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Author Topic: Newbie MMIC question  (Read 6444 times)
AB1JX
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« on: August 31, 2012, 08:04:07 PM »

This may border on a dumb question.  I've got some MAR6s ordered, one of which I want to use at the end of 200 Ft of RG6 as a receive preamp to overcome line loss.  All the preamp circuits I've run across for these use an RF choke and bypass capacitor on the output, assuming you're going to feed it direct to a receiver.

Can I bypass that step and connect my output to the coax center wire, then feed power to the coax with an RF choke in the house as well as pulling signal off with a coupling cap?  The coax would become part of the "tank" circuit for the MAR6.  It's probably either a crazy idea or the way everybody does it, I don't know which.

I just don't see jumping through hoops to separate DC and RF as though they were going different places, then turning around and putting them back together again, then separating again in the house.  I want the preamp to be powered over the coax anyway.

Thanks,

 Alan,  ab1jx
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W4OP
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« Reply #1 on: August 31, 2012, 09:14:04 PM »

If you are going to send DC out on the coax- that should be fine. But I think the MAR-6 will be driven into IM distortion  with very little signal input.
If this is for HF, I would not worry about the cable loss- your noise floor will still be limited by man made, galactic and atmospheric  noise.

Dale W4OP
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AB1JX
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« Reply #2 on: August 31, 2012, 10:48:49 PM »

Actually this is for amateur radio astronomy use.  I've got a T2FD antenna I built for 30-60 MHz use and the 200 feet of coax is to get away from house RF trash.  It's very quiet, bordering on deaf.  I see a couple of local 6 meter repeaters but that's about it.

I'm sweeping it with my IC-7000 and that shows intermod with the internal preamp(s) turned on.  There's a picture at http://ab1jx.webs.com/toys/jove/antennas/fan1/images/tue.jpg with the white vertical streaks being intermod.  The antenna there was a fan dipole, which seems quieter than the T2FD (I just put up the T2FD today).  Galactic noise would be welcome at this point.
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KA4POL
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« Reply #3 on: August 31, 2012, 11:21:24 PM »

It's good to see a fellow astronomy ham.
What you are planning to do is just shifting the output by including the coax on the MMIC side. No problem there. You did study the datasheet, I guess. So you know how to adjust the voltage correctly. It does not hurt to do some more decoupling close to the MMIC to attenuate oscillations.
Your planning can be seen at http://users.belgacom.net/hamradio/schemas/preamp_HF_VHF_UHF_SHF_wideband_MAR6.htm
Good luck.
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WX7G
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« Reply #4 on: September 01, 2012, 01:26:44 AM »

It should work well. You can skip the choke and use only a resistor for a 1 or 2 dB loss in gain.
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VK2TIL
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« Reply #5 on: September 01, 2012, 01:35:40 AM »

Yes, you can do what you described; TV pre-amps use the same principle of "DC up the co-ax".

MMICs are inherently broadband; your proposed amplifier will bombard the receiver with all kinds of signals from DC to daylight.  This is not a good thing.

I'm not a great fan of MMICs although I do use them sometimes for wideband amplifiers; they certainly have their place.  For narrowband (and better) amplifiers a discrete device (transistor or FET) will always out-perform an MMIC.  

I know nothing of radio astronomy but, if you are interested in a single frequency or a relatively narrow band of frequencies, an amplifier tuned for that frequency or band would be preferable.

This amplifier is interesting; www.ece.vt.edu/swe/lwa/memo/lwa0071.pdf

I built one of these, not for astronomy but for its low-noise characteristics; http://i47.tinypic.com/2076q74.jpg

Winding the transformers is, shall we say, character-building.  Smiley

For single-frequency/narrowband use, a filter on the input would make this a very good amplifier for your purposes.

Alternatively, a BF981/BF988/BF998 amplifier as used for VHF pre-amps (but suitably scaled for your frequency-of-interest) would work well; Google will find lots of these but see here for instance;

home.iprimus.com.au/toddemslie/bf981preamp.html



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VK2TIL
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« Reply #6 on: September 01, 2012, 01:56:18 AM »

Just a further thought.

