the filters became cheaper then it was quite a simple matter of a balanced modulator to get rid if the carrier followed by the filter to remove the unwanted sideband - a bit of frequency jiggling allowed you to remove the upper or lower sideband.

However a supply of surplus 9MHz crystal filters early on meant that the simplest methods resulted in the production of LSB below 9MHz and USB above - hence why hams stick with LSB in the lower bands although it's not strictly necessary now.

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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 9:08 pm 
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In general the two methods of producing SSB were 1.) using phasing to null out one of the sidebands and 2.) using a filter to only allow one sideband to progress down the signal path. Besides using less bandwidth in a sometimes crowded band segment for phone, and requiring less power for distribution, a big advantage to single sideband is that because it uses less bandwidth, there is less noise. With amateur band segments of 125 KHz for General class licenses, a 10 KHz am signal would only allow 12 or a few more operators at any one time. A 3 KHz SSB signal offers a lot more room for stations to participate on crowded bands.

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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 9:39 pm 
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For the question of range, my take is that so much of a comparison of SSB to AM depends on the equipment and the operator. For weak signal listening, most of the time I can hear the SSB station better. But a lot of that has to do with how Hams generally run their AM rigs. They sacrifice "punch" for smooth,wide range, clear audio. Overall level of modulation is lower. However, if that same operator decided to slap on an amplified D-104 mic with a more in the middle frequency response and crank it like most CB'ers he could be heard mostly as well as the SSB signal.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 9:57 pm 
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AM and SSB are both forms of amplitude modulation but out of habit hams often talk of AM vs SSB.

In both cases you are superimposing audio information, via amplitude variation at a rate in the audio frequency rate, on a RF signal. In the classic high level full carrier plus two sidebands AM transmitter this modulation occurs in the final amplifier. The amplifier is excited by RF energy and the voltage feed to the plate is varied in amplitude at a rate and level determined by the audio signal. So with a classic AM transmitter you have steady RF drive and vary the output via modulation of the voltage feed to anode (and screen grid with tetrode tubes). The result is a carrier and two sidebands. If you have a transmitter using a static high voltage supply of 1,000 volts and supply a 1 Khz. audio signal to the modulation section of this transmitter at a level sufficient to fully modulate the transmitter on modulation peaks (maximum positive amplitude of the modulating signal) the plate voltage will increase to 2,000 volts on peaks and the total output on peaks will be 4 times the unmodulated carrier level. In this setup the modulation transformer secondary is in series with the HV supply so that it alternatively increases and decreases the voltage source for the final as the modulating audio waveform changes in amplitude. If this transmitter is operating at a RF frequency of 3,800 Khz. without modulation there will be a single pure output frequency of 3,800 Khz. One the 1 Khz. modulating audio is supplied the 3,800 Khz. "carrier" remains and there are two new outputs at 3,799 and 3,801 khz. from the results of the modulation process. With complex speech waveforms there will be a near continuous spectrum of output centered around the carrier within the limits of the speech frequency and transmitter passband with the frequency and amplitude of the output based upon the frequency and amplitude of the speech components.

In the most commonly used "filter method" for generating SSB the modulation occurs within a balanced mixer referred to as a balanced modulator. Inputs to the mixer are the RF excitation frequency and the audio information and the output is the RF modulated by the audio frequency or again a spectrum of RF based upon the amplitude and frequency of the speech input. The balanced mixer design cancels the discrete RF excitation used as the input source and the modulating AF input is so far removed from the desired output frequency that it is typically fully rejected by the output coupling network of the mixer. So the output of the mixer is basically two sidebands and if you apply the same 3800 khz RF and 1 Khz audio from the above example to this balanced modulator you will end up with two sidebands of 3,799 and 3,801 and a greatly attenuated carrier at 3,800 Khz. Since balanced mixers aren't perfect some of the original RF excitation leaks through to the output. This is followed by a filter to strip one sideband and further attenuate the carrier. In this case if you wanted to transmit an upper sideband signal you would choose a filter with a -6db bandpass ranging from around 3,800.30 to 3803.00 which would provide for a recovered audio range of 300-3000 hz while putting the original carrier frequency down the slope of the filter providing further carrier suppression.

