dBmV vs dBµV vs dBm

Satellite engineers also use dBW, 1 Watt into 50 Ohms.


0 dBW = 30 dBm


And then there's all the antenna units- dBi, dBd. And sometimes dbC (dB referenced to the carrier).


Just remember that "dB" by itself is a relative term, i.e. a change in something like 10 dB, whereas dB with another letter is referenced to a standard, e.g. 0 dBmV = 1 mV into 75 Ohms (a unit of power).


As long as things are clearly stated and used consistently, all is OK. It's just a matter of adding/subtracting an offset to get into whatever frame of reference you are using.



Quote:
Originally Posted by GeekGirl /forum/post/0


...dbC (dB referenced to the carrier)....

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Eventually, we're going to start paying a lot more attention to dBc than we are now. Cable companies, when assessing the suitability of their amplifiers, have the luxury of dealing with a spectrum of signals that are "flat", but our residential antenna arrays develop signals that may vary by about 40dB from one another. Just yesterday, I had to process adjacent digital channels that varied in signal strength by as much as 25dB. While my amplifiers weren't considered overloaded by any of the benchmark overload levels that are referenced in manufacturer's literature, a digital signal that is 20 to 25 dB stronger than an adjacent one develops strong enough intermodulation distortion to render the weaker signal unusable even when the amplifier load is safely under the rather arbitrary maximum output level.


I have an off-air antenna targeted at Baltimore and located between Baltimore and Washington, such that the stronger, Washington signals hit the back of it at about 150 and 160 degrees (150 degrees from the towers themselves, and 160 degrees is the angle of the signals that bounce off Byrd Stadium, which are about equal in strength to the "direct" signals). The strongest signal developed by that antenna are my undesired analog channels 22 and 50 (25dBmV) 26 (24dBmV) and 20 (20dBmV). My undesired digital channels 48 and 50 are around 10dBmV, and everything else is weaker.


I have to recover and process Baltimore channels 38 (-18dBmV) and 40 (-23dBmV) even though there is a strong, Washington channel 39 (+2dBmV off this same antenna) in between them. Now, I can tune unamplified channels 38 and 40 with my Radio Shack Accurian tuner and develop signal "strength" percentages of 80% on one and 75% on the other, but if I preamplify them using any Winegard product - even my AP4727, which is supposed to have the same output capability as the AP4700 and AP4800, but with just 23dB of gain - and then bandpass filter and attenuate the single channel outputs, those channel 38 and 40 signals have been decimated by the amplifier beyond recovery.


Believe it or not, I get better performance from a Channel Master 0064DSB than from the Winegard that has identical, 23dB of UHF gain. Winegard's published overload ratings are a bad joke. Yet even with the 0064DSB, my analog channel 24 (3-4dBmV, coming off the antenna) suffers from some other analog picture sliding across it from left to right when it is amplified along with other analog signals that are a little over 20 dB stronger than it is.


Until we get a better handle on the development of intermodulation byproducts, we are going to be frustrated in a lot of situations in which the rated maximum output of an amplifier is not exceeded yet it makes weak signals worse, because a byproduct that is substantially weaker than the carriers it is derived from can still overwhelm a significantly weaker channel, and often does.



Quote:
Originally Posted by nybbler /forum/post/0


As I understand what you did, you basically used the two channel converters as bandpass filters and amplifiers, and ended up with 38 on 4, 39 on 5 (from both converters) and 40 on 6.


Would a single-channel trap to remove 39 have worked? Or would that have taken the 38 and 40 signals below the point of recovery.

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The 39 signal that is inadvertently downconverted to channel 5 by the 40-6 converter, and inadvertently converted to 72-78Mhz by the 38-4 converter, is unusable garbage. Independently, I am taking channel 39 off a directional antenna pointed at Washington that also receives channels 34, 35, 36 and 48 from the same tower cluster and amplifying it broadband.


I couldn't have processed this off-axis channel 39 signal even if I wanted to. Even though it was about 20dB stronger than the usable Baltimore signals, it was garbage even before I started to process it. A lot of people here who rely on antenna gain polar plots to "estimate" whether a so-called omnidirectional antenna is sufficient for their purposes do not realize that at oblique angles, the antenna gain is not flat across the channel, and the relative strength of a reflected, multipath signal which is close to the transmission line of a misaimed antenna will become great enough to degrade it, whereas the same multipath component would not be strong enough to degrade the signals from even a small antenna pointed correctly.


It is absolutely impossible to significantly weaken an overwhelming, adjacent channel at UHF frequencies through simple notch filtering, and I barely get away with processing these signals through the XUV converters, which are simple, one step heterodyne converters with input and output filters.. Holland and others make better quality, double conversion processors that reduce the desired signal to intermediate frequency (47 Mh? I forget) at which the adjacent channel can be filtered off more efficiently, but those cost over $500 each, whereas I have used XUVs lying around that are worth very little to me, and I can buy new ones for probably around $140 each.

