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post #1 of 139 Old 12-06-2014, 06:19 PM - Thread Starter
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RCA TVPRAMP1R Preamp: A Technical Review

The RCA TVPRAMP1R preamp is one of the few preamps available with separate or combined VHF/UHF inputs. Except for gain, there are no specifications available for this preamp. This post will correct that.

I purchased one of the preamps and performed various measurements with my Rigol spectrum analyzer, an Ailtech noise source and an HP 75 ohm directional coupler. Here are the measurements with analyzer images attached.

The power supply is 9V and the preamp draws 68ma.

1) Full band gain measurement. The UHF gain is 2-3 dB higher than claimed.
2) UHF gain to 1000 MHz. I wanted to see how badly a nearby cell phone tower might affect the preamp. The gain is => 20 dB up to 1000 MHz.
3) An expanded look at VHF and the FM trap. With the FM trap switched out the gain is flat from low to high VHF.
4) Isolation. This is referenced to 0 dB and shows how much signal leaks through the preamp with the power off. Strong signals, especially on VHF will still be received even with no power applied.
5) Noise Figure UHF. This is better than most preamps.
6) Noise Figure high VHF. Good enough for most situations.
7) Noise Figure low VHF. Better than required since the environmental noise will be higher than the noise figure.
8) Return Loss for low, high and UHF combination input. The markers show the lowest return loss (highest SWR) for each band. I converted those to SWR.
9) Return Loss for the separate VHF input. The markers show the lowest return loss (highest SWR) for the low and high VHF bands. I converted those to SWR.
10) Return Loss for the output port. The markers show the lowest return loss (highest SWR) for each band. I converted those to SWR. This is much better than I've seen on other preamps.

I also preformed a 1 dB gain compression test for VHF and UHF. There are no graphs for this test. P1dB on VHF is +15 dBm and on UHF is +10 dBm.

For such an inexpensive preamp I think the measurements are all good except for P1dB. I'd really like to see +20 dBm. My guess is that this preamp probably should not be used on a high gain UHF antenna with TV Fool Noise Margin predictions of much over +40 dB. This should be an excellent preamp where one is required.
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post #2 of 139 Old 12-07-2014, 07:23 AM
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The RCA preamp is supposedly similar to the original Channel Master 7778/7777 preamps in terms of design and offering both separate UHF and VHF inputs. The noise figure on the CM was claimed about 2.0, whereas the RCA is reportedly about 2.8 unless you state otherwise. But apparently a good inexpensive preamp that is supposedly resistant to overload, which should help if you have both strong and weak signals.
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post #3 of 139 Old 12-07-2014, 08:16 AM
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I've had this preamp for 4 years now, & it has helped at getting 4 LPTV UHF stations, & the only Class A VHF-LO station. The full power stations were never the problem, as I'm about 30 miles SE of Chicago. It has always been the LPTV stations that are weak in my direction that have been the problem, & the pre-amp has helped it. It especially helps with LPTV VHF stations that broadcast less than 1kw (maybe 2kw as well).
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post #4 of 139 Old 12-07-2014, 12:56 PM - Thread Starter
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Quote:
Originally Posted by tylerSC View Post
The RCA preamp is supposedly similar to the original Channel Master 7778/7777 preamps in terms of design and offering both separate UHF and VHF inputs. The noise figure on the CM was claimed about 2.0, whereas the RCA is reportedly about 2.8 unless you state otherwise. But apparently a good inexpensive preamp that is supposedly resistant to overload, which should help if you have both strong and weak signals.
I measured an old CM7777. I thought I had posted the results but I can't find it. If that post exists it will have the P1dB results. Most of the preamps I measured have P1dB in the mid teens. The Tin Lee I measured was +20 dBm, the best UHF preamp I found. A few old CM7777 measurements are attached. The RCA has a slightly better noise figure and slightly higher gain.
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post #5 of 139 Old 12-08-2014, 09:25 AM
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Chuck,

The DS815-TG can do a very good facsimile of a P1dB measurement and you can capture its screen. Use the power sweep function of the generator over a very tight frequency span and you'll get something that looks like the attachment. From the step display, it's fairly straight forward to determine the P1dB level.

