Desired Signal level (dBm) for OTA Dig TV? - AVS Forum
Forum Jump: 
 
Thread Tools
post #1 of 11 Old 05-19-2006, 08:40 AM - Thread Starter
Senior Member
 
etzeppy's Avatar
 
Join Date: Jan 2006
Location: Dallas
Posts: 216
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
I'm trying to find out the optimium signal level range in dBm for reliable Dig TV. I have access to an RF spectrum analyzer but I'm not sure what I'm looking for. I'm not far from my local stations in Dallas (10-15 miles) so I'm trying limp along with an attic antenna but I'm having a little trouble, mostly with FOX. Most of the FM stations are on the same antenna farm. Any need for an FM trap?

I would like to use the spec analyzer to figure out if I have my antenna oriented correctly and if it's possible to get enough signal with my current setup or if I'm even getting too much signal. I've done several searches but can seem to find the min and max signal levels for optimum performance.

Thanks

Keith
etzeppy is offline  
Sponsored Links
Advertisement
 
post #2 of 11 Old 05-19-2006, 09:45 AM
 
HDTVFanAtic's Avatar
 
Join Date: Mar 2004
Location: 3rd Rock from the Sun
Posts: 8,143
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
From the Code of Federal Regulation Title 47, FCC Rules and Regulations Part 73.622 which states:

(e) DTV Service Areas. (1) The service area of a DTV station is the

geographic area within the station's noise-limited F(50,90) contour

where its signal strength is predicted to exceed the noise-limited

service level. The noise-limited contour is the area in which the

predicted F(50,90) field strength of the station's signal, in dB above 1

microvolt per meter (dBu) as determined using the method in section

73.625(b) exceeds the following levels (these are the levels at which

reception of DTV service is limited by noise):



------------------------------------------------------------------------

dBu

------------------------------------------------------------------------

Channels 2-6................................................. 28

Channels 7-13................................................ 36

Channels 14-69............................................... 41

------------------------------------------------------------------------
HDTVFanAtic is offline  
post #3 of 11 Old 05-19-2006, 10:42 AM - Thread Starter
Senior Member
 
etzeppy's Avatar
 
Join Date: Jan 2006
Location: Dallas
Posts: 216
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
That at least helps me know what the FCC says the level should be. 41 db at 599MHz, center of channel FOX 4.1 (Ch 35 in Dallas), translates to -89.5dBm. However, I have no idea if that is really the min power I would be looking for at the input of a typical dig TV receiver. Based on past experience with the FCC, that 41 dBu contour value could be based on the input of a nominal antenna, input of the receiver, or just about anything else.
etzeppy is offline  
post #4 of 11 Old 05-19-2006, 10:51 AM
 
HDTVFanAtic's Avatar
 
Join Date: Mar 2004
Location: 3rd Rock from the Sun
Posts: 8,143
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
The FCC has a standard (as defined in 73.625(b) ) for measurement for signal that the stations use for determining their contours.

After that point, everything else will be influenced by the gain of the antenna, the length of cable and cable type not to mention each Receiver will also have different specs for its tuner section.

And just because most of the FM stations are out in the Cedar Hill Tower Farm with the TV stations, that does not mean you need an FM trap.
HDTVFanAtic is offline  
post #5 of 11 Old 05-20-2006, 04:05 AM
AVS Special Member
 
dr1394's Avatar
 
Join Date: May 2002
Location: Mizar 5
Posts: 3,160
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 17 Post(s)
Liked: 21
The minimum signal level for terrestrial (antenna pointed at the horizon) receivers is:

thermal noise floor + noise figure + required S/N ratio

The thermal noise floor is:

p=ktb

p = noise power
k = Boltzmann's constant (1.3807x10E-23)
t = antenna temperature (290K = 17C)
b = bandwidth (6 MHz)

p = 1.3807x10E-23 * 290 * 6000000

noise power in dBm (10 log p + 30) = -106.2

If you use a 3 dB preamp at the antenna, then your receiver noise figure will be essentially 3 dB. The required signal to noise ratio is around 16 db. So the minimum signal level is -87.2 dBm.

To convert from dBu to dBm, the equation for a 75 ohm antenna is:

P(dBm) = E (dBµV/m) + Gr(dBi) - 20log F(MHz) - 75.46

where Gr is the gain of the receiving antenna.

The DTV station is on channel 35 (599 MHz) and the typical antenna gain is 14 dBi. At the 41 dBu contour, the received power will be 41 + 14 - 55.54 - 75.46 = -76 dBm. With our -87.2 dBm receiver, the signal will be 11.2 dB above the minimum signal required.

Ron

HD MPEG-2 Test Patterns http://www.w6rz.net
dr1394 is offline  
post #6 of 11 Old 05-20-2006, 04:16 AM
AVS Special Member
 
dr1394's Avatar
 
Join Date: May 2002
Location: Mizar 5
Posts: 3,160
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 17 Post(s)
Liked: 21
The minimum signal level for terrestrial (antenna pointed at the horizon) receivers is:

thermal noise floor + noise figure + required S/N ratio

The thermal noise floor is:

p=ktb

p = noise power
k = Boltzmann's constant (1.3807x10E-23)
t = antenna temperature (290K = 17C)
b = bandwidth (6 MHz)

p = 1.3807x10E-23 * 290 * 6000000

noise power in dBm (10 log p + 30) = -106.2

If you use a 3 dB preamp at the antenna, then your receiver noise figure will be essentially 3 dB. The required signal to noise ratio is around 16 db. So the minimum signal level is -87.2 dBm.

