Baluns (Brands, Designs, Losses, DIY Loops, etc.) - Page 6 - Canadian TV, Computing and Home Theatre Forums

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post #76 of 811 (permalink) Old 2009-02-09, 11:32 AM
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Probably and most likely. But I dont like to assume that without looking at Channel Masters schematic of it. Its pretty easily to distort by changing a value or bending a coil. I know I can screw up a uhf tuner almost by looking at it, heh.
My CM0264 uses the BFG198 NPN 8Ghz wideband transistor. The example schematic in the data sheet for the BFG198 does show a 75 ohm input and output impedance being used.

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post #77 of 811 (permalink) Old 2009-02-09, 03:56 PM
 
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Quote:
Originally Posted by 300ohm View Post
True, theres a 300 ohm to something matching network in there. But at least its a Channel Master build quality internal matching network. That accounts for some positive in my book.
The CM Spartan preamps in general impress me as middle
of the road given the street price. SMD placement and assembly
seems a little rough. The mechanical design of the housing
leaves much to be desired for a unit intended to be mast mounted
IMHO as it isn't sealed and water can easily enter the housing
perimeter via capillary action, pooling in the case bottom.
I'd expect to at least find a conformal coating on the PCB with such
a housing design.

That aside the actual circuit design is better than most in
this class with the claimed performance actually supported by
the component data sheets. I derived a preliminary schematic
after identifying the active devices.

Note the FM trap only functions for the discrete VHF input and
not for the combined UHF-VHF input. Also it appears additional
reduction of the UHF noise figure is possible given the bias
of the BFG67 in the existing design.

I'd be happy to share the schematic with folks here however
either I lack the requisite forum superpowers to attach a file
or doing so isn't supported.
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post #78 of 811 (permalink) Old 2009-02-09, 04:43 PM
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Quote:
SMD placement and assembly
seems a little rough.
Yeah it looks hand made, as does the power supply.

Quote:
I'd be happy to share the schematic with folks here however
either I lack the requisite forum superpowers to attach a file
or doing so isn't supported.
Scan it in as a .jpg and post it as a picture.

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post #79 of 811 (permalink) Old 2009-02-09, 05:50 PM
 
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Quote:
Originally Posted by 300ohm View Post
Probably and most likely. But I dont like to assume that without looking at Channel Masters schematic of it.
It is a standard 4:1 balun winding on a ferrite binocular core.

Quote:
Its pretty easily to distort by changing a value or bending a coil. I know I can screw up a uhf tuner almost by looking at it, heh.
The only critical inductance in the preamp is ahead of the
monolithic FM band trap, namely a 10T hand tuned inductor
which is part of a user tunable tank.

Quote:
My CM0264 uses the BFG198 NPN 8Ghz wideband transistor. The example schematic in the data sheet for the BFG198 does show a 75 ohm input and output impedance being used.
The BFC198 is the VHF preamp. If I were going to use the
unit as a UHF-only amp I'd be tempted to remove the
choke supplying power to the BFC198 to quiesce the VHF
section and further reduce overall noise.
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post #80 of 811 (permalink) Old 2009-02-09, 05:54 PM
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Another improvement they could make to the 0264 model, would be to include band-pass filters on each of the UHF and VHF paths, before combining the two signals. Preferably after the individual pre-amps.

My experiments with it here indicate that they don't do that, and as a result the VHF and UHF antennas have to point in the same direction to avoid interfering with each other in the pre-amp.

Cheers
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post #81 of 811 (permalink) Old 2009-02-09, 05:59 PM
 
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Quote:
Originally Posted by 300ohm View Post
Scan it in as a .jpg and post it as a picture.
I'd already attempted doing so. The supported JPEG
resolution renders it illegible.

I've placed a copy below. You'll need to manually construct
the URL due to my being under the magic 10 post threshold
and as advertised the robotic censor will likely filter it:

freepdfhosting (dot) com (slash) f93234fe16 (dot) pdf

Note this is preliminary. Determination of the smd capacitor
and inductor values left as an exercise for the truly curious.

Last edited by stampeder; 2009-02-11 at 01:12 AM. Reason: Folks, copy this to your browser: http://freepdfhosting.com/f93234fe16.pdf
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post #82 of 811 (permalink) Old 2009-02-09, 06:11 PM
 
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Quote:
Originally Posted by mlord View Post
Another improvement they could make to the 0264 model, would be to include band-pass filters on each of the UHF and VHF paths, before combining the two signals. Preferably after the individual pre-amps.
Note both UHF and VHF sections have pre and post amplification
bandpass filtering. Combination occurs after post bandpass.

Quote:
My experiments with it here indicate that they don't do that, and as a result the VHF and UHF antennas have to point in the same direction to avoid interfering with each other in the pre-amp.
On what frequency are you seeing the interference?

The UHF section is designed to be capable of both UHF+VHF
amplification. So if you are receiving any unintentional VHF signal
from a UHF antenna this could cause your issue.
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post #83 of 811 (permalink) Old 2009-02-09, 06:23 PM
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Got it, thanks. Looks very nice.

Where are the BFG540 and BFG67 hidden in the preamp ? When I looked at mine, all I could find was the BFG198, which the datasheet I have has a chart plotting its gain from about 30mhz to over 1ghz.

