[Repeater-Builder] Occupied Bandwidth
Well friends and neighbors, I took my trusty IC-91AD and hooked it to my IFR COM120B and took some pictures. Burning questions like transmitter deviation, bandwidth, RF power output and so forth are answered herein. Photos contain explicit material and are recommended for mature audiences only. www.bosshardradio.com/dstar/spectrum/ or www.yahoogroups.com/group/dstar_digital/photos Use as you see fit. Steve -- Ham Radio Spoken Here !!! NU5D EM11 http://www.qrz.com/callsign/NU5D Nickel Under 5 Dollars
Re: [Repeater-Builder] Occupied Bandwidth.
Hi Bob, I didn't want to confuse a highly technical topic with the FM/PM thingagain. Personally, I find it easier to think of angle modulation in terms of frequency as opposed to phase. Sure, me too - - maybe most of us on the list. That's probablybecause we'reused to thinking in terms of sine waves. You draw a sine wave, and then you move the zero-crossing point left or rightto show how you can change the phase. The problem is thatyou canalso move the zero-crossing point left or right toshow how you can change thefrequency. That's what got me. So how do you showthe difference between phaseand frequency? The sine wave isn't the right tool to talk about phase, a rotating phasor diagram is. But that's okay. We can do this with sine waves if we look at a string of them. If you shift the phase by 10 degreesand then leave it that way for anumber of cycles,the frequency would change only momentarily. It would changeduringthe timethe phase was actually changing, not before or after. So,a"step" inphase angleresulted in a spike in frequency.Since frequency results from achange in phase,if you differentiate phase, you get frequency. This is just like differentiating velocity to get acceleration. The reverse works as well. If youchange the frequency, the phase must continuously change to accomplish that. And that's the whole difference between PM and FM. I didn't want to complicate the discussion of Carson's Rule TX bandwidth by having to explain how the numbers add up if using a phase modulated TX. For example, how do we determine the modulation index from phase deviation for use in calculating the sideband amplitudesfrom the Bessel function? I forwarded your message to Virgil and this is his response: Thetext book general case for theCarson rule does not state highest modulating frequency because the formulae is good for any modulating frequency. In the case below, they simply have used thebandwidth requirement to condition the formulae for finding the highest modulating frequency. The Carson rule works whether or not the modulating frequency is the highest modulating frequency. My issue is the math statement that implies that BW is found by using the highest modulating frequency. The definition below used the term CBR (bandwith requirement) which is the correct math form for defining the fm as the highest modulating frequency. Modulation Index for PM is MIp = k*Vm ( k = some constant as detemined by phase modulator ) For an frequency modulationDelta f = k*Vm ( k = some constant as determined by frequency modulator) Modulation Index for FM is MIf = Delta f / modulation frequency Modulation Index for PM is equal to Modulation Index for FM MIp = Mif Carson's Rule for: FM is BW = 2 times ( Delta f +modulating frequency ) By using the relationship between PM and FM modulation index. ( Delta f = modulation index times modulating frequency ) PM is BW = 2 times (modulatingfrequency) times ( modulation index plus one ) The Bessel function x axis is the modulation index -whether the modulation index is from PM or FM. 73, Bob Yahoo! Groups Links To visit your group on the web, go to:http://groups.yahoo.com/group/Repeater-Builder/ To unsubscribe from this group, send an email to:[EMAIL PROTECTED] Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.
Re: [Repeater-Builder] Occupied Bandwidth.