Several caveats:
1. Generator non-linearity. The TG on our Rigol is +/- about 1 dB. Pick a frequency where it's fairly accurate.
2. Depending on the amp to be tested, you may need either additional attenuation or amplification of the TG's -20 to 0 dBm range.

I tested the RCA about two years ago but haven't revisited it recently. Your results sound fairly consistent with what I recall.
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Last edited by ADTech; 12-08-2014 at 09:46 AM.
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post #6 of 139 Old 12-08-2014, 01:54 PM - Thread Starter
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Quote:
Originally Posted by ADTech View Post
Chuck,

The DS815-TG can do a very good facsimile of a P1dB measurement and you can capture its screen. Use the power sweep function of the generator over a very tight frequency span and you'll get something that looks like the attachment. From the step display, it's fairly straight forward to determine the P1dB level.

Several caveats:
1. Generator non-linearity. The TG on our Rigol is +/- about 1 dB. Pick a frequency where it's fairly accurate.
2. Depending on the amp to be tested, you may need either additional attenuation or amplification of the TG's -20 to 0 dBm range.

I tested the RCA about two years ago but haven't revisited it recently. Your results sound fairly consistent with what I recall.

Thanks for the tip. I've been using an HP-8640B as the signal source, a JFW 75 ohm 1 dB step attenuator and the Rigol as the power meter.
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post #7 of 139 Old 12-08-2014, 04:57 PM
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Calaveras, thank you for your detailed review of the RCA preamp and the follow-on measurements of the CM7777.

New data often raises questions::

1, 2, & 3 - Did you measure the separate UHF gain (down thru lower VHF)?

Curious how much FM gets through the independent UHF section.

4 - Wonder what the power-off isolation is thru the UHF section only (separate input)?

(Assuming your spectrum 4 was with combined mode).

6 & 7 - Did you see any difference in VHF noise figure when FM Trap was in?

9 - Did FM Trap affect VHF input Return Loss?

Seems you have some nice test equipment.

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

ADtech,

Thanks for the measurement data you have posted over several months.
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post #8 of 139 Old 12-08-2014, 06:52 PM - Thread Starter
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Quote:
Originally Posted by tripelo View Post
Calaveras, thank you for your detailed review of the RCA preamp and the follow-on measurements of the CM7777.

New data often raises questions::

1, 2, & 3 - Did you measure the separate UHF gain (down thru lower VHF)?

Curious how much FM gets through the independent UHF section.

4 - Wonder what the power-off isolation is thru the UHF section only (separate input)?

(Assuming your spectrum 4 was with combined mode).

6 & 7 - Did you see any difference in VHF noise figure when FM Trap was in?

9 - Did FM Trap affect VHF input Return Loss?

Seems you have some nice test equipment.

Your questions point out that with a preamp like this there are many possible combinations to measure. I did not do them all but I have a couple more which are now attached below.

1) VHF Gain with FM trap in Combined (yellow)/Separate (purple) inputs. They are virtually identical.
2) VHF return loss on VHF separate input.

When I get a little more time I'll review you questions that the additional measurements don't address.

The Rigol analyzer is the centerpiece of the test equipment. To convert the 50 ohm Rigol to 75 ohms I use a couple of Anzac TPX-75-4 transformers. After a long search I found an Ailtech 7618 noise source. As far as I can tell there's no such thing as a 75 ohm noise source so to use it in a 75 ohm system I added a JFW 57ZT-1G 75:50 ohm transformer with about 1 dB loss on UHF and 0.7 dB loss on VHF. For return loss I'm using an HP-86207A 75 ohm directional coupler.
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post #9 of 139 Old 12-09-2014, 03:43 AM
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Quote:
Originally Posted by Calaveras View Post
... with a preamp like this there are many possible combinations to measure.
Yes quite a few combinations, and such measurements can consume a good bit of time..

Thanks for the additional scans.

Quote:
...The Rigol analyzer is the centerpiece of the test equipment. .... For return loss I'm using an HP-86207A 75 ohm directional coupler.
Yes seems most precision lab test equipment is based on 50 Ohms. Often, hobbyists have to come up with innovative combinations to address 75 Ohm measurements. Looks like you have found your test combo to do the job.
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post #10 of 139 Old 12-09-2014, 07:39 AM
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The zip file attached below is the data I pulled from my RCA sample about a year and a half ago using the HP8970A Noise Figure Meter with an HP346A noise source.

