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SSB Carrier Supression

Discussion in 'General Electronics Chat' started by Space Varmint, Dec 3, 2008.

  1. flat5

    flat5 Member

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    My first receiver was a Navy BC457 (ARC-5). Then BC348.
    By the time I had the T-150 transmitter I was using a National NC183. In the car, I was 16 then, I had a Motorola 80D. First a 6 meter and then a 2 meter. The car was a 1958 Hillman Minx :)
    This was in Van Nuys, California.
     
  2. Space Varmint

    Space Varmint New Member

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    What did you do? Get them surplus? My first 2 meter was a Motorola HT-220. I had to pad down the oscillator and put new crystals in it. I used to fix them at the time. Along with all the other HT's Motorola made as well as Tactec (RCA) & Areotron and maybe a few others.

    edit:

    and pagers. We had the digital pagers but the first ones used to use reeds. Two tone audio frequency reeds that would un-squelch the pager.

    Ya know, I never had any luck trying to warp that crystal frequency down. Amazing! I can warp it up at least 5KHz and it remains locked by the crystal. The second I attempt to go below the frequency it grabs the natural frequency until I rotate the capacitor so far past, it will jump way down. Then I rotate it back up and as I approach the crystal's natural frequency You can look on the scope and see it phasing out where it looks like it is producing the crystal's natural frequency and a few other freqs in between the LC frequency.

    Must be the cut.

    May have to back up and do PLL.
     
    Last edited: Dec 18, 2008
  3. flat5

    flat5 Member

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    Hey! I had a HT220 too! In 1981 I was playing (sax) on the street in San Francisco on Powell Street. A security guard ran out of a department store and chased someone down the street. The trumpet player picked up the HT off the sidewalk and gave it to me after awhile. I had a job repairing Motorola communication equipment so was able to install repeater crystals and tune it up. Actually, I serviced maybe a hundred or more HT220s. MT300 were new back then. Trunking radios with microprocessors were very new at the time. Trucks would pull in to the shop with really strange radio problems but the fix was always the same. The radios connected to 12 volts through four copper strips. Two pairs. The fix was to clean the strips with an eraser! The 12 volts was not clean enough and the brains were getting confused :)
     
  4. dave

    Dave New Member

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  5. Space Varmint

    Space Varmint New Member

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    No kidding! We have similar backgrounds. I remember the MT300 and wasn't there a 440 and I know the was a 400. That was the Motorola Micors wasn't it? Yeah I remeber when they had the DVP (Digital Voice Protection). Only the FBI & US Martials had them. Pretty cool the way they worked, the PLL would lock up on the signal but sounded like nothing there. White noise. I think I know how they did it even though I never had to fix that part of it.

    I did not work in the bay but we had a mobile service dept. I was strictly portables and pagers.

    Anyway, I'm phsyched!!! I got the VXO to go under frequency with no phase noise. I can touch the variable capacitor metal shaft and the frequency does not move. The crystal is at 4MHz and right now I got it set at 3.985MHz solid! I only need to go below frequency by 1.5KHz and then run it through the crystal filter I made with two 4MHz crystals. So, 3 crystals on the same freq will do it. Cost about $1.50...lol

    Hey I play guitar. We need to jam...lol
     
  6. flat5

    flat5 Member

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    I'm happy for you that you were able to pull the xtal down 15khz at 4mhz. I'll try to remember that. You put a cap across it? What value? Any inductor involved?

    When I read your post above I missed the fact that you were repairing Motorola HT's and pagers. I was too much in a hurry to post :) Lucky for us it was Motorola. I saw some other brands and they seemed like junk in comparison. GE, RCA. I guess things are much different now. I had those codes and freq. that the DEA used. Never did anything with them. I never saw a HT400. Guess it was after I left that job. HT330 was the last one for me.
     
  7. Space Varmint

    Space Varmint New Member

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    Yeah I had ballance some parallel capacitance with about 22pf and give it al little more feedback.

    No I worked on GE's and Tactec (RCA) & Areotron too.
     
  8. Space Varmint

    Space Varmint New Member

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    Amazing how their is just some info out there that just is not there. I could not find nothing applicable to warping crystals. I had to clean it up even more. I've seen all sorts of signal come out of this thing. I even had it do an occasional delay based on the capacitor's time constant where it could be seen like restarting the signal of the crystal oscillator. This is a whole subject unto itself. I found a pretty good website on crystal jitter.