As Norm noted the other commonly used approach was the phasing approach which traded the expense of the crystal or mechanical filter for a somewhat more difficult method of splitting and phase-shifting the RF and audio inputs components used in modulation so that when combined the desired factors for output were summative while the undesired canceled. Either system works very well but it was easier to achieve and maintain excellent carrier and sideband suppression with the filter method and it became the most popular.

With both the filter and phasing method the modulation typically occurs at a low level stage in the transmitter and then all amplification of the signal then on must be done with linear amplifiers and any frequency conversion must be done with mixing rather than multiplication. Of course as you probably noticed from the previous posts AM modulation itself is a form of mixing where AF and RF frequencies are mixed together to generate the desired output and then all that is left is to removed the undesired products of the mixing process. In a traditional high level AM rig the final amplifier itself serves as a mixer.

Since all of the modulating energy is concentrated in the useful information containing "sidebands" the SSB system is more effective in use of its effective power by not wasting energy transmitting the carrier. With the proper demodulation setup a DSB (double sideband system without carrier) is as effective as a SSB system except it is very wasteful of spectrum space and can be a bit more difficult to tune compared to SSB. Another system, briefly popularized by GE, transmitted a small pilot carrier so that the receiver could easily synchronize with the transmitter taking away the complaint of tuning difficulty that some had when comparing SSB to full carrier AM. Neither DSB nor the reduced/pilot carrier system gained popularity and the major benefit of DSB was it was simple and cheap to generate compared to SSB since it is far simpler to remove the carrier than removing one of the sidebands that results from the modulation process.

And getting rid of the carrier allowed a light transformer and relatively small tube to produce much higher levels of RF output since the duty cycle of normal voice modulated SSB is much lower than AM with its continuous carrier. Some novice class transmitters came out with "controlled carrier AM" where the average carrier level varied with the modulation; sort of an AGC for the carrier. This allowed a simple AM system avoiding expensive heavy power supplies and modulation transformers but some don't care for it because it produces a continuously varying S meter level at the receiving end unlike traditional AM when there is no path fading occurring. Personally I have heard some excellent sounding setups using this controlled carrier system which is also very compatible with the typical light duty linear amplifiers that became common with SSB. But in terms of spectrum usage double sideband AM with carrier is wasteful of spectrum no matter how it was generated and the "squeal" of the beating between competing carriers is one of the more annoying sounds of a crowded band of AM signals.

With full carrier AM you also run into an issue of selective fading where the slight frequency difference between the carrier and audio sidebands allows propagation conditions to change the relative amplitude balance between the two results in distortion although a good synchronous detection overcome is helpful in this since like the typical SSB detector it uses a locally generated carrier for demodulation.

I operate a lot of vintage AM because I like the gear of the era but SSB replaced it for good reason with better power and spectrum efficiency and ultimately greatly reduced equipment cost.

Rodger WQ9E


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 09, 2018 10:40 pm 
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Modern generation of SSB is done on a computer. The audio is digitized just as
you would when recording music, the sampled audio is "upsampled" to a few MHz,
digitally filtered to remove the carried and the undesired sideband, and then
fed to an analog to digital convertor. At HF this could be at the actual transmit frequency,
otherwise its upconverted by same analog circuits as it was in the days of tubes.
Dedicated transceivers have the computer built in.

Sometimes the digital equivalent of phasing is used for the filters.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Sat 10, 2018 3:40 pm 
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As Roger pointed out SSB is a form of AM. That is also true of
CW (morse code).