Said by NIGHTHAWK:

"Not.


dBmV = 20 log (Vout/1mV).


dBmV is a ratio of two voltages. Period. There is nothing explictly or implicitly referencing power and it is independent of impedance."


Said by Targus: "No, see the post above yours, for a clue."


Disbelievers heh? I guess I'll have to quote chapter and verse, but before I begin, let me state my qualifications. I'm a retired CATV engineer with over 36 years in the business. I also hold an FCC General Class Radio Telephone lisence entitling me to run a TV or Radio station.


Nighthawk, while your formula is corrrect, it is incomplete as it should refference 75 ohms. That is always understood. And yes it is dependant on impedance by deffinition. You have to understand that the term "dBmV" was invented by the CATV industry. Back in the early 1950's it was determined that an RF level of 1 millivolt at the TV set would produce a clean noise free picture. That level was set as the refference for 0 dBmV.


Now as to the math behind the term dBmV I would suggest that you read the Technical Handbook For CATV Systems by Ken Simons. Chapter 1 discusses decibels and dBmV. For the uninitiated, Ken Simons was a very highly respected engineer employed by Jerrold Eletronics in the 1950s and 60s. Ken wrote the book on CATV mathmatics. He invented the venerable 704 Field Strength Meter which was the standard of the industry for years.


If you are disputing my statement that the correction factor between dBmV and dBm is 48.75, you are really showing your ignorance. Any CATV technical refference book will confirm that statement. Ken's book, Third Edition, page 72 paragraph 5 states: a. To convert dBmV to dBm: subtract 48.75 dB. b. To convert dBm to dBmV: add 48.75 dB.


Blonder Tongue's CATV Refference Guide dated 6/1/99 Rev 7 on page 6.7 states 0 dBm = 48.75 dBmV /75 ohms.


I can quote even more from my extensive library but by now it should be clear. But I will quote one more from the book "Modern Cable Television Technology" 2nd Edition by Walter Ciciora, James Farmer, David Large and Michael Adams, all highly respected CATV engineers. I n the Glossary on page 1005 " dBmV Decibels with respect to one milliwatt in a 75-ohm system. The unit of RF power used in cable television work in North America. In a 75 ohm system, 0 dBmV = -48.75 dBm."


So there you have it, since we all agree that dBm is a measurement of power, it follows that if there is a direct corelation to dBmV, than dBmV is also a measurement of power.


Next time you challange my statements make sure you know what you are talking about.



Quote:
Originally Posted by Rick0725 /forum/post/0


...at my home, the local ch 24 analog at 2490 kw creates interference on ch 14 low power, ch 35 low power and others. when the station goes off the air everything is fine.


...only problem, can not filter ch 24 because there is a 85kw digital ch 25 pbs that I enjoy watching.

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You could buy a pre-tuned channel 25 in, selected lowband channel out XUV on the internet for $160 or less and mix its output with your broadband output, but optimizing its performance will take more finagling than calculation. The XUV has a crappy noise figure of probably 8 to 10 dB. Its gain varies quite a bit from one manufacturer to another.


You should start by connecting your downlead directly into the XUV input with no preamp. If that doesn't work for you, meaning, if the VHF output cannot reliably be locked by your receiver, then the next remedy would be to try pre-boosting its input with your low gain amp. If that still doesn't work, you can try putting a really cheap bandpass filter, like channel 24 Jointenna, between your antenna and the preamp. The cheap, low gain bandpass filter will have less on-channel insertion loss than will a better quality filter. It won't reduce channels 23 or 25, but it will attenuate nearly every other channel to some degree or other, and that might be the most you can accomplish as far as limiting the debilitating effects of pre-amp intermodulation without fatally weakening the weakest signals is concerned. The Blonder Tongue lowband commercial bandpass filters, which cost about $500 each, commonly have about 10dB of insertion loss, for example.


If you need to combine the low gain preamp with the preamp input filter, then you probably can split the antenna downlead with a tap, with the preamp circuit getting the thru connection, and the rest of your spectrum being sourced by the tap. You may or may not need a preamp on that broadband downlead, depending on the strength of your second weakest or most troublesome, desired channel, but if you do, then you should be able to use some kind of cheap channel 24 notch. I know you've posted you channel lineup elsewhere, but I don't remember it, so I don't know if you could get away with a channel 23, 24 or 25 Jointenna used as a bandstop filter in front of your broadband preamp, or if you'd need something any tighter than that to avoid damaging other channels in the low twenties or upper twenties.


If you can't succeed with any of those combinations, I know an MATV dealer who bought a bunch of Holland agile heterodyne double conversion processors he doesn't need which, I think, can be tuned to any broadcast input channel and output any broadcast or cable TV channel simply by setting dip switches, whereas retuning the XUVs requires a spectrum analyzer and the inputs are limited to fairly narrow bands (low, mid, high and super). Last I knew, he had six left, and wanted $350 for one or $300 each for all six.