Inexpensive WB inline amps

[AntAltMike]

I'm going to be installing a lot of WB68 unpowered multiswitches that will be connected to home run coaxes that will vary in length from about 50 feet to 270 feet. On the input side, there will be Sonora HRPID1441 polarity lockers and probably Sonora LA142a dual inline amps to lighten the current draw of the receivers and to utilize the power developing capacity of the WB68s to their max, but I'm concerned that on the longer runs, the signal level will still sometimes drop below -60 dBm. I'd like to boost it at the receiver a little, but at present, the least expensive inline amp I have found that amplifies the entire DirecTV WB spectrum of 250 MHz to 2,150 MHz is the Sonora LA141a, which sells for nearly $30 each.

The LA141a is a medium power amplifier that can be used near the LNB because it has a maximum input of -28 dBm, but at the end of 200+ foot RG-6 home runs, all I need is a low power inline amp because my inputs will be more than 20 dB below that. The $5 or $6 bullet amps work just fine in that application for DBS L-band frequencies of 950-1,450 MHz and even in stacked Ku L-band systems that go up to 2,150. Has anyone started making residential grade, low power wideband inline "bullet" amps priced comparable to what we have been paying for DBS frequency inline amps?

[AntAltMike]

It took less time for me to find them than it did to compose the above post asking about them. Pico/Macom has broadened its, "LA2150" product line to now include four models: LA2150A is a flat, 20dB gain, wideband low-power inline amp, LA2150B is wideband but with 4dB of slope, LA2150C amplifies 950 to 2,150 MHz by 20dB, and LA2150D is also L-band but with 4dB of slope. I can get them for about $7.50 each. Yay!

[AntAltMike]

Pico's published "spec" for these amps is that they have a maximum output of 105 dBuV (decibel-microvolts). What does that tell us?

Well, to convert from decibel microvolts to decibel millivolts, or dBmV, we subtract 60, and then to convert from decibel milliwatts, or dBm, we subtract 48.75, so the "maximum output" level is -3.75 dBm. Sort of. The specs don't say this, but that is probably the level at which one single analog carrier will develop a certain reference level of Third Order Intermodulation Distortion (3rd Order IMD). We use that as a starting point to try to determine just how much power it can develop in our application without contributing excessive 3rd order IMD..

First of all, every time "load" of the amplifier is doubled, the maximum output level goes down by 3 dB, so with a typical, DBS satellite L-band load of 16 transponders, the maximum output is reduced by 2 to the fourth power, or by 12 dB, making the maximum output -15.75 dBm. The load with some additional Ka transponders will be greater, but I haven't yet "scoped" out such an output, so I don't know how many additional transponders I am likely to find, or whether they are at the same power level as the Ku transponders. For this analysis, I'll assume that the Ka transponders effectively doubles my load and derate the inline amp's output by another 3 dB, down to -18.75 dBm.

Now, remember that I said that the amplifier's output spec utilized an unstated benchmark 3rd Order IMD level? Many amplification products use -35 dB as the 3rd Order IMD reference level because that was a useful level for analog signal performance, but with digital signals, we try to keep it under -40 dB. 3rd Order IMD also varies with the output level. I have no way to measure 3rd order IMD, so I must rely on calculation methodology I have been given, but unfortunately, satellite product sales literature most often claims that 3rd Order IMD goes down by two dB for every one dB of decrease in power, whereas academic literature claims it goes down by three dB for every one dB decrease in power.

I'm going to assume for this calculation that the manufacturer's sales literature had it right, if only because I will be holding them responsible if their product seems to cause a system failure in this application, so to reduce the calculated 3rd Order IMD developed by these inline amp by another 5 dB down to -40 dBm, I will have to further reduce its output level by another 2.5 dB, or down to -21.25 dBm. If I had, instead, assumed that 3rd Order IMD goes down by 3 dB per 1 dB drop in amplifier power, then the output level would have to be reduced to -20.41 dBm, but it is easier to manage and follow the decimal equivalent of "halves" and "quarters" than "thirds", so we'll go with the calculated -21.25 dBm safe threshold at this point for exposition's sake.

