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Author Topic: An Interesting Driver IC  (Read 1472 times)
TANAKASAN
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Posts: 933




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« on: June 07, 2013, 04:27:07 AM »

I realize that the majority of the discussions here are on QRO matters but once in a while the subject drifts off to the quality of our output rather than the quantity. So, here's something interesting for the stages earlier on in the P.A. chain:

http://www.ti.com/lit/gpn/lmh6703

The highlights:

About 0.4 watts into a 100 ohm load using a +/- 5v supply
1.8 GHz bandwidth
2nd Harmonic -87dB below carrier @ 5 MHz, -69 dBc @ 20 MHz
3rd Harmonic -100 dB below carrier @ 5 MHz, -90 dBc @ 20 MHz
IMD -80 dB below carrier @ 50 MHz 5 dBm/tone

Available in quantity at about two bucks each.

So, we can use these as the lower stages of the P.A. chain, what can we use further down the line where real power is involved?

Tanakasan
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WX7G
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Posts: 5920




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« Reply #1 on: June 07, 2013, 05:48:08 AM »

Into 50 ohms it hits its current spec at 200 mW. I would say it's really a 0.1 watt part but it's a clean part.
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N3QE
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Posts: 2094




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« Reply #2 on: June 07, 2013, 06:11:15 AM »

In the $20 range there are even better parts (usually modules) for CATV distribution systems. Zenki would be very happy the extreme effort those guys go to, to reduce IMD to tiny tiny levels. CATV distribution guys hate the thought that anything of theirs could ever splatter.
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TANAKASAN
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Posts: 933




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« Reply #3 on: June 07, 2013, 07:05:53 AM »

Yeah, those CATV modules are nice for 50 MHz and above but parts that can amplify HF are as rare as rocking horse droppings. Communications Concepts had an amplifier kit on their site for years that used an MHW device and it was always out of stock.

Tanakasan
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NM3G
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Posts: 46




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« Reply #4 on: June 07, 2013, 09:10:45 AM »

I think what you want is a CATV reverse channel amplifier. You can start your search by looking for R0605300. While minimum quantity from manufacturer is 50, they're under $20.00 each and low frequency is stated at either 0 or 5 MHz, depending on what you want.

http://www.rfmw.com/paw.aspx?type=198 has an entire listing of CATV amplifier modules and ICs, and you may be able to discuss sampling with them.

If you are interested, the Motorola MHW592 is running around $180.00 each, when you can get them ... partially because they were used in many of the GE MRI amplifiers (Erbtec 1.0 and 1.5T) as the input amplifier (this is the amp that has the YC-156 driven by a 3CX800A7). Might be a few sitting around in warehouses yet ... I know GE was scrapping all the ones they replaced with the solid state amps.

If you really NEED an MHW592, drop me an e-mail and we can discuss off-line.

73
Rick
NM3G

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ZENKI
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Posts: 906




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« Reply #5 on: June 07, 2013, 04:54:52 PM »

You cant beat real engineers doing their job properly. Maybe someone can email the links to some of the ham companies. Everyone whines about how hard it is to design a clean TX chain.  Yet we have known about these
CATV amplifiers for years. Even the late Helge Granberg  used to use these modules for ultra linear PA's, and that was  40 years ago. The ham radio markets IMD is going down to the abyss, and the opposite is happening
in the real world of engineering. Its really a poor reflection of how poor ham technical skills are becoming and nothing else. Its then  no surprise we have 10,000 dollar radios with IMD performance of a truck stop CB radios.

We also have to address one of the root causes of splatter in ham transceiver designs. That is poor ALC design, and using ALC as a form of compression. We have known about this issue for years, however we still have current
model transceiver design with this issue. Very few hams can remember the days when high power SSB could be routinely heard on the air running 40 to 100kw SSB.  All these commercial transmitters used tubes and the IMD  performance of these systems were superb. I still have a OLD TMC transmitter driver that emits a beautifully clean signal from all this old technology.   I wish I could buy a transceiver that is as clean as the old TMC driver

This CATV module driving a 50 volt FET in class A and then driving a serrated 600 watt PA, you certainly wont have a  IMD issue. So easy.  I know I know they prefer the RM Italy brand its da best.


In the $20 range there are even better parts (usually modules) for CATV distribution systems. Zenki would be very happy the extreme effort those guys go to, to reduce IMD to tiny tiny levels. CATV distribution guys hate the thought that anything of theirs could ever splatter.
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M0HCN
Member

Posts: 473




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« Reply #6 on: June 09, 2013, 08:36:13 AM »

The gotcha with CATV gain blocks is that they are pretty much all **very** high bias class A with commensurate levels of heat generation.

I would have thought that a CFB video opamp (With suitable transformer on the front to make the impedances match and to optimize noise performance), followed by a push/pull class A stage to a few watts, followed by 50V LDMOS (Maybe MRFE6VP6300 or similar held back to maybe 17dB or so gain by feedback) would get you reasonable performance@250W or so if the details were right.

If you were feeling fancy, a modest relay controlled attenuator between the video opamp and the driver stage would allow the noise performance to be optimized when running reduced power.

One of these days I will have to prototype it.

If looking, there are similar high speed CFB opamps from all the usual suspects, and TAPR Hermes does the push/pull thing into a transformer to get a half watt from 4 or the things.

Regards, Dan.
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