To convert from dBu to dBm, the equation for a 75 ohm antenna is:

P(dBm) = E (dBµV/m) + Gr(dBi) - 20log F(MHz) - 75.46

where Gr is the gain of the receiving antenna.

The DTV station is on channel 35 (599 MHz) and the typical antenna gain is 14 dBi. At the 41 dBu contour, the received power will be 41 + 14 - 55.54 - 75.46 = -76 dBm. With our -87.2 dBm receiver, the signal will be 11.2 dB above the minimum signal required.

Ron

HD MPEG-2 Test Patterns http://www.w6rz.net
dr1394 is offline  
post #7 of 11 Old 05-20-2006, 06:39 AM - Thread Starter
Senior Member
 
etzeppy's Avatar
 
Join Date: Jan 2006
Location: Dallas
Posts: 216
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Originally Posted by dr1394
The minimum signal level for terrestrial (antenna pointed at the horizon) receivers is:

thermal noise floor + noise figure + required S/N ratio

The thermal noise floor is:

p=ktb

p = noise power
k = Boltzmann's constant (1.3807x10E-23)
t = antenna temperature (290K = 17C)
b = bandwidth (6 MHz)

p = 1.3807x10E-23 * 290 * 6000000

noise power in dBm (10 log p + 30) = -106.2

If you use a 3 dB preamp at the antenna, then your receiver noise figure will be essentially 3 dB. The required signal to noise ratio is around 16 db. So the minimum signal level is -87.2 dBm.

To convert from dBu to dBm, the equation for a 75 ohm antenna is:

P(dBm) = E (dBµV/m) + Gr(dBi) - 20log F(MHz) - 75.46

where Gr is the gain of the receiving antenna.

The DTV station is on channel 35 (599 MHz) and the typical antenna gain is 14 dBi. At the 41 dBu contour, the received power will be 41 + 14 - 55.54 - 75.46 = -76 dBm. With our -87.2 dBm receiver, the signal will be 11.2 dB above the minimum signal required.

Ron
Thank you Ron! I've seen this type of calculation for land mobile radio but just couldn't find it referenced to 75 ohm and TV equipment. You're the man!

Thanks
etzeppy is offline  
post #8 of 11 Old 05-20-2006, 06:49 AM - Thread Starter
Senior Member
 
etzeppy's Avatar
 
Join Date: Jan 2006
Location: Dallas
Posts: 216
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Ron,

As a follow-up, I'm measuring levels in the range of -60 dBm to -45 dBm for the local TV carriers. I'm also seeing FM and a few other signals at -20 dBm and higher. At what level would you expect to exceed the dynamic range of the receiver and start generating intermod or causing receiver desense?
etzeppy is offline  
post #9 of 11 Old 05-20-2006, 09:26 AM
AVS Special Member
 
dr1394's Avatar
 
Join Date: May 2002
Location: Mizar 5
Posts: 3,160
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 17 Post(s)
Liked: 21
Quote:
Originally Posted by etzeppy
Ron,

As a follow-up, I'm measuring levels in the range of -60 dBm to -45 dBm for the local TV carriers. I'm also seeing FM and a few other signals at -20 dBm and higher. At what level would you expect to exceed the dynamic range of the receiver and start generating intermod or causing receiver desense?
The ATSC receiver performance guideline states -83 dBm to -8 dBm.

http://www.atsc.org/standards/practices/a_74_rfs.pdf

-83 dBm is the noise floor a receiver with a 7 dB noise figure. -8 dBm is where preamps start to overload. Just guessing, but I'd say most receivers will overload long before -8 dBm.

Unless you're trying to receive a weak channel 6, an FM trap is almost always a good idea. Other huge signals are often in the paging band around 150 MHz.

Ron

HD MPEG-2 Test Patterns http://www.w6rz.net
dr1394 is offline  
post #10 of 11 Old 05-20-2006, 11:02 AM - Thread Starter
Senior Member
 
etzeppy's Avatar
 
Join Date: Jan 2006
Location: Dallas
Posts: 216
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Thanks for all of the info Ron. This has been an amazing experience. I disconnected my directional VHF/UHF and replaced it with a set top VHF rabbit ear & UHF loop combo unit (no amp). I also added an FM trap and variable 20 dB pad. I have everything tunned in at around -65 dBm with no unwanted signals higher than about -40 dBm.

I was about to purchase an outdoor antenna and put myself through a ton of hassle, for nothing. Too much signal; what a concept.
etzeppy is offline  
post #11 of 11 Old 05-20-2006, 11:45 AM
AVS Special Member
 
AntAltMike's Avatar
 
Join Date: Nov 2001
Location: College Park, MD
Posts: 3,579
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 23 Post(s)
Liked: 25
FYI, for those who get their hands on a spectrum analyzer but aren't all that familiar with one, bandwidth correction must be factored into the signal strength level indicated in the screen.

I've used a few AVCOM spectrum analyzers that had selectable bandwidths of 3Mz and 300Kz. If you are using the 3Mz setting, you need to add 3dB to what you see on the screen. If you are using the 300Kz setting, you need to add 13dB. When evaluating antenna reception quality, I always use the finer resolution because it doesn't mask over notches in the antennas reception spectrum.

Somewhere way back in the Local Antenna Reception thread, I think hol_lands posted a list of the tested input thresholds of five different, fairly late model tuners at different frequencies. As I recall, the minimum input of those receivers and channels tested was in the range of about -79dBm to -82 or 83dBm.

I, too, would be very surprised if a tuner could function properly with an input signal level of, or anywhere near -8dBm.
AntAltMike is offline  
Closed Thread HDTV Technical

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off