Quote:
Note this is preliminary. Determination of the smd capacitor
and inductor values left as an exercise for the truly curious.
Heh, I have no plans to build it. The power supply on the other hand, I could duplicate very easily if need be.

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post #84 of 811 (permalink) Old 2009-02-09, 06:57 PM
 
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Quote:
Originally Posted by 300ohm View Post
Got it, thanks. Looks very nice.

Where are the BFG540 and BFG67 hidden in the preamp ? When I looked at mine, all I could find was the BFG198, which the datasheet I have has a chart plotting its gain from about 30mhz to over 1ghz.
They are 4-pin SOT143B smd packages about 0805 sized.
If you casually viewed the board you probably mistook them
for passive components. I use a jeweler's loupe when
dissecting an assembly of this size.

Note the diagram reflects the components found on the
particular 0265 unit at hand with the following SMD codes:

CODE MFG DEVICE PACKAGE
--------------------------------------------------
V3p NXP BFG67 SOT143
N37 NXP BFG540 SOT143
p5C PHIL PMBD7000 SOT23
BFG198 NXP BFG198 SOT223

Quote:
Heh, I have no plans to build it. The power supply on the other hand, I could duplicate very easily if need be.
My curiosity was of how close the advertised marketing
numbers met the device data -- which in this case does
seem reasonable.

The BFG67 measured collector current is ~17.8ma which
seems rather high given where NXP characterizes the
device. It appears a reduction of 0.5db in NF is possible
by reducing Ic to 5ma. Unsure ATM how this will impact
maximum output level from the BFG540.
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post #85 of 811 (permalink) Old 2009-02-09, 09:24 PM
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Quote:
My curiosity was of how close the advertised marketing
numbers met the device data -- which in this case does
seem reasonable.
Yeah, I was a little surprised by the vintage of the components. GerryB on this site was able to purchase some newer, lower noise devices for just a few dollars. When is someone going to come out with a preamp with components designed/made in this millenia, for a reasonable price ??

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post #86 of 811 (permalink) Old 2009-02-10, 09:21 AM
 
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Quote:
Originally Posted by uhmgawa View Post
My curiosity was of how close the advertised marketing numbers met the device data -- which in this case does
seem reasonable.
But looking at the preamp chart (http://www.digitalhome.ca/forum/atta...0&d=1229323619), I wonder whether the noise figures include balun loss. What I think may be the same preamp circuits but with different input impedances (CM0064 and CM0068) are listed as having identical noise figures. That can't be true unless the input balun is lossless since its loss directly adds to the noise figure. In fact, the only 300-ohm preamp listed with a higher noise figure is the CM3039. This suggests to me that the noise figures may have come from semiconductor datasheets, not measurement of actual preamps.

Brian
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post #87 of 811 (permalink) Old 2009-02-10, 09:52 AM
 
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Quote:
Originally Posted by 300ohm View Post
Yeah, I was a little surprised by the vintage of the components. GerryB on this site was able to purchase some newer, lower noise devices for just a few dollars. When is someone going to come out with a preamp with components designed/made in this millenia, for a reasonable price ??
I expect there isn't much commercial engineering effort expended
in the OTA broadcast market outside of the magic antenna crap
stuffing the shelves of the local MalWart. So we're probably lucky
to have the currently available selection.

Don't be too put off by the vintage of the components in the
preamp under discussion. The engineering challenge of
economical fringe VHF/UHF signal reception was a done deal
way before these devices were even thunk up.

That said the Avago ATF-551M4 (or sibling device) with which
I hazard GerryB was experimenting, looks to have the potential
to consume a few weekends of hacking. That's quite impressive
performance at a piddling price. Guess those are the bread
crumbs which fall off the table where folks are trying to solve
the same issues at 5.8GHz.
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post #88 of 811 (permalink) Old 2009-02-10, 10:25 AM
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Quote:
This suggests to me that the noise figures may have come from semiconductor datasheets, not measurement of actual preamps.
Could be, like noted before, the soldering on it looks handmade. Maybe they have limited test equipment ? The datasheet also shows how much the gain is affected by input voltage. From what Ive read from other users of the CM0264 and its cousins, the power supply voltage varies from individual unit to unit. I have 18 vdc on mine, some have reported 22 vdc and one reported 12 vdc, heh.

Im not ragging on the unit, it works very very well. Compared to my old Radio Shack preamp its like night and day. I do have pictures of the RS cut apart, posted in the preamp thread, if you want to get a good laugh.

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post #89 of 811 (permalink) Old 2009-02-10, 10:39 AM
 
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Originally Posted by 300ohm View Post
I do have pictures of the RS cut apart, posted in the preamp thread, if you want to get a good laugh.
I'll take a look. You guys have got me so curious about the CM preamps that I think I'll take down my mast today, remove my old CM0064 (the one I found in a neighbor's orchard), photograph the circuitry, and see if I can figure out why it runs on AC power. I also need to add that drip loop I forgot. I think I'm going to remove the twin-lead twists while I'm at it because they can only increase the response to horizontally polarized signals. Although I haven't verified this, I believe all the UHF-TV signals around here are horizontally polarized.

Brian
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post #90 of 811 (permalink) Old 2009-02-10, 10:47 AM
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Although I haven't verified this, I believe all the UHF-TV signals around here are horizontally polarized.
Yes, all North American TV is horizontally polarized and has been since about the begining, except for those cities now experimenting with circular polarization.

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