Amplifiers in a cascade tend to multiply the 3rd Order IMD. Each time the number of amplifiers in a cascade that has been balanced such that they each would be contributing the same amount of 3rd Order IMD is doubled, the 3rd Order IMD increases by 6 dB. In a cascade in which I will be using an LA2150A, there will seemingly be three amplifiers: the LA142a, the internal, loss compensation amp in the WB68, and the LA2150A, itself. So since three amps in a cascade increases the 3rd Order IMD by about 10 dB, then each of these three amps must have its own 3rd Order IMD reduced to -50 dB so that the cumulative 3rd order IMD remains at -40 dB. Using the formula that a 1 dB reduction in amplifier power reduced the 3rd order IMD by 2 dB, that means we derate each amplifier in the cascade by 5 dB, so in this application, our LA2150A inline amp must not develop an output of over -26.25 dBm.

Since I am figuring that I will only be inserting these LA2150A amps at the end of long coax lines where the signal will be well below -50 dBm, and since they only amplify by 20 dB, then their maximum output will always be below -30 dBm and in most applications, my signal levels in that application will likely be ten dB below that, so I appear to be safely below my calculated -26.25 dBm maximum output level for the LA2150As.


Except for one other vexing concern. All of the reference levels I have seen for 3rd Order IMD were for narrow analog carriers, but I am loading these amplifiers with really wide digital transponders. I don't know how much more RF energy is contained in a wideband (about 27 Mhz wide for Ku band, and I think about 36 MHz wide for Ka), but I suspect that there is ten times as much RF energy or more in a DBS transponder than there is in an analog carrier of the same signal level. That would mean that, even though I have performed this calculation to the hundredth of a dB, my estimate of maximum tolerable power levels may be substantially overestimated. If the Ku/Ka channel load has maybe ten times the RF energy in it that was assumed in published maximum output level, then maybe I have to further derate this component and others by another 5dB to maintain the 3rd Order IMD at my target levels. It may therefore be that, with a typical DBS Ku/Ka channel load, my LA2150As might develop -40 dBm of 3rd Order IMD when their output is only -31.25 dBm, rather than at -26.25 dBm, as I had initially calculated above, and so, in this three amp cascade, maybe I shouldn't use it in any application in which its output will exceed -41.75 dBm, since I still must allow for the 10 dB of cascade intermodulation!

I'm still not sweating using an LA2150A in this application since the DBS receiver manufacturer's recommendation of a maximum 3rd Order IMD level of -40 dB is surely conservative, and, in this multiple dwelling unit application, I doubt I will ever be seeing signals that would push this amplifier beyond its safe operating limits, but as you can see, there is less margin for error than you otherwise might have expected, and so those residential DBS installers among you who might be tempted to bid on large jobs in multiple dwelling unit buildings would be well advised to get your Lever III installer certification before you wind up being responsible for a distribution system that just doesn't work even though each and every component in it seems to be in good operating condition.

[veryoldschool]

With only two cups of coffee, I'm only on my "first pass" on this but:
1) what amp is in the WB68? It's a switch.
2) From doing a lot of third order testing, by nature of the signal, and increase in one [dBm] will increase it by 3 [dBm]. It's not a third order if it doesn't.
3) From working with non military products, they typically don't have a lot of reserve in their specs.
4) marketing and engineering fight all of the time over "specs".

[AntAltMike]

...
1) what amp is in the WB68? It's a switch..

All DBS multiswitches I have examined have weak amplifiers in them to compensate for their internal splitter loss. Eight output multiswitches, for example, might typically introduce 12 to 15 dB of splitter loss on each divided input, but no switch whose specs I have examined had any more than a few dB of L-band signal loss. I don't think that any residential products ever had a "loss" of more than 4dB, and many had no loss at all. I have admittedly have not yet evaluated my WB68 input-to-output ratios, but I surely will this week.

Holland makes inexpensive 3x4 model APE switches that amplify the satellite RF but not the diplexed off-air RF, so the satellite RF on the output ports actually "loses" about ten dB more than does the off-air RF. I use lots of these in a building I service where some of the Holland APE switches diplex in the eight channel master antenna system, whereas some others diplex in the 860MHz, 135 channel cable system. The beauty of leaving the TV frequency channels unamplified is that you don't inadvertently have them mucking up the DBS loss compensation amplification. I can externally amplify them as is warranted by the circumstances.


2) From doing a lot of third order testing, by nature of the signal, and increase in one [dBm] will increase it by 3 [dBm]. It's not a third order if it doesn't.

