: Signal Amplifiers (Amps, Preamps, Distro Amps) - See Chart in Post #1



bentoronto
2010-03-28, 08:51 AM
At first reading, that seems like a defencible kind of experiment done in-situ with simple tools (pads) and using a metric ("dB margin") that can be applied broadly. Good R&D.

BTW, some old FM capacitor front-end tuners have lovely mechanical meters that can be helpful for aiming antennas to a certain transmitter. While maybe not the same parameter as HDTV signal strength meters use (whatever that may be), good for some functions and readable to maybe 5% (1:20) changes.

holl_ands
2010-03-28, 01:12 PM
FCC OET measured performance for over 100 CECB Converter Boxes. Statistical analysis:
http://www.fcc.gov/oet/info/documents/reports/9TR1003-ConverterBoxTestReport.pdf
Average Sensitivity was -85 dBm (+/- 1.25 dB UHF and +/- 2 dB VHF).

In fol. test by MSW for NAB, Apex DT250 was found to be 1 dB more sensitive than the
LG/Zenith CECB Prototype and the best CECB tested was 4 dB more sensitive:
http://www.nabfastroad.org/NAB-STV%20Digital%20Converter%20Box%20Evaluation/Manuals-summary-report1.html
Since ALL had to meet -83 dBm CECB spec, the Prototype was probably right at the spec point.
Hence APEX DT250 likely has a sensitivity of about -84 dBm, so:
NF = -(-106.2 Thermal Noise) - (15.2 Required CNR) +(-84 Sensitivity) = 7.2 dB

If you want more accuracy, you're going to have to measure your actual box anyway.....

IDRick
2010-03-28, 03:21 PM
FCC OET measured performance for over 100 CECB Converter Boxes. Statistical analysis:
http://www.fcc.gov/oet/info/documents/reports/9TR1003-ConverterBoxTestReport.pdf
Average Sensitivity was -85 dBm (+/- 1.25 dB UHF and +/- 2 dB VHF).

In fol. test by MSW for NAB, Apex DT250 was found to be 1 dB more sensitive than the
LG/Zenith CECB Prototype and the best CECB tested was 4 dB more sensitive:
http://www.nabfastroad.org/NAB-STV%20Digital%20Converter%20Box%20Evaluation/Manuals-summary-report1.html
Since ALL had to meet -83 dBm CECB spec, the Prototype was probably right at the spec point.
Hence APEX DT250 likely has a sensitivity of about -84 dBm, so:
NF = -106.2 (Thermal Noise) + 15.2 (Required CNR) -(-84) = 7.2 dB

If you want more accuracy, you're going to have to measure your actual box anyway.....
Holl_ands, thank you for the links and equation for estimating NF. You're always so incredibly helpful! :)

Additional info, I also measured power level using a Sadelco 719c during the test. Special thanks to Rabbit73 for loaning the Sadelco, very helpful! During that particular test, I observed the following:

Sadelco reading with pre-amp: +8.5 dBmv
Converted to dBm = (+8.5-48.8) = -40.3 dBm
Power at dropout = -40.3 - 49.1 = -89.4 dBm

Sadelco reading without pre-amp: -14.5 dBmv
Converted to dBm = (-14.5-48.8) = -63.3 dBm
Power at dropout = -63.3 - 26.2 = -89.5 dBm

Overall, I have 9 estimates for this channel: Mean = -89.3, SE = 0.35

Using the equation provided by Holl_ands, estimated NF for my Apex 502: -106.2 + 15.2 - (-89.4) = -1.6

Hmmm, not a good answer... While my estimate appears to be consistent across measurements, it does not appear to be accurate estimate for power level at dropout. Any thoughts on how to improve my estimate?

