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Author Topic: Weird Ham Questions  (Read 3128 times)
KX8W
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Posts: 7




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« on: August 03, 2011, 07:06:46 PM »

I got to talk to a couple of ham friends while at Scout camp and I have two questions
for all you older hams....First, why can we have frequency modulated single side band? Or at least FM without the carrier?
and secondly, if SSB is so efficient, why don't we have SSB 2 meter HTs?

73,

KD8QGJ
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M0HCN
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Posts: 473




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« Reply #1 on: August 03, 2011, 07:50:07 PM »

Well firstly frequency modulation, by definition requires something to carry the information about the frequency, this surprisingly is called the carrier!
Actually the subtle thing about FM is that there are certain values for the modulation index at which the carrier completely nulls out, and only the sidebands are radiated, but this only happens when the modulation index (Deviation/Modulating frequency) hits very precise values given by the roots of a bessel function of the first type, and for anything except a sine wave as the modulating signal the math gets hairy, unlike the  case with SSB the two side bands do not contain two identical copies of the information so removing one will cause distortion.

Secondly, SSB requires linear amplifiers where FM can use class C (non linear but efficient) amplifiers which are smaller, simpler, cheaper and result in far less of your battery ending up as heat in any given transmission, also FM receivers are simpler then SSB ones. You do sometimes see SSB offered on vehicle mounted radios that do not have the power and thermal constraints of a HT.
SSB also requires very precise tuning where FM can work even with substantial frequency errors (and can have simple automatic frequency control to pull it  onto frequency, important in a radio that has to work channelized over say -40 to +30 degrees C.

HTH.

Regards, Dan.
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N2EY
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« Reply #2 on: August 04, 2011, 05:41:38 AM »

why can we have frequency modulated single side band? Or at least FM without the carrier?

We can't have those things. The basic math and physics of modulation prevent it.


and secondly, if SSB is so efficient, why don't we have SSB 2 meter HTs?


Several reasons:

1) Amateur 2 meter FM is based on the standards of the land-mobile services of the 1950s and 1960s. It used to be common for hams to convert old 150-174 MHz commercial land-mobile FM sets to 2 meters.

2) SSB requires that the frequency of the tx and rx be very accurately set - including a repeater, if used. Until recently that sort of accuracy wasn't easy or cheap to put in an HT.

3) FM capture effect and impulse noise immunity make a big difference.

4) Saving spectrum isn't such a big deal on VHF/UHF. All of the amateur HF/MF bands from 160 to 10 put together have less space than 2 meters alone.

73 de Jim, N2EY
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G3RZP
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« Reply #3 on: August 04, 2011, 06:09:20 AM »

For once Jim, I'm going to disagree. Back in the 1960's, I was searching for some information in what was then the Marconi Company library at the Great Baddow Research Labs, and I came across an article of about 1961 translated from Russian. It had a full analysis of how to mathematically produce an FM signal with one sideband, and a block diagram of how to do it., and how to demodulate it. As I recall, it was pretty complex, being of course, all analogue, and there were a lot of component drift possibilities that would screw it up. The original idea of the research was improved spectrum utilisation for military tactical comms, but the conclusion was that it was impractical, at least in those days.

Now of course, digital modes have taken over so it doesn't really have any application, even if you could do it all in DSP. And No, I don't have a clue about which journal it was in.
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KE7IZL
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« Reply #4 on: August 04, 2011, 09:30:06 PM »

SSB in the digital age is easy. Instead of turning tuning capacitor plates or some other error prone analog method it uses a digital signal generator. So when your radio says (on its nice LCD display) that the frequency is 144MHz, it means 144,000,000Hz, not 144,000,001Hz. With this accuracy, it would be very easy to do SSB transmitting and receiving in any band, regardless of if it's HF, VHF, or UHF.
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G3RZP
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« Reply #5 on: August 05, 2011, 12:56:56 AM »

Except you would need a frequency standard accuracy of one part in 10 to the minus 9 to know that it really was 144,000,001
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KE7IZL
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« Reply #6 on: August 05, 2011, 01:35:44 AM »

Except you would need a frequency standard accuracy of one part in 10 to the minus 9 to know that it really was 144,000,001

I'm sure that modern radios with digital tuners are built to super high precision standards. Remember DACs that output a digitally synthesized waveform are controlled via a CPU/microcontroller. Said microcontroller is clocked by a crystal oscillator, and these things are known to have VERY high precisions good enough for SSB operation (or they may be even better still if they happen to be of the "oven controlled" variety for temperature stability, though I don't know how many radios have CPUs clocked with "oven controlled" crystals).
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N2EY
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« Reply #7 on: August 05, 2011, 05:24:27 AM »

SSB in the digital age is easy. Instead of turning tuning capacitor plates or some other error prone analog method it uses a digital signal generator. So when your radio says (on its nice LCD display) that the frequency is 144MHz, it means 144,000,000Hz, not 144,000,001Hz. With this accuracy, it would be very easy to do SSB transmitting and receiving in any band, regardless of if it's HF, VHF, or UHF.