The simplest explanation for how it works is based on full blown AM.
This consists of a carrier and two side bands. One is above the carrier
and one below the carrier seperated by the modulation frequency.
In other words, if your carrier is 1 MC and a modulation frequency
is 1 KC the sidebands will be at 999 and 1001 KC. Simple enough.
Another important thing to consider is in true AM one half of the
transmitted power is in the carrier and 1/4 in each sideband.

SSB, no matter how its derived, eliminates the carrier and one sideband.
If you listen to a SSB signal on an AM radio you won't be able to understand
what is being transmitted. However if a BFO is used in the receiver, which
in effect re-inserts the carrier so the AM detector works properly, all
is well.

One reason SSB is far more efficent than AM is the transmitter can
put ALL of its output power into one sideband. Thats a 4 times the
effective power. Also the available power is concentrated in a smaller
bandwidth.

Now to confuse things even further.... It is possible to transmit two or even
three seperate channels of information within the bandwidth used by
a classic AM signal. This is called ISB or independent sideband. Primarily
used by the military and commercial stations. And its just an extension
of SSB as used by hams. However each sideband is modulated independently.
Taken one small step further, a supressed carrier can be introduced
and modulated as well.

As it turns out I just completed the restoration of a commercial receiver
that does this. Its the RCA R3, circa 1958 or so. RCA used them
for point to point service (SW) as did the VOA. In both cases two
independent program channels were transmitted and the supressed
carrier was used at the receiver to keep the receiver tuned (exactly)
to the transmitter via an AFC circuit.
Steve

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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Sat 10, 2018 10:47 pm 
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The best, most detailed and comprehensive book about SSB was issued by Collins Radio (the main promoters of SSB) in 1957 under the tittle "Fundamentals of SSB". A rare and collectible (read expensive) book, it is now available for free download on the Collins Collectors Association website:

http://www.collinsradio.org/cca-collins ... side-band/

This thick technically-oriented book goes very deeply into the topic but the 1st chapter about the basic principles of SSB, how it works and compare to standard AM is the clearest and most understandable description you may find. A highly recommended read.

ARRL also issued a book dedicated to SSB under the tittle "Single Sideband for the Radio Amateur" which is also very complete and includes many schematics and practical circuits to build by the amateur. Written in the classic "ARRL style" (very likely a compendium of previously published articles) there were many editions (well into the 1970ies) , most of them are now widely available for free downoad on the web.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Sun 11, 2018 2:42 am 
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Greetings to 10.7Megahertz and the Forum:

I put together a power-point presentation on how SSB works for the local ham club. Lots of good pictures. The Power-Point notes for each slide form a reasonably good explanation of SSB to acompany the pictures and diagrams.

If you would like a copy (or anyone else) PM me with your E-mail address and I will send it to you.

Regards,

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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Wed 14, 2018 5:29 pm 
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majoco wrote:
There were many schemes in the past for producing SSB because the simple methods we use now were inhibited by the cost of suitable crystal filters.

Once the filters became cheaper then it was quite a simple matter of a balanced modulator to get rid if the carrier followed by the filter to remove the unwanted sideband - a bit of frequency jiggling allowed you to remove the upper or lower sideband.

However a supply of surplus 9MHz crystal filters early on meant that the simplest methods resulted in the production of LSB below 9MHz and USB above - hence why hams stick with LSB in the lower bands although it's not strictly necessary now.

"There were many schemes in the past for producing SSB because the simple methods we use now were inhibited by the cost of suitable crystal filters."
The simplest I saw was where a tube was used for the diodes. Of course, it usually helps when the books add a bit more detail and I never quite understood why they don't do this. It stated the forward resistances of the diode bridges are equal and that the RF is in parallel but no RF is able to reach the output. So, naturally, I traced the path from the RF coil to the diodes and investigated where the + and - waveforms would be blocked or passed. The basic idea is the bias from the AF signals is fed to the diodes and allows them to conduct.
At the moment I decided to go over old ground so I've been looking at good old plate AM modulation. This was evaluated as pretty efficient but expensive. Even so, the carrier wave does require power, which is gotten around when it comes to S.S.B.
In fact, all the various possibilities for amateur modulation are addressed. Not that I'm a Ham operator but I figured the easiest way seems to be grid modulation. Not that much power efficiency in watts but probably cheaper and less complex to adjust.
I should point out I hardly read anything modern. All my textbooks are either sixties or even earlier.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Wed 14, 2018 5:48 pm 
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zarco wrote:
As Roger pointed out SSB is a form of AM. That is also true of
CW (morse code).