The individual .txt file names are self-explanatory.

A note about the data from the 8970A: If the measured noise figure value exceeds the calibrated limit of the meter, usually between 30 and 35 dB, the meter inserts wildly invalid values for both gain and noise figure. When I pull the NF meter data into an Excel spreadsheet, I usually do a search for these values and replace them with a standardized value, for example 35 dB for NF and 0 for the gain if I want to keep the graph from having extraordinary spikes.

The Rigol has a built in conversion function for 75 ohm measurements when using a standard 5.7 dB insertion loss minimum loss pad.

This app note may be of interest: http://www.maximintegrated.com/en/ap...ex.mvp/id/3250
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File Type: zip RCA TVPRAMP1R 50-1000 MHZ VHF input FM filter ON.zip (48.8 KB, 508 views)

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post #11 of 139 Old 12-09-2014, 08:36 AM
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UHF Noise Figure?

I am not an engineer and these graphs and charts confuse me. So, long story short, what was the UHF noise figure on the RCA preamp determined to be? I had read where it reportedly was around 2.8, whereas the old Channel Master was claimed to be about 2.0. All I know is the lower the better.
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post #12 of 139 Old 12-09-2014, 09:40 AM
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Read the text files I posted for my readings. They're all in there.

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post #13 of 139 Old 12-09-2014, 09:46 AM - Thread Starter
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Quote:
Originally Posted by tylerSC View Post
I am not an engineer and these graphs and charts confuse me. So, long story short, what was the UHF noise figure on the RCA preamp determined to be? I had read where it reportedly was around 2.8, whereas the old Channel Master was claimed to be about 2.0. All I know is the lower the better.
I measured 1.9 to 2.2 dB which are the values I calculated from the display and I added to the image.

I use the noise source On/Off measured on a spectrum analyzer method to calculate noise figure. This method requires the Excess Noise Ratio (ENR) of the noise source to be known. Most noise sources have a table of ENR vs frequency on the device. The HP-8970A that ADTech uses has this table entered into the unit. ENR is interpolated between frequency points.

In this post I briefly explain the method and include the formula to calculate noise figure using the noise On/Off difference and ENR.

https://www.avsforum.com/forum/25-hdt...topic-524.html

Basically the higher the noise figure of the device being measured, the smaller the noise On/Off difference.
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post #14 of 139 Old 12-09-2014, 10:08 AM - Thread Starter
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Quote:
Originally Posted by tripelo View Post
New data often raises questions::

1, 2, & 3 - Did you measure the separate UHF gain (down thru lower VHF)?

Curious how much FM gets through the independent UHF section.

4 - Wonder what the power-off isolation is thru the UHF section only (separate input)?

(Assuming your spectrum 4 was with combined mode).

6 & 7 - Did you see any difference in VHF noise figure when FM Trap was in?

9 - Did FM Trap affect VHF input Return Loss?

I think I've addressed your questions. New images attached.

1) Gain - VHF through UHF using the VHF separate input
2) Gain - VHF through UHF using the UHF input with input switch set to separate
3) Isolation - VHF Separate Input
4) VHF Noise Figure Trap In/Out (Purple - Noise off, Yellow Noise on with trap in, Cyan - Noise on with trap out) No difference in low/high VHF noise figure. Marker 1 is at 88 MHz and Marker 2 is at 174 MHz.
5) VHF Return Loss with trap In/Out (It looks much different but the SWR is good either way
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post #15 of 139 Old 12-09-2014, 10:16 AM
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Quote:
Originally Posted by ADTech View Post
The zip file attached below is the data I pulled from my RCA sample about a year and a half ago using the HP8970A Noise Figure Meter with an HP346A noise source.
Thanks for the data files.

Quote:
... Excel spreadsheet, I usually do a search for these values...
If no big mistakes, think the attached images reflect your data over the VHF and UHF bands.





One can see that the FM trap does not cover the lower end of the FM band (the response is above the 90 MHz marker).

Quote:
...app note may be of interest: http://www.maximintegrated.com/en/ap...ex.mvp/id/3250 ...
Yes, good info. One can set up a spreadsheet utilizing those equations, or similar, and convert 50 Ohm derived S-parameters to 75 Ohm S-parameters and actual impedance values.

