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Author Topic: Supressed carrier "Simple Sidebander" qu  (Read 3464 times)
KC9KEP
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« on: April 10, 2009, 07:06:53 AM »

Hello all,

I’ve been considering building one of several SBB transmitter tube rigs for the sake of fun & learning.
One design that I’m considering, "The Simple Sidebander", was published in June 1962 of Popular Electronics magazine.
The author was Hartland B. Smith, W8VVD.
It transmits on 80 and 40 and uses three tubes; 12AX7, 6AR8 and 6AG7 and features a separate power supply using a 5U4.
Hartland was able to make minimize the complexity of the circuit by allowing both sidebands to be transmitted and suppressed only the carrier.
Can anyone tell me what the disadvantages to creating sideband transmission in this fashion are?
I would assume that this method occupies more bandwidth than is necessary and uses up transmitter power to create an un-needed sideband.
Will anyone complain that I’m sending both upper & lower sidebands if I use this design to transmit?

Thank you & 73’s!

--KC9KEP
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AA8LL
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« Reply #1 on: April 10, 2009, 07:14:44 AM »

Unless you're driving a KW amplifier in the DX window, most won't notice unless you tell them.  Yes, "someone" will complain.  Someone complains about everything, in case you didn't notice.  Complaining is one of the most popular aspects of amateur radio.  (And I'm not complaining about it, nope!) You might as well do what you want as long as it is legal.
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KB1LKR
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« Reply #2 on: April 10, 2009, 07:15:09 AM »

It would occupy as much bandwidth as an AM signal (but for a narrow slot at the carrier -- maybe you could sneak a PSK31 signal in there <grin>), but would tune like an SSB signal.

Could you build it as an AM transmitter, leaving the carrier, and making it simpler still?
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WA3SKN
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« Reply #3 on: April 10, 2009, 08:09:33 AM »

What you are talking about is "double sideband" modulation.  I don't know why you would want to build one, though.
The AM guys will complain, the SSB guys will complain, and you have don't minimize bandwidth much... plus they still have to re-insert the carrier.
Why not build something useful?

-Mike.
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G3TXQ
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« Reply #4 on: April 10, 2009, 08:29:54 AM »

I built a 160m DSB transmitter like that back in the 1960s as a first step on the road to SSB. It used two 807s in a push-pull PA. The 807s were driven in parallel by the RF carrier, and had audio applied differentially to the screens. As others have said, you might get criticism these days for using twice the bandwidth necessary.

Why not try the "Imp" design that featured in the May 1960 edition of QST. It was a 3 valve SSB transmitter using a 2-crystal half-lattice filter:

V1 6U8A Audio amp + carrier oscillator
V2 6U8A Mixer + VXO
V3 6CL6 PA

Balanced modulator was a pair of 1N34As.

Fuller details at:
http://p1k.arrl.org/cgi-bin/topdf.cgi?id=45475&pub=qst

Steve
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KC9KEP
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« Reply #5 on: April 10, 2009, 11:01:54 AM »

Thanks everyone for all the ideas.

And, I’d agree that the Simple Sidebander would not be very “useful” ..

The “Imp” looks very interesting.  Very nice design :-)  The only “problem” is that I’d need to build a final for it (which I may just do).

A couple other designs that I found compelling:

June 1967 QST – “A 50-Watt P.E.P. Output Transceiver for 75”.  It’s actually a mobile rig and .. I don’t need the receiver.  I guess I could just build the xmitter portion.  It features a 12AT7 Balanced Modulator (and I’d rather us the 7360 beam deflection tube) and a 6146B for output.

December 1973 QST – “How to Build an SSB Transmitter”.  (or “50 Watts on 15 Meters”)
It also uses a 6146B for output & features a pi filter.

I do have some crystals for the Jun 1967 design, but need to do a parts hunt for the 1973 design.

Any familiarity or comments about these two designs?

73'S!

--KC9KEP
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VK1OD
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« Reply #6 on: April 10, 2009, 01:22:33 PM »


One of the attractions of DSBSC (as it was known) was that of a very simple transmitter, based on a high level balanced modulator.

A pair of 6146s might deliver around 50W PEP DSBSC. 100% of the transmitted energy contained information. The necessary bandwidth is twice the highest modulating frequency.

The DSBSC signal could be received on an SSB receiver which filtered out only one sideband. Otherwise, for good results, a synchronous detector was required.

The SSB receiver was the most common solution, and yes, only half the received power was used... so a 50W DSBSC tx gave similar S/N to a 25W SSBSC tx in that case.

By comparison, a plate and screen modulated 150W DC input, 100W carrier AM transmitter fully modulated also transmitted 25W of power for each of the sidebands, and delivered similar S/N on an SSB receiver.

Today, on crowded bands, the bandwidth might be about as popular as an AM station parking in a congested band segment.

Owen

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G3TXQ
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« Reply #7 on: April 10, 2009, 01:52:22 PM »

>>VK1OD wrote: The SSB receiver was the most common solution, and yes, only half the received power was used... so a 50W DSBSC tx gave similar S/N to a 25W SSBSC tx in that case.<<

Owen,

If I remember my modulation theory, that's correct if you're comparing on the basis of average powers. But if you compare the techniques on the basis of PEP (probably the more practical power limit) the difference was even more pronounced i.e.  [SSB 25W PEP] produces the same S/N as [DSB-SC 100W PEP] when received on an SSB receiver.