The simplest explanation for how it works is based on full blown AM.
This consists of a carrier and two side bands. One is above the carrier
and one below the carrier seperated by the modulation frequency.
In other words, if your carrier is 1 MC and a modulation frequency
is 1 KC the sidebands will be at 999 and 1001 KC. Simple enough.
Another important thing to consider is in true AM one half of the
transmitted power is in the carrier and 1/4 in each sideband.

SSB, no matter how its derived, eliminates the carrier and one sideband.
If you listen to a SSB signal on an AM radio you won't be able to understand
what is being transmitted. However if a BFO is used in the receiver, which
in effect re-inserts the carrier so the AM detector works properly, all
is well.

One reason SSB is far more efficent than AM is the transmitter can
put ALL of its output power into one sideband. Thats a 4 times the
effective power. Also the available power is concentrated in a smaller
bandwidth.

Now to confuse things even further.... It is possible to transmit two or even
three seperate channels of information within the bandwidth used by
a classic AM signal. This is called ISB or independent sideband. Primarily
used by the military and commercial stations. And its just an extension
of SSB as used by hams. However each sideband is modulated independently.
Taken one small step further, a supressed carrier can be introduced
and modulated as well.

As it turns out I just completed the restoration of a commercial receiver
that does this. Its the RCA R3, circa 1958 or so. RCA used them
for point to point service (SW) as did the VOA. In both cases two
independent program channels were transmitted and the supressed
carrier was used at the receiver to keep the receiver tuned (exactly)
to the transmitter via an AFC circuit.
Steve

"The simplest explanation for how it works is based on full blown AM.
This consists of a carrier and two side bands. One is above the carrier
and one below the carrier seperated by the modulation frequency.
In other words, if your carrier is 1 MC and a modulation frequency
is 1 KC the sidebands will be at 999 and 1001 KC. Simple enough.
Another important thing to consider is in true AM one half of the
transmitted power is in the carrier and 1/4 in each sideband."

I agree with you. The basic plate modulation system takes some getting into. At the time of vintage Ham it seemed to be classed as a more expensive option presumably because you needed a good deal of power to drive the RF amplifier. The sidebands are added to the RF amplifier in such a way as 100 watts RF would become 150 watts (adding the sidebands). So, looking at it as a pessimist, a whole 100 watts is just being transmitted a bit like a surf wave that carries the surfer.
The voltages appear to be pretty hefty in plate modulation.
For ages I never really understood about grid current and class C RF amplifiers. I never got my head around how they so often write "with class C amps, grid current flows". It was only 2 days ago I was reading up on plate modulation and class C amps and it simply stated the grid can be driven very positive by strong signal voltage and it then acts a bit like a plate in relation to the cathode. That is, electrons flow from cathode to the grid and out. I'd always thought they were talking about grid leak!
Anyway, from the old book I have usually they have a class C RF amplifier and the drivers and modulators. There's a winding that couples the sideband wattage to add to the RF amp. I have no idea how much it would cost to make such a system today. Back then it was classed as expensive but nowadays I cannot say.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 16, 2018 4:35 pm 
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Very interesting information on grid modulation in my ARRL, 1960 textbook. It uses an example of the RF amp with a plate power dissipation of 55 watts (2 tubes used to get 110 watts total). Yet when the modulation process is applied through grid modulation, it's explained how you wind up with a mere 33 per cent carrier efficiency in watts. In other words, the carrier sacrifices much of its wattage as the sidebands are added. Of course, things may have changed since 1960. If I understand correctly, though, grid modulation in the 1960's was maybe easier to set up for the amateur cost-wise. No need for the huge voltages often used in plate modulation.
S.S.B., of course, greatly improves upon the power limitations you get with conventional modulation. In the course of the weeks I'll be revising modulation in all its formats and that includes S.S.B.
Anyone who wants to add their own diagrams or comments on S.S.B. or experiences feel free to chip in.
By the way, FM modulation was also addressed and this appeared to be not so popular in the sixties mainly because the power losses to the carrier appear to be a bit more than with grid modulation. However, the book shows a basic reactance modulator and states such a system was not too difficult to set up.
Bear in mind too my information is always a bit dated as I tend to only read retro books. Things must have moved on a great deal since 1960.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 16, 2018 4:55 pm 
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Not much has changed since the 60s in terms of classic grid modulation efficiency. It is a low cost way to get AM but the usable output from a given tube will be considerably less compared to plate modulation.