Quote:
Originally Posted by tylerSC View Post
...what was the UHF noise figure on the RCA preamp determined to be? ...
tylerSC,

Calaveras' measurement:

Quote:
Originally Posted by Calaveras View Post
...I measured 1.9 to 2.2 dB...
Those numbers are a bit lower than ADtech's (close to 2.5 dB), that is pretty good agreement considering measurement variables, equipment variables, and unit-to-unit variables.
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post #16 of 139 Old 12-09-2014, 10:59 AM
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Here are a few other images from that period of testing that were in the same folder. All were done with our older HP8569B analyzer. I haven't revisited them for accuracy or expanded explanation.
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post #17 of 139 Old 12-09-2014, 03:46 PM - Thread Starter
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Quote:
Originally Posted by tripelo View Post
Thanks for the data files.

Those numbers are a bit lower than ADtech's (close to 2.5 dB), that is pretty good agreement considering measurement variables, equipment variables, and unit-to-unit variables.

The exact numbers depend on the ENR I used with the noise source. The problem is determining the loss of the transformer plus the adapters. I was using the numbers from the transformer spec but closer examination shows that might be as much as 0.5 dB too high. That means the NF numbers could be up to 0.5 dB too low. Low noise figures are hard to measure to within 0.1 dB because it's hard to know the ENR that accurately. Fortunately 0.5 db isn't going to be noticed by anyone watching TV.
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post #18 of 139 Old 12-09-2014, 05:29 PM
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Quote:
Originally Posted by Calaveras View Post
I think I've addressed your questions. New images attached.
Yes, thanks for the additional measurements.

Quite interesting to me. May help with a puzzling question I had in mind. Need to think a bit more about the results shown in the graphs and how they relate to the actual preamp circuitry.

Quote:
Originally Posted by ADTech View Post
... a few other images from that period of testing that were in the same folder...
Good.

Quote:
Originally Posted by Calaveras View Post
... Low noise figures are hard to measure to within 0.1 dB...
Yes, understood.
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post #19 of 139 Old 12-10-2014, 07:50 AM
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Chuck,

Quote:
The problem is determining the loss of the transformer plus the adapters.
Additionally, the response of the transformers isn't going to be linear across the test band, in most situations.

When using the NF meter with min loss pads, all those extraneous insertion losses are calibrated out or accounted for during meter setup. For the min loss pads, since they have a very flat insertion loss (resistive elements), their insertion loss is known and it doesn't vary across the test frequency band. Additionally, the DUT's temperature must be accounted for and there's an entry that has to be made to account for that since the device's physical temperature will also affect the meter readings.

Quote:
Fortunately 0.5 db isn't going to be noticed by anyone watching TV.
This is so true but is overlooked for so many who are looking for that last little bit of margin. Kind of like trying to squeeze an addition couple of horsepower out of an engine that's already near its limits.

You're certainly to be commended for putting together your numbers with your setup. You are definitely in the ball park!

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post #20 of 139 Old 12-10-2014, 09:24 PM - Thread Starter
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Quote:
Originally Posted by ADTech View Post
Chuck,

Additionally, the response of the transformers isn't going to be linear across the test band, in most situations.

When using the NF meter with min loss pads, all those extraneous insertion losses are calibrated out or accounted for during meter setup. For the min loss pads, since they have a very flat insertion loss (resistive elements), their insertion loss is known and it doesn't vary across the test frequency band. Additionally, the DUT's temperature must be accounted for and there's an entry that has to be made to account for that since the device's physical temperature will also affect the meter readings.
I agree with you but there are tradeoffs to the methods. Although not applicable to what we're doing, a min loss pad lowers the noise source ENR and therefore lowers how high of a noise figure that can be measured. High noise figures require high ENR sources. It's been a couple of decades since I used an HP-8970A but I seem to remember that it ran a 28V square wave into the noise source so the meter measured noise on/off just like the spectrum analyzer method. High noise figures with a low ENR source produce a very small difference between noise on/off in turn producing large changes in the noise figure for tiny changes in the noise. We had a unit at work with such a high noise noise figure we had to use a noise source with an ENR of 25 dB.