When I looked into this a year or so ago, I did come across a verbal pissing match thread between two genuine experts which had deteriorated to the point where they were more intent on insulting each other than agreeing on evaluation methodology long before they exceeded my ability to comprehend their arguments.

3) From working with non military products, they typically don't have a lot of reserve in their specs.

But consumer electronic products sometimes do, because they are being used by people with no test equipment, little technical knowledge, and strong motivation to try to violate the published performance limits of a product. DBS receivers use to come with published specs saying that the input signals should be between -30dBm and -60dBm, but a few years later, they more often said the window should be -35dBm to -55dBm, yet I probably have never seen a receiver fail because of too much or too little signal as long as it was between about -20dBm and -65dBm. The conservative specs in consumer electronic equipment force the consumer to try to hit the middle of the range and tend to indemnify the manufacturer against unreasonable performance complaints of the consumer.


4) marketing and engineering fight all of the time over "specs"

Radio Shack held onto touting "peak power" for its car radios for years. Spec for consumer RF products are bad jokes: Gain over isotropic versus gain over dipole; average gain of a broadcast antenna, when its gain over the band of its intended use may vary by more than half a dozen dB; antenna gain measured over a test range that probably is set up so that ground bounce will fortuitously be added to direct reception all are used to "sell" a product rather than to assure its proper implementation.

Notice that lots of preamplifier manufacturers will furnish certain overload numbers for 5% sync compression, then in another year's catalog, those same values might be given for those same products as the limits for "cross modulation" or for "IMD". I think that if the people producing the sales literature hear that some dealers are tending to ask more for one parameter rather than another, they just change the labels on the same numerical specs tables to satisfy them.

The common, Pico/Holland/TruSpec/put-your-name-here broadband high powered distribution amplifiers typically called CA300/450 all make the same ridiculous claim of being able to develop an output of 66dBmV with a ten channel load. If they could do that, I'd use them to light Cristmas trees.

Over at AVSForums, there are people who have love affairs with products that they have only used once or not at all, largely because of their favorable published specs. The Winegard preamp overload specs are bad jokes. I compared two Winegard AP4727, 23dB gain preamps with two Channel Master OSD0065 23dB gain preamps and even though the Winegard published specs claimed it could develop 10dB more than the Channel Master without overloading, the Channel Master decisively beat it, but not by exceeding my own expectations of it.

Back when everybody and his brother who could fabricate formed mesh was making C-band dishes, Dr. Frank Baylin wrote that he doubted that any of these manufacturers had any means to test the gain of any of them, that they more likely just copied the published gain figured for the products they were attempting to duplicate.

But as far as this 2dB/3dB input to 3rd Order IMD distortion "discrepancy" is concerned, I see two likely roots of it. 1) Someone paraphrased someone else's technical literature and copied a sentence from some other (2nd Order?)intermodulation byproduct tutorial which has since been copied ad infinitum, or 2) sometimes, certain remedies or technical considerations are commonly implemented or applied simultaneously, and sometimes certain related considerations get linked together as a practical matter. If I need to reduce my 3rd Order IMD by, say, 9 dB, but to do so required that I reduced my output by 3dB, then, if I am doing cerebral simultaneous calculations to make all this work, I am still faced with the fact that I now have a weaker system then I was originally trying to develop, and now I often have to do something to bring the signal power back up, which will bring the 3rd Order IMD part way back up. Maybe, along the way, someone observed that if he reduced the IMD by 9dB by reducing the input by 3dB, then his reduction in 3rd Order IMD only exceeded the reduction in signal power by 6dB, which would be an apples to oranges comparison, but it may have been the source of the confusion.

[veryoldschool]

I'm not going to post my resume here, but I've been married to an HP 8566 longer than most. I've been a thorn in the side of many an engineer since I test what they designed.
I've heard such BS as "that's instantaneous dynamic range" for an excuse why they used a step attenuator to meet a 65 dB proposal [contract] spec with a system that had 35 dB.
Two tone third order is used for a reason. One dB compression point is another. These limit the performance range.
I questioned an engineer at a cable equipment company about the use of "6 dB" for a 2-way power splitter. Reply: well yes it's 3 dB BUT since it's measured in dBmV it's 6 dB.
After working with a few Cable equipment companies, they barely make published specs. They seem to be able to get away with it since their own field engineers are even worse.
If you want to know how something will work: cobble up a test setup and measure it.
If your test setup is valid, you know what you're doing, then you'll have valid data.
Everything else is just "smoke".