The old ARRL "Understanding Amateur Radio" publications from the same era as your 1960 handbook do an excellent job of providing a simple explanation of the various methods, benefits, and drawbacks of accomplishing AM modulation. For a more in-depth view the West Coast, later referred to as Orr, handbooks are a step up in quality from the ARRL handbooks although the ARRL books from that era were quite good. And another step up is the excellent series of Terman Engineering handbooks. You can find many examples of these in PDF form for free download from American Radio History and other sites.

I believe the best setup for getting a usably powerful AM signal out of a low cost and light weight transmitter is some variety of controlled carrier modulation as used in rigs such as the popular Heathkit DX-60, Knight T-60, Drake 4 line, etc. It is very easy on the power supply and tubes, is very compatible with the duty cycle capabilities of typical external linear amplifiers, and can be received with a normal AM receiver. The carrier level does vary and is noticeable if one watches the S meter or listens to the quieting (or lack thereof) of background noise with weaker signals but it can provide excellent audio quality and is a much better choice for lower cost gear than a basic grid modulated system. It is also very easy to set up unlike some of the classic efficiency modulation systems.

For more reading fun take a look at the various "super modulation" systems using separate "carrier" and "peak" tubes, the Doherty system, and the somewhat bizarre and difficult to adjust RCA "Ampliphase" system.

Today various "switch mode" modulation techniques are used to provide very high efficiency using solid state devices with no big modulation transformers, high voltage supplies, etc.

But it is a lot of fun operating a classic plate modulated rig. When I was doing initial testing of my Johnson Desk KW during its restoration I nearly gave myself a heart attack due to the noise of the very loud relay used to short the modulation transformer when used in CW mode. When you are watching, sniffing, and listening for trouble a sudden very loud CLACK gets your attention :)

Rodger WQ9E


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 16, 2018 6:08 pm 
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"I nearly gave myself a heart attack due to the noise of the very loud relay used to short the modulation transformer when used in CW mode. When you are watching, sniffing, and listening for trouble a sudden very loud CLACK gets your attention :)"