Again it's been a long time, but I seem to remember that we went with the spectrum analyzer method because we had units that downconverted the input, say 16-18 GHz in to 2-4 GHz out. I don't think the HP-8970A could handle that situation, noise source at one frequency range and the measurement at a different frequency. The spectrum analyzer didn't care.

I looked at the issue of noise varying with temperature. Although it does vary, it takes a pretty big temperature change to see much difference. It's not like you're going to receive stations when it's 0°C out and not when it's 30°c.


Quote:
Originally Posted by ADTech View Post
This is so true but is overlooked for so many who are looking for that last little bit of margin. Kind of like trying to squeeze an addition couple of horsepower out of an engine that's already near its limits.
I'm guilty of this but I've found that squeezing out the last dB only buys you a few more reception minutes when the station is fading or when it's about to come back in. Signals just don't seem to sit nicely at 1 dB above the cliff for long.

There's kind of an equivalent in DTV to analog TV. An increase of 3 dB was about the minimum change you could barely detect in the analog picture. 6 dB was definitely noticeable and 10 dB was a real difference. In DTV 3 dB will improve the time a weak station can be decoded but nowhere near enough to make it reliable. 6 dB is a big improvement but the station will likely still have bad periods. A 10 dB increase can make a weak station pretty close to 100%.
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post #21 of 139 Old 12-11-2014, 04:24 AM
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RCA TVPRAMP1R - UHF Input Transistor

There is some variability in noise figure from unit-to-unit.

When components change from one unit to another, this can affect the noise figure. For Example:

Pete Higgins reported on an RCA preamp in TVfool forums back in August 2013. The left portion of the image below is image is an extract from his image of the circuitry, representing the 1st transistor in the UHF section. Thanks Pete, for your special effort supplying photos and part code numbers.

A unit from Radio Shack was purchased two days ago, the right-hand portion of the image below is a similar portion of the recent UHF circuit.




The location of the date code is shown below.





The PCB has been relaid out with a few changes. Likely most were for productivity and cost reduction (for example; 0603 smd parts are used in the newer board, whereas the older appeared to have 0805 parts).

The first transistor in the UHF preamp was changed from a BFG67 to a BFG520xr. For the newer units, this could have reduced the noise figure by about 0.2 dB.

The BFG67 noise figure at ~20 mA is specified to be:

~1.85 @ 900 MHz
~1.6 @ 500 MHz

Whereas, the BFG520xr is specified at ~20 mA to be:

~1.6 @ 900 MHz
~1.5 @ 500 MHz

This indicates that not all units of a particular preamplifier model will have the same measured noise figure.

This physical difference in units may help support the measurement differences in noise figures found by Calaveras and ADtech.

.
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Last edited by tripelo; 12-11-2014 at 04:40 AM. Reason: clarify
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post #22 of 139 Old 12-11-2014, 11:21 AM
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For anyone thinking they can measure Noise Figure down to tenths of a dB, fol. is RECOMMENDED READING, esp. Appendix A: Noise Figure Measurement Uncertainty Curves:
http://cp.literature.agilent.com/lit...5091-4801E.pdf

Also note BFG520/BFG520XR manufacturer specifies a 0.5 dB variation between their TYPICAL NF and the MAXIMUM ALLOWED NF...so there is considerable variation from device to device as they are produced, PASS the Quality Assurance tests (i.e. does not exceed the MAX, usually done on a few SAMPLES...NOT each and every unit) and end up in Preamps...which DEGRADE the NF a bit due to Input Filter Losses:
http://www.farnell.com/datasheets/109990.pdf

FYI: Includes specs for 1 dB Compression and 3rd Order IMD Intercept. HOWEVER, this is for ONE particular set of test conditions. Note that the PERFORMANCE Curves indicate Distortion is minimized for 20-30 mA Collector current...which also results in HIGHER Noise Figure. I don't think we can read the Resistor Component values to determine how the TVPRAMP1 is ACTUALLY biased....and it probably varies from device to device, as is typical for semiconductors.

Last edited by holl_ands; 12-11-2014 at 11:37 AM.
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post #23 of 139 Old 12-11-2014, 12:08 PM
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Thanks tripelo for pointing me to this very informative column. I’m embarrassingly envious of Calaveras’s array!