Or was I asleep at the back of class again Smiley

Steve
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KB4QAA
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« Reply #8 on: April 10, 2009, 02:51:00 PM »

It seems like an easy fix for the Simple Sidebander would be to add a Low Pass filter that strips the USB and passes the LSB.  

A couple of components located immediately after the modulator would be the obvious locaction.  But I havent' seen the schematics.

Sounds like a great project.  I don't think it would meet the spirit of current engineering practice to transmit using DSB.
Bill
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KC9KEP
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« Reply #9 on: April 10, 2009, 03:22:57 PM »

Bill,

From the June 1962 Popular Electronics page 54 is
the schematic of the Simple Sidebander:

http://www.bignick.net/temp/SimpleSidebander.jpg

73's

--KC9KEP
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VK1OD
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« Reply #10 on: April 10, 2009, 03:28:54 PM »


Yes Steve, that was a bit confusing.

I should have used PEP for a clearer comparison, I should have said:

The SSB receiver was the most common solution, and yes, only half the received mean power was used... so a 100W PEP DSBSC tx gave similar S/N to a 25W PEP SSBSC tx in that case.

By comparison, a plate and screen modulated 150W DC input, 100W carrier AM transmitter fully modulated also transmitted 25W PEP for each of the sidebands, and delivered similar S/N on an SSB receiver.

Does that fix it?

Owen

PS: I played with one of these things, but as a kid at school, my budget didn't run to 6146s, but 6DQ6As pulled from TVs worked a treat on 10m during the height of cycle 21.
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W8JI
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« Reply #11 on: April 11, 2009, 03:38:35 AM »

Tom,

That is a fun project provided you use your head and don't get on crowded bands, especially near weak signals.

There are a few issues. One was already pointed out, it has both sidebands. A second issue is there is nothing that limits bandwidth of the transmitter except a frequency response and distortion characteristics of everything from the PA stage back to the microphone!!! This is actually a worse problem than the second sideband.

Normal filtered SSB transmitters use a good RF crystal or mechanical filter to limit bandwidth. It doesn't matter much how much distortion you have or what the mic response is, the filter cleans up the off frequency crap. As such, only the linearity of stages after the filter have a large effect on bandwidth.

Your system will pass everything including distortion in the RF stages right back to the audio input out o the antenna, so the bandwidth is almost certain to be very much worse than twice a filtered SSB rig.

It will be about like an old AM rig with low level junk extending out a considerable distance beyond what some might imagine.

This does not mean you can't have fun or learn something. It just means you have to be careful when and where you place the center of that signal. It probably is not a good idea to use something like that when activity is high or going to be high soon on a band, but it is loads of fun for everyone at other times. It is a lot like driving a 1966 Pontiac Gran Prix.

73 Tom
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G3TXQ
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« Reply #12 on: April 11, 2009, 10:26:26 AM »

>>VK1OD wrote: Does that fix it? <<

Owen,

Yes, I think so! I'm away from my notes at the moment decorating my daughter's cottage, but a quick look at the vector diagrams suggests the following S/N advantages:

1) Equal PEP. Signal demodulated as SSB.
SSB: Ref [100W]
DSB-SC: -6dB [25W+25W]
AM: -12dB [25W + 2 x 6.25W]

2) Equal PEP. Signal demodulated optimally.
SSB: Ref [100W]
DSB-SC: -3dB [25W+25W]
AM: -9dB [25W + 2 x 6.25W]

3) Equal average power. Signal demodulated as SSB.
SSB: Ref [100W]
DSB-SC: -3dB [50W+50W]
AM: -7.8dB [66.7W + 2 x 16.6W]

4)  Equal average power. Signal demodulated optimally.
SSB: Ref [100W]
DSB-SC: 0dB [50W+50W]
AM: -4.8dB [66.7W + 2 x 16.6W]

73,
Steve

 
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VK1OD
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« Reply #13 on: April 11, 2009, 02:21:04 PM »


Steve,

Your implied peak to average ratio is correct for a single sine wave at 100% amplitude modulation.

The numbers for AM change significantly under full telephony modulation where the average power in a sideband might be around 3% of that for the sine wave case.

For example, if the peak to average ratio for a typical telephony waveform is say, 30:1 (15dB), then the mean power in each sideband would be 0.8%, and the mean power of a fully modulated AM telephony wave with 100W carrier is 101.7W. The power limit for AM in VK is specified at 120WpY (mean power), which by the above explanation means 118WpZ (carrier) and 472WpX (PEP). It is difficult to understand why the limit is not specified as PEP (pX) which is much easier to measure than mean power (pY). (I have lobbied for a modeless limit of 400WpX (PEP).)

Curiously, DSBSC power limit in VK is 120WpY, which using the 30:1 peak to average ratio for voice, and applying the 2:1 factor for DSBSC, means that a telephony signal of of up to 7.2kW PEP would comply with the 120WpY limit. That would be a S/N advantage in an SSB receiver of 6.5dB over a 400WpX SSB telephony transmitter.

Owen

Owen
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G3TXQ
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« Reply #14 on: April 11, 2009, 02:37:01 PM »

Owen,

Yes, the numbers related to a single sinusoid modulating signal, and to 100% modulation in the case of AM.

I'm interested to hear of the mode-specific power limits in VK. Here in UK everything is specified as PEP. Interestingly a footnote to the power limit tables specifies PEP as:

"Peak envelope power is the average power supplied to the antenna by a transmitter during one radio frequency cycle at the crest of the modulation envelope taken under normal operating conditions."

So it looks like we can legally increase power to overcome any tuner and feedline losses Smiley

Steve
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