This is a range of discrete-device amplifiers that can be tuned/adapted to almost any frequency;

http://www.minikits.com.au/kits2.html#eme173a6

Mark VK5EME is happy to do overseas orders.  Smiley
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KA4POL
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« Reply #7 on: September 01, 2012, 03:44:30 AM »

If you now should think about a discrete preamp, take a look at http://w7iuv.com/preamp50/preamp_r50.pdf
There are no exotic parts and it performs just great.
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AB1JX
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« Reply #8 on: September 01, 2012, 08:24:18 PM »

I'm sweeping 30 - 60 MHz about 10 times a minute so single frequency or tuned amps are out of the question.  A bandpass filter might be a good thing though.  The choice of 30 - 60 MHz is because that's one of the ranges my IC-7000 will tune without any relay clicks, so I think it's all done with varactors/varicap diodes.  See my page at http://ab1jx.webs.com/toys/spectra/index.html  The picture at the bottom of that page shows a 24 hour 30 - 60 MHz scan using my longwire.  Life was more exciting that way, but most of it is manmade, including the intermod.

I already had a copy of the on6mu preamp page.  What I'm basing some of this on is the wavelab page at http://wavelab.homestead.com/HF_VHF_multi_index.html.  I've got a dozen MC13135 chips on order as well as my MAR6s.  He's using a MAR6 in his preamp between a pair of T2FDs and the coax and another in the signal splitter indoors where he feeds 7 front ends (actually MC13135 receivers).
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KA4POL
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« Reply #9 on: September 01, 2012, 10:14:36 PM »

I hope you are living far away from all manmade noise.
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VK2TIL
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« Reply #10 on: September 02, 2012, 02:01:18 AM »

An interesting project; a filter will help a great deal.

Google "Elsie" and you will find Jim Tonne's wonderful filter design program.

I just did a "quickie" in Elsie and got good results for your frequency-band-of-interest.

A Butterworth filter;

http://i48.tinypic.com/10wjoko.jpg

If you allow 1dB of ripple (fine for your application), a Chebyshev filter with steeper skirts can be used;

http://i49.tinypic.com/1zn1hqu.jpg

These designs are just "quick & dirty"; you will probably get better results with some experimenting with Elsie.

I think that these filters will sweep OK at 6 seconds sweep time but you would have to try them; they are not difficult to build.
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W8JI
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« Reply #11 on: September 02, 2012, 05:34:21 AM »

MMIC's are just fine, if you use one strong enough.

The real problem is an open widow of RF in front of the MMIC might increase the power levels to the point where you have distortion issues.

Sweep TIME has nothing to do with filters, because the filter is fixed tuned. Just use any filter with a passband slightly wider than the maximum sweep width, and do not worry about sweep at all.

As for preamps, you probably want low noise figure first (because you have a lossy antenna on a quiet frequency range) and dynamic range second. I have no idea what your local noise floor power level is within the receiver filter bandwidth you intend on using.

I would look at a good bandpass filter design for the swept frequency range, which might actually require a combination low-pass and high-pass in tandem rather than a true bandpass.

You can have a transmission line after the MMIC, but be wary of parasitics if the line reflects back any odd impedances. You probably won't need anything exotic if you use a filter ahead of the amplifier stage.

73 Tom
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AB1JX
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« Reply #12 on: September 02, 2012, 07:44:18 AM »

My question was about powering an MMIC, now we're wandering. I probably won't stay with 30 - 60 MHz because this ties up my IC-7000 and I can't afford another.  Hopefully with the MC13135s I can come up with something else, either several of them on different frequencies or sweeping with 1 since they're VCO based.  If I can make a varactor tuned frontend for it that tracks using the VCO voltage that would be the ultimate.  The really good machines that do this sweep ten times per second, not per minute.  I'm not sure how the time constants on the VCO/varactor control line will work out.  I don't need to use a PLL, so there isn't a lock time to worry about.  Just a range of voltages with trimpots for setting the endpoints should be good enough, especially if I can rig up a marker or two using crystals.  Like a sweep generator in reverse.

I might get steeper skirts with the high and low pass filter combination. As for odd impedances I expect to use 75 ohm, since I got 500 feet of RG6 for $32 at Home Depot.  I have a bunch of RG59 also which is more manageable indoors and has copper conductors.  The MAR6 data sheet says its designed for 50 ohms but hopefully 75 is close enough and is widely used.