At 4/29/2004 10:05 AM, you wrote: Hi Bob, Fortunately, voice emission PSDs are spread fairly evenly across the entire 300 to 3000 Hz modulation band (thanks to pre-emphasis, otherwise the PSD would be sloped toward the low end, yes we're talking FM not PM Bob don't go there!!) How could I turn down such a nice invitation? :-) Communication is difficult without definitions. PSD is some sort of distortion measurement? Sorry: PSD=power spectral density: how the power is distributed as a function of frequency. I should have spelled out the term first then used the abbreviation. General comments: Carson's Rule holds true for all types of angle modulation, FM and PM included. And yes, the male voice peaks somewhere around 300-400 Hz, so preemphasis does spread the energy over a wdier spectrum. Beyond that, I'm not sure what FM not PM means. I didn't want to confuse a highly technical topic with the FM/PM thing again. Personally, I find it easier to think of angle modulation in terms of frequency as opposed to phase. It's just easier for me to envision an EM wave changing frequency how that looks on a deviation scope as opposed to changing phase. This is probably again due to the lack of practical phase modulators (I've never seen a scope that directly displays phase vs. time). Don't think that I don't like PM, though. All my 440 MHz TXs are phase modulated. The G.E. phase modulators seem to do fairly well at UHF. To achieve flat audio, I simply de-emphasize all the way down to 60 Hz or so. Applying this audio to a phase modulator makes for a very flat overall system response. There may be a small amount of distortion at low frequencies, but my ears can't hear it. Virgil and I have been trying for a long time to show that FM and PM aren't two entirely different modulation schemes; one is just the derivitive of the other. With that close of a relationship, it just seems counterproductive to keep talking about the two as if they were as distantly related as AM and FM. Of course. Sorry if I made it sound otherwise, but I didn't want to complicate the discussion of Carson's Rule TX bandwidth by having to explain how the numbers add up if using a phase modulated TX. For example, how do we determine the modulation index from phase deviation for use in calculating the sideband amplitudes from the Bessel function? Bob Yahoo! Groups Links * To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ * To unsubscribe from this group, send an email to: [EMAIL PROTECTED] * Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
[Repeater-Builder] Occupied Bandwidth.
CARSON'S RULE BANDWIDTH = 2 X (PEAK DEVIATION + HIGHEST MODULATING FREQUENCY) Most 2M off the shelf radios I have seen lately in wide band FM hit at least +/- 5kc peak deviation, and 6 to 6 ½ is not at all uncommon. Measurements using HP8921 or IFR COM120B. I have not run a swept transmit audio response test using an audio sweep generator into the mic input @ 60% full system deviation, but I suspect it would not come to a dead stop at 3 kc of audio response. Hence the 16 khz or more of occupied band width. Now add a little bit for slightly off frequency operation, and a little over modulation and you really crowd the channel bandwidth. Add to that a little excessive high pass in the post limiting audio filter and the bandwidth again increases. The selectivity of a VHF FM radio is largely determined by the IF section, and crystal filters. The front end helical resonators or tuned lines from the old days, are going to be more than 200 kc wide at 3 db points. Even a couple of band pass cavities in front of the receiver will be very large in bandwidth compared to the overlap of the two repeater input frequencies. Using a MastrIII or MSF5000 would have better front end performance, but adjacent channel rejection (when the adjacent channel shares part of the desired channel) is not going to be very good. Going to a Micor or M2 or adding a helical preselector will help, but mostly by adding insertion loss and not in bandwidth shrinkage. At best, changing the IF filters might narrow the response, or offsetting the mixer frequency on the receiver, or mistuning the IF away from the neighboring input might help, at the expense of degrading performance on the desired operating frequency. Introducing loss in the front end may also help. I wonder how a signal generator at the input of the repeater generating on the offending frequency would act? If a -80 dbm. Signal at the antenna port modulated at 1 Kc tone with 4 or 5 kc of deviation bothers the repeater then it will be difficult to ask your neighbor to turn things down. Anyhow, best luck with the project and let me know how it turns out, Steve Yahoo! Groups Links * To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ * To unsubscribe from this group, send an email to: [EMAIL PROTECTED] * Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/ attachment: winmail.dat
Re: [Repeater-Builder] Occupied Bandwidth.