    I was glad to find this website too:International Radio - crystal radio filters, performance enhancements and publications for ham radio (Kenwood, Yaesu, Icom) and other radio enthusiasts

    then click crystals

    I wanted to see what I could get away with because I noticed if I get too close to the crystal frequency it might jump back on the Xtal freq. Possible loading could cause it. It looks as if it should be exactly 1.5 KHz or very close to it. I think I'm ready to put the chip in...the MC1496. If this goes well I should be close to buzzing this thing out! I got some old amps I built years back and hooked them up and did some CW QSOs. After a few bad signal reports I cleaned up any rf leaking back into the circuit from the antenna. The hams will tell you, you got a problem in your power supply...lol. It does make the same hum sound you would get from poor filtering. It was crazy too. I got a 2-way antenna switch to switch from transmit to receive and i guess it was the cause, but what happened was, I was running the receiver off an old power supply I built into a plastic tool case so of course no shielding there but it was only the receiver. So what I suppose happened is the rf leaked through the antenna switch back to the transmitter modulator which at the time was an old PLL that generates various frequencies on the 40 meter band. I put a regulator on another power cable and ran it off the same power supply as the transmitter and it cleared it up. The power supply is mucho clean. A Kenwood I picked up at a ham fest. So everything works. I got about 75 watts out and it is clean. So all I will need is a nice modulator signal and about 1/2 watt to drive the two amplifiers and I'm talking!
    It's been a while but I sure do enjoy it. It was so much fun just operating CW last weekend. Hope to do both shortly.

    I would consider more of you people to get your ham license because the internet is not gonna be what it has been. The gov't is moving to an "internet II". It will be filtered and many blog sights will not be legal anymore. There is a war on free speech coming fast. Obama here in the USA will undoubtedly pass the "Fairness Doctrine" which will give them legal leverage to pull this stuff off.
     
  9. Space Varmint

    Space Varmint New Member

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    Anyway, here is what I ended up with:
     

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  10. flat5

    flat5 Member

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    I may have to re-draw that to understand it. I've only just looked at it.
     
  11. Mikebits

    Mikebits Well-Known Member

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    I looked at your circuit and decided to redraw it to make out what you have going on. See attached.
    I can see why your unable to pull your crystal as your varicap is hanging on your output. Not sure if taking output off the feedback path is a good idea as the next stage will cause loading and detuning. What value cap do you have in parallel with the varicap?

    I am not sure why your hanging a .01 cap in parallel with a 30pf cap on one side of your crystal. This would seem to put a rf ground on the one side of the xtal so the 30pf is useless.

    I thought for a hartley osc the crystal should be in the feedback path, not shunt, but then again I do not have much knowledge in osc design.
     

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  12. Space Varmint

    Space Varmint New Member

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    Those are good questions. In oscillators it is the AC feedback path that determines the type of oscillator it is. Most people automatically assume if it has a crystal, it is a Pierce oscillator. Not so. So the Hartley gets it positive feedback off of the tap on the coil. The .01's are just coupling caps, like AC shorts. The small value capacitors to ground are to cancel out the parallel capacitance of the crystal. Understand they are frequency responsive. Sometimes when filtering potential rf from the power bus or other parts of a circuit, it is necessary to use two or thee different values to capture all of the rf/ac interference.

    It works very well, but the only draw back to the circuit is, when I turn off the power and turn it back on the frequency will jump above the crystal frequency approx 27 Hz. So I have to retune it each time I restart the oscillator. But it does remain locked once set, until power is cut again. I have used heat on it and squeezed the variable cap and it stays on freq. So I will have to watch the loading. I assume this chip has a good input buffer.
     
    Last edited: Dec 20, 2008
  13. Space Varmint

    Space Varmint New Member

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    Actually I take it back. I can get it to come up locked below crystal frequency. I replaced C10 with a variable cap. It turns out to be a balancing act between c10 & c14.

    What is so tough about it is getting it to remain stable at such a close frequency to the crystal's natural frequency. It is spec'ed at 1.5KHz from crystal filter. I'm attempting to use 3 or 4 crystal of the same frequency for the whole set up.
     
  14. Space Varmint

    Space Varmint New Member

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    Got a final update...lol.

    Now I feel secure in proceeding. What it would do orginally is warp up in frequency but was very difficult to get it under freq. I had tried series inductance but over looked parallel inductance. Turns out to be the ticket. It will now fire up right on frequency, has the most beautiful sinusoidal wave-form and when you tune it it now does not want to go to it's natuaral frequency but will stop right before. Or what the frequency is, is 4MHz, it will tune smoothly right up to 3.999KHz. It has no jitter, when I test it with receiver, there is only one frequency.

    The update:
     

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  15. xanadunow

    xanadunow New Member

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    This should not turn into a brawl. The original article by Varmint stated - he liked the sound of it. Indeed it is "nice".

    But efficiency was the aim of the SSB game, not the sound of it.

    Pondering about the power consumption was the leading factor to discovery that in the AM modulation - 50% of the transmitter's power is consumed by the "carrier" frequency and 25% for the "side-band" each. There are two of them, the upper (USB) and the lower (LSB).

    It was also discovered, that for legible reception we do not need a carrier (50% of power saving) and out of the two - only one side band is required (another 25% of power saving). It meant, that the whole whooping 100% of transmitter's power can be dedicated to transmit the only thing required for a legible reception of the voice - a ONE Singe Side Band (hence the SSB). USB or LSB - to be more exact.