I once measured voltage between the HT can - terminal and chassis on a 1950s radio. There was a resultant loud crack like a firework. Scared the hell out of me. This radio had spent decades in a farmer's barn and I was working on it to get it operational again. I ended up bypassing the HT can and fitted a new one.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Mon 19, 2018 6:27 pm 
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I've had some interesting nights. I came to the conclusion that maybe the easiest way to build a modulator is the grid modulation system using a triode as RF amp. I guess many here will know screens or suppressor grids can be used as well as just the regular grid.
As yet I have no immediate plan to actually build a transmitter but I did find it challenging to start to think about how I would do it (just using the ARRL).
O.K., for a start, the grid modulation system is only 33 per cent efficient. So, by comparison with SSB it's a poor substitute. However, what did intrigue me was there was an 8 watt modulator shown that would easily drive a triode RF amp in grid modulation. I think it used a split phase driver. And the circuit seemed very typical of triode systems.
Someone here has probably used this kind of transmitter and my guess is it would have had its limitations.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Tue 20, 2018 5:27 pm 
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Noticed something about the info given in ARRL. The most detailed examples in my 1960 book on modulators tend to explain only the class B type and it's stressed the bias must be from batteries. The reason given is that grid current flows and the grids are driven into positive values. I'd just assumed all modulators followed this pattern. That is, class B with fixed battery bias. Then, last night, I noticed they had a circuit for a low power modulator. It was classed as ideal for grid modulation. I then happened to notice there were two 6AQ5A in push-pull and the bias was simply through resistance. I just Google searched and figure it's a class AB1. At this stage I guess there are a few loose ends I have to tie up. Also, I thought all class C RF amps required class B modulators?
Of course, in those days in the 1960s these ARRL books would have been used in classes I guess and the students would have asked the engineering teachers.


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Tue 20, 2018 9:51 pm 
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Greetings to 10.7Megahertz and the Forum:

It might be a good idea to go back and review classes of operation and what they mean. The idea when amplifying audio for the purpose of modulating a transmitter is to produce audio of sufficient power to do the job with minimal distortion. How one goes about this is dependent purely on one's preferences. It is perfectly feasible to have a class A amplifier to provide the modulation power... and it would sound great. It is also quite wasteful of power and requires the largest investment in power supply, tubes and other components because of the low efficiency and the higher DC power required to overcome this inefficiency.

Classes Ab1, Ab2, etc. are modifications of the bias point for what is essentially a class A amplifier. They give up a certain amount of distortion in order to get better efficiency with the accompanying lower cost.

Class B amplifiers are biased at cutoff. Therefore, they can amplify only 1/2 half of a sine wave; the other half (which makes the grid even more negative) is lost. The resulting distortion is severe. However, by employing two tubes in a "push-pull" configuration, each tube amplifies 1/2 of the sine wave... one amplifies the positive going half while the other amplifies the negative going half. In this way, one can realize the efficiencies of class B (no idling current) and still have relatively low distortion. Note that one can bias a push-pull amplifier as any of the variants of class A if one wishes; but the trade-offs are distortion versus efficiency. Since most tubes are a bit non-linear just coming off of cut-off, a pure class B push-pull amplifier will have a bit more distortion than one that is biased so as to have some idling current.

Class C has the tube biased well beyond cutoff and cannot amplify a sine wave without severe distortion. The output of a class C amplifier is a series of pulses representing the most positive peaks of an input sine wave. Thus, a class C amplifier is wholly unsuitable for audio. However, when a class C amplifier has a resonant "tank circuit" as its output circuit, the "flywheel effect" of a resonant circuit will reconstruct the sine wave from a series of pulses, just as a child's swing describes a smooth motion even though an adult pushing the swing applies only brief pulses of power at only one part of the swing's travel.

Note that this effect occurs at a single frequency (or very narrow band of frequencies) where the tank circuit is resonant. Since we are transmitting RF at a single frequency, this works fine. Audio, however, represents a frequency variation of several octaves and cannot be reconstructed by a resonant circuit, so class C is strictly RF.

So, the class of operation of a modulator stage is completely independent of the class of operation of the stage it is modulating.... it depends only on amplifier configuration... a choice of the designer.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Wed 21, 2018 1:39 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 699
Location: Tokyo
Three of the ARRL SSB handbooks (1954, 1965, 1970) are available here:

http://www.acrayara.com.ar/cms/RM?actio ... braryFiles

The articles were taken from QST and are fascinating to read, even if you dislike SSB (as I do).

The above site belongs to Acrayara, an Argentine radio society. Most of the books are in English, but many simply duplicate what is available elsewhere.

Rob


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