Steve S. Bosshard (NU5D) wrote: CARSON'S RULE BANDWIDTH = 2 X (PEAK DEVIATION + HIGHEST MODULATING FREQUENCY) Thank you for quoting what I've been saying the past two posts. Most 2M off the shelf radios I have seen lately in wide band FM hit at least +/- 5kc peak deviation, and 6 to 6 ½ is not at all uncommon. Measurements using HP8921 or IFR COM120B. I have not run a swept transmit audio response test using an audio sweep generator into the mic input @ 60% full system deviation, but I suspect it would not come to a dead stop at 3 kc of audio response. Yes, I agree that most off the shelf radios are not properly set up. BUT, what does that have to do with a PROPERLY set up system? You don't solve this problem by saying that 20 kHz bandwidth is OK because that's what we are seeing as a spoon-fed limit. The minute you do that, you will then see 25 kHz BW radios being made. Where do you stop? The ONLY answer is to educate these so-called *trained* radio technicians (licensed hams). If they can't solve the problem, they should at least be taught to be able to identify a problem and take it to someone who CAN fix it. Of course, on this list we are for the most part preaching to the choir. It's the non-technical hams that need it. But the point is that you NEVER cater to the lowest common denominator, you would would never have gone below 60 kHz spacing that you had in the 60s or early 70s. If you see a problem, and can show that the TX is not properly aligned (be it user OR repeater), you tell the operator that and let them decide how to solve it (either by doing it themselves, or having it aligned by someone with the proper equipment, or discontinuing use). I recently got an FT-736R. Nice radio, but it has no provision for limiting the deviation to any calibrated settings. This is a manufacturing flaw, IMHO. There should be a way to set the deviation other than a vague meter setting. But, I got it for SSB/CW anyway, so I'll use it there and not on FM (except perhaps on 440 where it won't matter as much). Going to a Micor or M2 or adding a helical preselector will help, but mostly by adding insertion loss and not in bandwidth shrinkage. You're not going to see that much insertion loss in a preselector! But, I also agree that you're not going to solve that much with one, either. At best, changing the IF filters might narrow the response, Great idea! I wish I'd said that. OH WAIT! I DID! (in my last post) 8-O I'm surprised about the 180 turn you did from saying Has nothing to do with receiver to the above comments. :-) Joe M. Yahoo! Groups Links * To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ * To unsubscribe from this group, send an email to: [EMAIL PROTECTED] * Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
Re: [Repeater-Builder] Occupied Bandwidth.
At 4/28/2004 12:34 AM, you wrote: Steve S. Bosshard (NU5D) wrote: CARSON'S RULE BANDWIDTH = 2 X (PEAK DEVIATION + HIGHEST MODULATING FREQUENCY) Thank you for quoting what I've been saying the past two posts. Here's a question: just what amount of the transmitter's TPO is Carson's Rule bandwidth? -20 dBc? -26 dBc? -30 dBc? I've seen all 3 in various definitions. If you use the Bessel functions to calculate bandwidth for worst-case single-tone emissions, using 3 kHz tone @ 5 kHz deviation, the 3rd sideband (J3) @ +/- 9 kHz is only 21.85 dB down from the unmodulated carrier. Fortunately, voice emission PSDs are spread fairly evenly across the entire 300 to 3000 Hz modulation band (thanks to pre-emphasis, otherwise the PSD would be sloped toward the low end, yes we're talking FM not PM Bob don't go there!!), in addition there may be an assumption in Carson's Rule that the upper modulation limit is down by 3 dB due to conventional analog low-pass filtering, so a 3 kHz tone would only deviate the TX 3.54 kHz, not 5 kHz. Using 3.54 kHz deviation 3 kHz modulating freq. gives a modulation index of 1.178, corresponding J3 at -30.1 dB J2 (+/- 6 kHz sideband) at -16.2 dB. That's more like it! Using linear interpolation (don't know if that's legit here, but I'm just ranting at this point), +/- 7.5 kHz would be at -23.15 dB. As a (in)sanity check, let's try 2 kHz tone modulation. Let's say the post-limiter filter doesn't kick in yet, so we have full 5 kHz deviation corresponding modulation index of 2.5: J3 (the sidebands at +/- 6 kHz) are 13.3 dB down from the unmodulated carrier, J4 (+/- 8 kHz) are at -22.64 dB. So my guess is that Carson's Rule specifies the 22 to 23 dB bandwidth of a conventional NBFM voice TX. Am I close? Bob NO6B Yahoo! Groups Links * To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ * To unsubscribe from this group, send an email to: [EMAIL PROTECTED] * Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
