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CATV line amps as RF power amps?

P

Pimpom

Jan 1, 1970
0
Is anyone familiar with Philips' BGxxxx series of RF amplifier
modules? They are widely used as line amplifiers in the cable TV
business, with a flat gain over the commercial TV broadcast
bands.

Most of them use a 24V supply with current consumptions of a few
hundred mAs, but their max output levels are specced to be about
60dBmV only for Dim (= intermodulation distortion?) of -60dB. I
guess this is because it's important to avoid cross modulation of
the many different channels and their transfer characteristics
are made as linear as possible. No Class C stuff. The datasheets
do not give even a block diagram and say nothing about input and
output impedances (I assume 50 ohms), max output power, etc.

I was wondering if it would be practicable to use these modules
at higher signal levels to amplify a narrow-band signal.
Harmonics could be suppressed with filters. Here's an example:
http://www.nxp.com/documents/data_sheet/BGY887.pdf
What do you think?
 
P

Pimpom

Jan 1, 1970
0
datasheet say 75 Ohm, not so surprising since it is for tv

Oops. I didn't see that line just above each table. I looked
again after you pointed it out. Don't know how I could have
missed it. Thanks.
 
P

Pimpom

Jan 1, 1970
0
Tim said:
75 ohms, as pointed out.

The ones I've seen specified the total intermodulation
distortion
with a multi-tone test -- i.e., an answer to the question "how
many
channels, and how tightly must the channel amplitudes must be
controlled?". And yes, they want to keep the distortion down.

I would expect that power dissipation would be an issue, and
the
limiting factor.

The way they're constructed, it shouldn't be a problem to bolt
the device on to a good-sized heatsink.
The only way to know for sure would be to either
find a friendly apps engineer and ask, take some apart and
look, or
just build some amps and see how well they survive.

Yes, I'll try it out. I wanted informed opinions to see if
there's some factor I hadn't thought of that makes it a complete
waste of time. At least, the sourcing problem I continually face
in this place is not an issue. These devices are locally
available at ~$7, as against the ~$40 at Digikey and $60 at the
Indian branch of Farnell.
 
P

Pimpom

Jan 1, 1970
0
Michael said:
The CATV amp modules I've used were hybrids on an aluminum
heat
spreader, and epoxy filled.

Yes, the mechanical design makes it very convenient to bolt them
on to a heatsink.
 
P

Pimpom

Jan 1, 1970
0
Michael said:
They are push pull.




They are 75 ohms. The top of page 5 says: ZL = 75 Ohms. A
block
diagram would just be a couple cascaded P-P stages and a power
pin.




I think they won't last long. I used to repair CATV line &
trunk
amplifers. They are picky about impedance matching, bypassing
and
circuit layout.

I don't have a lot of experience in those things, but I know the
basic principles involved. This will be a good excuse to build up
some experience. They cost only ~$7 US apiece here, so it's not a
big deal if I fry a couple of them in the process.
60 dBmV output is 1 volt at 75 ohms.

I'll see if I can get around 1W output at a single frequency.
If you have a decent internet connection, here is some useful
CATV
data. It is a little over 12 MB download.

<http://www.blondertongue.com/UserFiles/file/documents/2009 BBand Ref Guide.pdf>

Thanks. It took just a few minutes even with my 512kb/s
connection. I used to have a 2Mb/s line - the fastest available
here, but my kids came home from college and the 2.5GB monthly
cap was nowhere near enough. So I switched to the lower speed
with unlimited usage. I'm getting a separate 2Mb/s account for my
own use.
 
P

Pimpom

Jan 1, 1970
0
Mark said:
should be fine as long as you keep the baseplate temp under
control
and don't drive the input so hard that the supply current
increases
significantly.

Yes, monitoring the supply current is a good idea. Thanks.
someone else already mentioned they are 75 Ohms.

As I said earlier, I don't know how I could have missed seeing
that part.
 
P

Pimpom

Jan 1, 1970
0
Five and a half-and-a-bit-more watts max power consumption.
What's
your load look like?

They're intended to boost a CATV signal downstream of a
splitter to
just strong enough that a TV can see it clearly, right? That's
not
what I'd call a "power" app, but MMMV...


Active cooling worthwhile?


Built-in pass/notch filtering? Suppose you need to boost a CW
signal
*between* two channels or one that FMs over several? I know it
says
"flat response", but is that also the bits between channels?

Response flat within +/-0.2dB 40-860MHz. Return losses given for
octave frequency blocks of 40-80MHz, 80-160MHz, 160-320MHz, etc.
Does that sound like there are band-pass filters and
inter-channel notches?
Mechanical sensitivity aka microphonics? Portable app?
"Automotive
qualified products".

Substrate diodes? ;>)
Come ON!
 
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