The performance predictions extrapolated from Calaveras’s, ADTech’s and your data coincide with my real world experience. The cabling distance from my antennas to the distribution point in my garage is ~125’ so in my weak signal environment I definitely need to run pre-amps.

As you know, I live in a challenging signal environment with several strong local FM & one strong TV station located on Box Springs Mountain 3.4 miles out my back door. I have to point my main antenna array right through those strong stations to receive the 1 & 2 edge Los Angeles stations 58 miles away. I have tried an old Channel Master, and several Winegard models including their HDP-269 and even their new LNA-200’s all of which overloaded either from signal ingress or input level.

The $23 price alone would have kept me from trying the RCA TVPRAMP1R. After reading ADTech’s published results on his test unit I half-heartedly ordered one to try. To my amazement, the cheap RCA out performed all the more expensive main-stream amps. It was significantly less prone to overload, and had the flexibility of a switchable FM trap and switchable single/dual VHF/UHF inputs. I have since bought 4 and tried 5 (one of my original 4 started “motor-boating” and was returned) and they all seem to perform excellently. I have one each on my 10 element FM antenna, my LA array, my San Diego array and the 4th on an HDB8-X mounted on my aluminum Ham tower.

For what it’s worth, I replaced the one I opened for you yesterday with a brand new LNA-200 on the array I leave pointed towards San Diego (not pointed through Box Springs Mountain) and didn’t see any change in signal strength or SNR’s on the three San Diego UHF stations checked.

If I understand the gist of this thread, lowering an already low 2.0 dB – 3.0 dB Noise Figure (at least one close to the external noise floor) is beneficial but environmental factors such as fading & multipath will probably pretty quickly mitigate any benefit derived. A little like peeing in the ocean trying to raise the level?

Thanks guys, a thoroughly enjoyable read.

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RCA TVPRAMP1R - Circuit

In case some of you guys that are familiar with circuit design, and interested:

The choice of 20 mA in a preceding image for the current of the 1st UHF transistor was determined by:

- Drawing a schematic
- Making some Voltage measurements
- Some calculations

It is not good to feed trolls, but for others, you can see the resistor values around Q2 in the image (attached below).





If truly interested persons want a complete schematic, PM me.
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Last edited by tripelo; 12-12-2014 at 09:54 AM. Reason: clarify
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post #25 of 139 Old 01-05-2015, 09:42 PM
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Would the FM trap on this preamp cause it to block out channel 7 television? The reason I ask is because I just installed this amp and now my signal went from about 50% to zero signal on channel 7 only. This channel is pretty important to me as it is 1 of the only 2 networks that I get local (60 miles away) news on. Otherwise, I was quite pleased with its performance getting 2 more channels and noticeable improvement on 5 channels which had been marginal (all located 60 to 70 miles away).

If this is a possibility, it would reason that I should turn off the FM trap to get channel 7 back. Unfortunately, I tried that, and it appears to have made no difference. I'm now wondering if I have a bad switch on the FM trap (similar to others having problems with the UHF/combiner switch) making it stay on all the time.

If a defective FM trap switch wouldn't cause this issue, does anyone have any idea what else could have caused this issue?

BTW... It isn't a problem with my VHF connection as channels 9 and 12 both have been properly amplified.

Thanx,
medoug.

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Quote:
Originally Posted by medoug View Post
Would the FM trap on this preamp cause it to block out channel 7 television. . . . . . . does anyone have any idea what else could have caused this issue?
Assuming you still live in Madison, WI the likely issue is 2nd harmonic distortion from WERN 88.7 FM. The FM trap in the amp does not provide much attenuation on the lower FM frequencies. Insert a full band FM trap between the antenna & pre-amp to add additional attenuation for that strong local signal.
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Only Antennas-Direct and MCM "Trap", are actual FM FULL BAND Filters providing significant attenuation in Lower FM Band. Radio-Shack and FM Trap in TVPRAMP1 are designed to allow reception of Ch6 and hence have minimal attenuation in Lower part of FM Band, see fol thread: https://www.avsforum.com/forum/25-hdt...fm-filter.html
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Quote:
Originally Posted by Digital Rules View Post
Assuming you still live in Madison, WI the likely issue is 2nd harmonic distortion from WERN 88.7 FM. The FM trap in the amp does not provide much attenuation on the lower FM frequencies. Insert a full band FM trap between the antenna & pre-amp to add additional attenuation for that strong local signal.
I've never lived in Madison. I used to live about 50 miles south of there so maybe that's what I used in my profile.