If we ignore my estimate of NF and use Holl_ands estimate (always a wise choice to use a highly respected communications engineer's estimate over a dairy scientist's...) then the estimated NF on the winegard pre-map is:

difference in margin minus pre-amp gain = 22.9 - 19 = 3.9

(NFapex - NFpreamp) = 3.9
NFpre-amp = 7.1-3.9 = 3.2

Final result is close to Winegard's published NF (2.8 dB) for the AP 8700. Holl_ands estimate is much better... :)

IDRick
2010-03-28, 03:47 PM
Ah ha! Holl_ands link provided the answer! The FCC used a different measure: "For the tests performed in this program, TOV (threshold of visibility) was selected as the level at which at least one picture impairment was visible in each of two consecutive 20-second observation intervals." In my testing, I determined the point where the picture was completely lost. Picture impairment started when signal quality on the Apex 502 dropped below 100%, which was 4 to 5 dB higher than dropout point. To match the FCC, I need to either use a different threshold point or change the required CNR. I can easily estimate TOV by simply decreasing the regression intercept by 4.5 dB.

holl_ands
2010-03-28, 07:19 PM
I corrected the above formula...it had been calculating negative NF.
Now it calculates positive NF.....the same say I actually do it....

In formal ATSC tests, TOV is typically defined as 3x10-6 bit error rate (BER)
over a "meaningful" measurement interval...which depends on whether it
is a Gaussian Noise Test (quick), a Multipath Test (longer) or an Ensemble.
Which requires access to the decoded MPEG2 data stream...and a known pattern.

You also have to be careful....you might get different answers whether you
measure the point at which you LOSE lock...or the point at which you ACQUIRE lock.

IDRick
2010-03-28, 08:13 PM
Thanks for correcting the formula. For my purposes, the current regression works well for most applications. Probably not worth the effort to redo the data collection process since it likely only applies to my particular Apex... I have developed regression equations for two Apex boxes. On UHF, the slopes were identical but intercepts differed between converter boxes.

holl_ands
2010-03-28, 09:09 PM
Fol. articles from Potomac Instruments summarizes NTSC and ATSC Field Strength Predictions
for Class A/B Service Contours and how to adjust signal level readings for ATSC, using
Potomac Inst. FIM-71 & FIM-72 Analog Field Strength Meters:
http://www.pi-usa.com/support.htm

ATSC Tuner Sensitivity is assumed to be -81.2 dBm (VHF) and -84.2 dBm (UHF)
plus they ass-u-me "standard" values for antenna gain and cable loss to derive the
ATSC Minimum Planning Factor Field Strengths:
27.6 dBuV (Lo-VHF), 35.6 dBuV (Hi-VHF) and 40.6 dBuV (UHF).

We can compare these to NTSC Class B Planning Factor Field Strengths:
47.0 dBuV (Lo-VHF), 56.0 dBuV (Hi-VHF) and 64.0 dBuV (UHF).
From the difference between these two, we can conclude the assumed
NTSC Grade B Sensitivities are -61.8 dBm (Lo-VHF), -60.8 dBm (Hi-VHF and UHF).

These are ATSC and NTSC Class B Contour Lines where half the residents fail to receive a signal
and signal degradations are "not TOO bad"....whatever that means....(I'm paraphrasing)....
which is equivalent to a ITU-R Grade 3.

FYI: Difference between NTSC Grade A and Grade B is 21 dB (Lo-VHF), 15 dB (Hi-VHF) and 10 dB (UHF), so
NTSC Grade A Sensitivities are -40.8 dBm (Lo-VHF), -45.8 dBm (Hi-VHF) and -50.8 dBm (UHF).

===================================
Fol. June 1974 FCC Report 2229-63 is very out of date, but might apply to an VERY OLD TV:
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/reports/R2229-63.pdf
TASO Picture Quality Grading scale is explained on pg10.
Figures 29 and 30 plot statistics for 47 different TV Receivers on Ch34: TASO Grade vs Sensitivity.

Grade 1 "Excellent, no snow" was met at -43 dBm level for half the TVs....and not at all for some.
NTSC signal source was limited to only 40 dB Video SNR and might have been a limiting factor....

Grade 2 "Fine, barely perceptible snow" was met at -50 dBm level for half the Antique TVs.
For TVs bought in 1972 the BEST was -57 dBm and WORST was -37 dBm (Yikes!!!).

Grade 3 "Passable, snow not objectionable" was met at -57 dBm level for half the Antique TVs.
BEST was -67 dBm and WORST was -46 dBm.