I bookmarked the Elsie download page.  I already have a program called Adlab which is similar.

So on front ends, is a 40673 about as good as a BF988?  I wish I could frequency track the remote preamp, but the plans I have use a BF988 as a front end for each MC13135.

Any insights on suitable broadband antennas?  I tried a fan dipole with 17 dipoles (http://ab1jx.webs.com/toys/jove/antennas/fan1/index.html) and now a T2FD.  I might try a really stubby LPDA sitting on the ground pointed straight up.  I don't really know much about radio astronomy, I'm just starting and trying to get antennas up before snow.
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WB6BYU
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« Reply #13 on: September 02, 2012, 10:12:43 AM »

Quote from: AB1JX

... Just a range of voltages with trimpots for setting the endpoints should be good enough, especially if I can rig up a marker or two using crystals...




This likely would work, depending on your application.  How critical is the frequency accuracy?
If the actual frequencies aren't that important, you could just use sequentially-switched crystals,
such as those from a scanner.  I often find boxes of odd frequencies at hamfests.  The same
switching signals could be used to choose separate trimmer capacitors in the input tuned circuit.

Or just get a cheap scanner receiver, which is designed to do exactly this.


We still have a lot of users in the 30 - 50 MHz range in this area, and any TV stations still on
the low VHF channels may swamp your preamp.  You certainly want to filter out the FM broadcast
band, as that is one of the most saturated portions of the RF spectrum.



Quote

So on front ends, is a 40673 about as good as a BF988? 



The 40673 and other metal can dual-gate FETs are no longer made, though they
are still around.  There were many other similar parts (40822, 3N200, 3N206, etc.)
and, looking though the RCA data book, there is little practical difference among them. 
The BF998 is current production, and would be the best choice for future parts availability
if you can work with SMD devices.  The operating voltages are different, but at that
frequency range I don't think you'll notice any performance difference, and the same
circuits should work with slight changes to the DC voltage and biasing.



Quote

Any insights on suitable broadband antennas?  I tried a fan dipole with 17 dipoles (http://ab1jx.webs.com/toys/jove/antennas/fan1/index.html) and now a T2FD.  I might try a really stubby LPDA sitting on the ground pointed straight up.  I don't really know much about radio astronomy, I'm just starting and trying to get antennas up before snow.



How much gain do you need?  What is your noise floor?  How much do you need to
reduce terrestrial pickup?  How narrow of a pattern do you want?  Point straight up?

The T2FD will have some loss, perhaps 5dB or so when it is more than 1/3 wavelength long,
and more at lower frequencies  or shorter antennas.  It isn't a bad idea if you want good SWR,
but not as good for high sensitivity or efficiency.  But you can get better performance and
reasonably wide bandwidth with a true fan dipole.

No, not the kind with separate wires for each frequency, but the old-fashioned type that
is a derivative of a biconical dipole.  In this case, take 10 wires each 11' long.  Connect
5 of them to each side of a 4 : 1 balun that covers the frequency range of interest.  Space
the ends on each evenly so the furthest ends are about 11 feet apart.  Mount the whole
thing about 5' off the ground.  (A good reflector plane underneath will help - perhaps a layer
of chicken wire.)  That should give you an SWR better than 3 : 1 from 30 to 60 MHz (depending
to some extent on the ground underneath.)


If you want to try a Log Periodic array, I'd suggest looking at the Zig-Zag Log Periodic:

http://www.cebik.com/content/lpda/zzlp3.pdf

Once you do the initial math, these are fairly easy to build by winding wire back and forth
between sets of nails on a wood frame.  You can get good performance as long as you
use enough elements:  in the article W4RNL used 20 elements to cover 50 - 200 MHz.
I built one with 12 elements to cover 200 - 500 MHz, but the rear pattern wasn't well
controlled.  That might not be as much of a problem if you are aiming it straight up.

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WX7G
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« Reply #14 on: September 02, 2012, 10:42:23 AM »

Heatsinking the MMIC:

The MAR-6 needs to have the two GND leads attached to a bit of copper to heatsink it. If you're building on perf board provide 1/2" x 1/2" squares of copper foil for each ground lead. If built on a PCB do the same.
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