I actually live close to Dubuque, IA now. I receive mostly VHF stations from about 60 miles from the southwest with towers at Iowa City and near Cedar Rapids/Waterloo, IA. I receive mostly UHF stations from about 70 miles from the northeast with towers near Madison, WI.

Would it really be likely that strong FM stations would result in overwhelming the pre-amp such that the result would be a zero % signal on both my TV and converter box tuners for channel 7? I guess I would have expected interference in the signal, not a complete block-out on the tuners.

I will have to look at FMfool to see if I have strong FM channels to my southwest where I have my VHF antenna pointed. I was kind of under the impression that strong FM signals on the lower end of the band wouldn't be a problem with channel 7 since they would be on the opposite end of the FM frequency range.

Holl_ands, thanks for the information on the Full-FM traps if I decide to go in that direction to try to fix the problem.
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post #29 of 139 Old 01-06-2015, 11:46 PM
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Quote:
Originally Posted by medoug View Post
Would the FM trap on this preamp cause it to block out channel 7 television? The reason I ask is because I just installed this amp and now my signal went from about 50% to zero signal on channel 7 only. This channel is pretty important to me as it is 1 of the only 2 networks that I get local (60 miles away) news on. Otherwise, I was quite pleased with its performance getting 2 more channels and noticeable improvement on 5 channels which had been marginal (all located 60 to 70 miles away).

If this is a possibility, it would reason that I should turn off the FM trap to get channel 7 back. Unfortunately, I tried that, and it appears to have made no difference. I'm now wondering if I have a bad switch on the FM trap (similar to others having problems with the UHF/combiner switch) making it stay on all the time.

If a defective FM trap switch wouldn't cause this issue, does anyone have any idea what else could have caused this issue?

BTW... It isn't a problem with my VHF connection as channels 9 and 12 both have been properly amplified.

Thanx,
medoug.
I can't find either a www.tvfool.com or a www.fmfool.com Report for your location....it would help if you could provide both.

Looking back at your older posts, you mentioned that you visited the Radio Shack in Prairie du Chein, WI. I entered zipcode 83821 (location of your local Post Office?) into both TVFool and FMFool....I see very weak signals with NM=6.6 dB on Ch7. I also see an EXTREMELY STRONG FM station on 94.3 MHz that's only 2.5-mi from 83821 location and VERY STRONG FM station on 104.3 MHz. Second Harmonics are 400 kHz wide signals centered on 188.6 MHz (Ch9) and 208.6 MHz (Ch12)... which falls directly on top of Ch9 (189 MHz Center), but NOT Ch7 (177 MHz Center).

HOWEVER, the much stronger 188.2 MHz (bottom edge of 2nd Harmonic) is only 8.2 MHz away from 180 MHz (top edge of Ch7)....which means the FM Station is acting as EXTREMELY STRONG NEXT ADJACENT CHANNEL INTERFERENCE which can prevent Ch7 reception. I'm surprised you can presumably receive Ch9, but then we don't have an accurate TVFool and FMFool Reports for your ACTUAL location, the interference level is probably lower....and there can be local variations...

Note that FM Trap in RCA TVAMPR1 is only minimally effective against 94.3 MHz. You should try inserting either Antennas-Direct or MCM FULL BAND FM BAND FILTERS on input of Preamp. However, since the FM Station is about 87 dB STRONGER, with 2nd Harmonic Interference considerably lower, we don't really KNOW how much FM Band attenuation you need....it is possible you would need TWO such filters in series.

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Quote:
Originally Posted by medoug View Post
Would it really be likely that strong FM stations would result in overwhelming the pre-amp such that the result would be a zero % signal on both my TV and converter box tuners for channel 7?
Absolutely, strong FM can cause issues even w/o a preamp. I could not receive a signal on channel 8 only 4 miles away unless I used a full band trap at my previous residence. The culprit was a station on 90.9 (2nd harmonic for channel 8)
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