It's not that simple. G3RZP is right.

A digital frequency system is only as accurate as the time base oscillator - the clock - that drives the system. If it is off, the frequency is off. What the LCD display says doesn't mean a thing unless the clock is just as accurate.

In order to get 1 Hz accuracy at 100 MHz, the time base oscillator would have to be accurate to within 1 part in 100 million. Getting 1 Hz accuracy at 144 MHz requires even more accuracy. This accuracy would have to hold over time, temperature, etc.- in an HT. FM doesn't need to be anywhere near that accurate.

1 part in 100 million accuracy is equivalent to a clock that gains or loses less than 1 second every three years, without any external reference. And every tx and rx in the system has to be that accurate.

This sort of accuracy can be done today with high-precision TCXOs. But remember (as previously stated) that the FM standards we hams use today are based on commercial land-mobile practice of decades ago.

73 de Jim, N2EY
« Last Edit: August 05, 2011, 06:49:30 AM by N2EY » Logged
AA4PB
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Posts: 12770




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« Reply #8 on: August 05, 2011, 06:24:46 AM »

Many people make the assumption that because the digital dial reads down to 1Hz that the frequency calibration is accurate to 1Hz. That is very often not true. That last digit serves a purpose in being able to reset a frequency in the short term, but you can't depend on the absolute value of the readout. The same is often true for multimeters and other test equipment. Just because your multimeter digital readout goes down to .001V doesn't mean that the meter is accurate to .001V.
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M0HCN
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« Reply #9 on: August 05, 2011, 07:41:43 AM »

"Excess precision" is the term, but actually for a SSB radio absolute precision is less important then having a small enough tuning step to allow the signal to be tuned close enough to make it ineligible ~1Hz tuning steps are easy these days, and quite sufficient.

That said, I don't really see the upside, it is not like you  are going to be doing weak signal work with a HT after all. 

Regards, Dan.
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K7KBN
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« Reply #10 on: August 05, 2011, 11:34:50 AM »

Ever notice that the "accuracy" of test equipment, panel and switchboard meters and other devices is universally stated something like:  ACCURACY: ±5%.

Which to me implies an INACCURACY of ±95%.

While I know that this actually means "maximum allowable error at ±5% of full-scale value", I have to wonder why the manufacturers don't state it that way.
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73
Pat K7KBN
CWO4 USNR Ret.
KE7IZL
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Posts: 47




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« Reply #11 on: August 05, 2011, 11:50:00 PM »

Is it legal to use SSB in the VHF UHF bands?
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G3RZP
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« Reply #12 on: August 06, 2011, 05:17:16 AM »

Yes. And CW and even AM
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WB9QVR
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Posts: 28




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« Reply #13 on: August 08, 2011, 04:33:30 PM »

Actually there have been some attempts to market some SSB HTs.  Mizuho made several models in the late '70s and early '80s.  Santec also produced the LS-202A FM/SSB HT in the mid-'80s.  Icom and other manufacturers have also built 'luggable' SSB portable units (they weren't technically HTs, though).  However, none appeared to be very successful.  One reason might be the higher price tag of these units as compared to their FM counterparts.  Another reason might be the overall lack of SSB operation on the VHF/UHF bands (except during contests and the occasional band opening).

In short, there's no technical reason why an SSB HT couldn't be produced.  It just isn't likely due to the small market for such an item.
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W0FM
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« Reply #14 on: August 09, 2011, 02:54:58 PM »

Some amateur equipment manufacturers took a stab at 2M SSB HT's years ago, but they never seemed to catch on.  Probably because the majority of hams already were caught up in the FM craze at the time.

Also, in the commercial two way radio business there were attempts at using Amplitude Compandored Side Band (ACSB) modulation for Land Mobile Radio applications starting in the late 1970's..  It quickly was abandoned, partially because the temperature extremes in mobile applications made it very difficult to keep the radios precisely on the required frequency.

73,

Terry, WØFM
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