    For a legible reception, the carrier frequency has to be reconstituted within the receiver. In ham's terms, this role within the receiver is performed by a BFO (Beat Frequency Oscillator). It restores the carrier at the precise position (frequency wise), where it would originally be within the transmitter and if set exactly at that point, there is no difference to the sound, it sounds exactly like an AM transmission.

    In real life nothing is perfect, BFO can be tuned by hand to achieve the perfection, but SSB techniques do not use variable BFOs, instead - a crystal filter oscillator (XFO). There are actually two of them and we choose which one to use, depending on whether we want to listen to upper or lower side band (by hams convention LSB is used on bands below 14MHz and USB 0n 14 MHz and above). The exact position of the suppressed carrier in the transmitter and the position of the reconstituted carrier in the receiver is solely responsible for the pitch of the sound we hear in the speaker. It may be shifted by mere 50 to 300 Hz and sound funny but not illegible. I say again, no distortion is present in pitch of the sound if the carrier is reconstituted at the exact point it should be. Any "funnies" are resulting from a possible frequency drift of the transmitter and receiver and the selection of crystals.

    To achieve the efficiency of SSB, both - transmitter and receiver incorporate a balanced mixer (this is where a carrier is done away with within the transmitter or reconstituted in the receiver), a crystal oscillator (XFO), and - a crystal filter.

    The role of the crystal filter (it is a band pass filter) is to shape the side band in such way, as to permit only the sound frequencies between 300 to 3000Hz (normal SSB) or 300 to 2700Hz (narrow SSB). Legibility requirements for the speech state that the minimum audio frequency can be no more than 300Hz and no less than 2700Hz, thus the efficiency can be shaped within the audio stages and IF stages of both the transmitter and the receiver to do just that. As a side benefit to this shaping we can also cut out the second side-band.

    The crystal filter forms the IF stage of every SSB transmitter and receiver and it is a "heart" (an expensive one comparing against standard AM gear) of every SSB device.

    IF stages are one and the same in both transmitter (TX) and the receiver (RX) and are relatively expensive, thus a transceiver was born (TRX) utilizing one only (and the same) IF stage, bringing in savings in expense in addition to SSB's efficiency. These qualities become recognized, gained popularity and have become a norm in the radio voice communication. As a side benefit, SSB occupies only half the amount of RF spectrum the AM does. In the "nutshell", this is all it is about.
     
    Last edited: Dec 21, 2008
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    But the sound quality is so bad that my wife and I cannot hear human speech. It is ducks quacking or whistles noises.
     
  17. xanadunow

    xanadunow New Member

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    Communication's task is to "pass the message" and to do it with the max efficiency. The quality thus can be sacrificed. This is why CW will never die. It can get through when nothing else can. The quality has it's deserved space in listening to the music and hence the FM mode in the radio communication. But FM is wastefull on the RF spectrum and this is why it is used only on frequencies well above the shortways. In turn, it has limits to the "sure" range of reception. I am absolutely sure that the goal behind SSB is retained not in the quality but in the requirement to "pass the message" in the most power efficient manner :)
     
    Last edited: Dec 21, 2008
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My wife and I are used to hearing 20hz to 20kHz speech. The "narrow band" 50Hz to 15kHz of FM and TV is ok.

    Maybe "hams" have a hearing disability so they can understand speech when most of the important frequencies are missing.
     
  19. Leftyretro

    Leftyretro New Member

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    You state the case well why it's the most popular modulation mode used in the HF ham bands. There are ham groups that like to restore old AM gear and shoot the shift with each other but they are a minority.

    I kind of like the sound of SSB, it always reminds me of those great scenes in the movie Dr. Strangelove with the crew of the B-52.

    Lefty
     
    Last edited: Dec 21, 2008
  20. xanadunow

    xanadunow New Member

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    Ability to hear any range of (audio) frequencies between 20Hz and 20kHz is very much an individual matter of a human and is rarely attributed to a particular "hobby" as implied in the message by uncle above. If anything - the age, the listening habits and perhaps, the occupation (namely the protective gear for the ears) - are the major factors in the deterioration of frequency boundries we can hear as individuals during the span of our life.
    But it is well outside this particular thread, we were talking about SSB here.
     
    Last edited: Dec 21, 2008
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    No we aren't.
    We are talking about a "radio" that has an audio low frequency cutoff of 33kHz (to avoid hum) and a high frequency cutoff of 500Hz (to avoid hiss). Therefore the audio bandwidth is only about 200Hz. None of its transistors have negative feedback so the distortion is extremely high. Most of its audio circuit is attenuation and very high gain.

    Also its BFO wanders all over the place. It seems to be modulated by the audio.

    No wonder it sounds horrible.
     

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