For more recent NTSC Tuners, UHF Grade 2 Sensitivity of -61 dBm or better might be in the ballpark
with UHF Grade 3 Sensitivity of perhaps -71 dBm.

IDRick
2010-03-29, 01:29 AM
Holl_ands or anyone,

Can you explain the relationship between cascaded NF and my margin to dropout method? Using Tom's spreadsheet for my situation, the cascaded NF is 5.29 dB with an AP8700 and 17.50 dB without a pre-amp. As stated earlier in this thread, margin to dropout for ch 17 was 49.1 dB with the pre-amp and 26.2 dB without a pre-amp. I saw a net improvement of 22.9 dB by adding the pre-amp. Please excuse my lack of knowledge, but I don't see the relationship between these two measures....

Thanks!

Rick

dave1480
2010-03-29, 09:45 AM
hello i can get both at the same price,live in the toronto city but have a db8 indoors with many hi-rises in the way ,south facing.most channels are solid but 2 series breaks up at times,would also like to get the 51 series,it reads about 20% but no pic.the dbm power is around -109 for this location.the kitz will boost to around -80dbm,the cm7777 will boost it to -60dbm.at -80 im at the edge of break-up.any ideas.thanks

Jase88
2010-03-29, 11:13 AM
I believe this has been mentioned a few times in the amplifier thread...however, here goes:

The purpose of a pre-amp is to overcome line loss in marginal reception areas, where the cable between the antenna and tuner is a longer run. Secondly, a pre-amp is sometimes used to reduce tuner noise.

Given that you're indoors, I'm assuming that your cable run is shorter (i.e. under 40 feet)? If you have no reception due to blockage, etc., a pre-amp won't help. You can't amplify what isn't there: 0 x 26dB = 0.

DdDave
2010-03-29, 01:34 PM
dave1480, in addition to Jase88's comments above, you would probably want to play around with aim more than pre-amps. Your DB8 is a pretty directional antenna, and from your location 51 is not south, it is probably south-east or possibly even ESE. Take a look at your TVfool results to see where you have to aim.

EDIT: took a quick look on tvfool, and 51 is probably at around 128 from downtown, and "south" in downtown is probably aound 165 or 170, so it's a 35-40 rotation required.

holl_ands
2010-03-29, 02:21 PM
A GOOD tuner might have a NF=7 (CECB Sensitivity spec is equivalent to 8 dB),
(and the BEST might be 2 dB lower).

If a 12 dB Preamp w NF=3 (e.g. HDP-269) is inserted with short, 0 dB loss cables,
without Preamp NF=7 and with Preamp SYSTEM NF=3.5, which is 3.5 dB better.

If there is 2 dB loss in the cables, NF=7+2=9 dB is reduced by the Preamp to
a SYSTEM NF=3.9 dB, which is 5.1 dB better.

This presumes that input signal levels are low enough that the Preamp is not generating
Intermod Distortion Noice Products higher than the Thermal Noise Floor......

Also, when a Preamp is embedded into an Indoor Antenna, it is easier to match the SWR with
LESS LOSS than a matching network in a passive Indoor Antenna (e.g. RCA ANT111 RELoop).

IDRick
2010-03-29, 02:45 PM
A GOOD tuner might have a NF=7 (CECB Sensitivity spec is equivalent to 8 dB),
(and the BEST might be 2 dB lower).

If a 12 dB Preamp w NF=3 (e.g. HDP-269) is inserted with short, 0 dB loss cables,
without Preamp NF=7 and with Preamp SYSTEM NF=3.5, which is 3.5 dB better.

If there is 2 dB loss in the cables, NF=7+2=9 dB is reduced by the Preamp to
a SYSTEM NF=3.9 dB, which is 5.1 dB better.

This presumes that input signal levels are low enough that the Preamp is not generating
Intermod Distortion Noice Products higher than the Thermal Noise Floor......

Also, when a Preamp is embedded into an Indoor Antenna, it is easier to match the SWR with
LESS LOSS than a matching network in a passive Indoor Antenna (e.g. RCA ANT111 RELoop).
Holl_ands, I assume your response was for me?

I understand how to calculate the NF data but to me, it seems that NF approach underestimates the change in signal strength. In my case, NF method suggests signal strength improves by 12.2 dB but I observed a 23 dB increase on both the Sadelco719c and Apex margin to dropout method. Why the discrepancy?

Jase88
2010-03-29, 02:54 PM
I did a quick search, but didn't see any posts regarding this...

Did anyone notice that Tin Lee is now offering pre-amps?

RadarBoy
2010-03-29, 10:04 PM
Hi All;

I am looking for some help on an ideal pre-amp to add to my system: http://www.digitalhome.ca/forum/showpost.php?p=1083494&postcount=1015

Despite years of working with WWII electronics veterans, the finer points of calculating the right amp (or if I am better without) are beyond my meager skills. Anyone able to help me out?

Panamark has had success with a CM7777 in his setup, but he is 10km further away from the strong sources i live right under. Anythoughts?

Radarboy

holl_ands
2010-03-29, 10:49 PM
Tin-Lee (and other) Preamp specs were compared here (read back a few posts for context):
http://www.digitalhome.ca/forum/showthread.php?p=880155

Preamp Specs (modified Solid Signal chart) and Overload Calculator to determine "Best" Preamp:
http://imageevent.com/holl_ands/files/ota

majortom
2010-03-29, 11:38 PM
Holl_ands or anyone,

Can you explain the relationship between cascaded NF and my margin to dropout method? Using Tom's spreadsheet for my situation, the cascaded NF is 5.29 dB with an AP8700 and 17.50 dB without a pre-amp. As stated earlier in this thread, margin to dropout for ch 17 was 49.1 dB with the pre-amp and 26.2 dB without a pre-amp. I saw a net improvement of 22.9 dB by adding the pre-amp. Please excuse my lack of knowledge, but I don't see the relationship between these two measures....

When your padding it down with attenuators, your padding the entire spectrum with the same amount, fades don't work that way in the real world,
unless comparing say outdoor to indoor reception, etc. i.e. ur knocking down potential interferers down by the same amount, etc.

Do you have any weaker signals to do your test on? Thinkin 26 dB margin without a pre-amp before losing the signal is a pretty good signal to start with. Realistically not at much risk of fading to point of complete dropout in real world conditions without a pre-amp. Is that a pretty reliable signal for you at your location? Can ya repeat your "fade margin" test on a weaker signal? Is there something that ya can barely receive now without a pre-amp? As most people are using a pre-amp to pull a marginal desired distant signal out of the dirt to the point it can received reliably.

IDRick
2010-03-30, 12:43 AM
When your padding it down with attenuators, your padding the entire spectrum with the same amount, fades don't work that way in the real world,
unless comparing say outdoor to indoor reception, etc. i.e. ur knocking down potential interferers down by the same amount, etc.

Do you have any weaker signals to do your test on? Thinkin 26 dB margin without a pre-amp before losing the signal is a pretty good signal to start with. Realistically not at much risk of fading to point of complete dropout in real world conditions without a pre-amp. Is that a pretty reliable signal for you at your location? Can ya repeat your "fade margin" test on a weaker signal? Is there something that ya can barely receive now without a pre-amp? As most people are using a pre-amp to pull a marginal desired distant signal out of the dirt to the point it can received reliably.
Yes, you're correct. Ch 17 is rock solid without a pre-amp. I do have one station that does not come in without a pre-amp (ch 47) and another that has a margin of 10 dB without a pre-amp. Both ch 15 and 17 had a 23 dB increase in margin after adding the pre-amp. Other channels have smaller increases (21 to 22 dB). I need do more tests to verify response and calculate average response. Margin estimates do vary by plus or minus 1.5 dB when repeated over time. My antenna is mounted in the attic but all towers are LOS and range from 30 to 45 miles away.

majortom
2010-03-30, 07:22 AM
forgot to ask, what did u say u were using to pad the signal?
using the best of laboratory test equipment, I wouldn't expect
anything closer than +/- 1 dB in any test result.

IDRick
2010-03-30, 11:31 AM
Majortom,

I'm using Tru-Spec attenuators. I purchased 2-20's, 1-10, 1-6, 1-3, an 2-1's. Much cheaper than buying a variable attenuator although the variable attenuator would be more user friendly.

Best,

Rick