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David

when you put this 12 dB gain in front of that mixer, the effect is to reduce the IIP3 of the mixer by 12 dB.? if you compare the input of the mixer (reduced by 12 dB) to the output IP3 of the galli 6 - you should see that the mixer is dominating the linearity of the network. (which is not surprising).? the goodness of this amplifier is that you are adding some gain (that I suspect will be nice on 28 MHz) without degrading the front end linearity, except for the 12 dB gain that is added.? in effect you are making the mixer handle 12 dB larger signals, so this is why your IIP3 measurement is coming out in this way.? see if it represents a 12 dB reduced IIP3 when adding the amplifier?

cool stuff this RF - you have constructed a wonderful instrument.

Curt

I saw the link in a post from 2018,

So the idle current draw may be higher now that there is a large display to power.? Maybe someone here has done a recent bias adjustment...

Curt,

Thanks for the quick reply.? To test the Two-Tone generator, I fed its output through an attenuator to my uBITX and tuned to the expected distortion product frequency.? I couldn't hear anything so believe the generator and uBITX are behaving well.? I'll take a look at Farhan's calculation tool.

Here is a scan with the GALI-6+ in the chain.? I wonder if the GALI-6+ OIP3 is worse at lower frequencies (since it was specified at 1 GHz).


--
David KV4FP

David

I cannot easily see from here - but possibly you are only looking at the amplifier ITOI by itself, and not its impact on the chain?

Farhan had placed a cumulative calculation tool at his website (and I can also assist if you email me).? The presence of gain between stages does impact the overall calculation.?

BTW I don't have any linearity issues with my uBitx, but I don't have a huge many element beam high in the air.? Its fine for many of us with normal one and two element antennas.?

On you nice spectrum analyzer - keep the trick in mind of adding 3 dB attenuation (or whatever you have) in various spots to insure the instrument itself isn't limiting your measurement.? Nice work all around and thanks for sharing

Curt wb8yyy

Greetings from North Carolina.? I'm a uBITX owner and have lurked on this group for some time.? Lots of good discussion.

I have built a fairly close copy of Farhan's Specan analyzer, plus the Two-Tone generator adapted to 40m.? The calculated IIP3 with the generator tones at -20 dBm is around 5 dB (distortion products down around 50 dB), much worse than the expected 15 dB.? Experimentation has shown the culprit is the U2 amplifier which is in the signal chain following the first mixer.? I couldn't find a reliable source of the MAV11 so substituted a GALI 6+.? The datasheet claims a low frequency gain of 12.2 dB and an OIP3 of 35.5 dB at 1 GHz.? Removing/bypassing U2 improved the measured IIP3 to 14 dB which is near the capability of the ADE-1 mixers.

Does anyone have a suggestion of where I went wrong the GALI-6+???
--
David KV4FP

Barry,

The best way is to measure the current is to first rotate both bias pots to full clockwise position, ?note the current, then increase each pot one at a time to get the total current 200 ms above the full clockwise of both pot values.?

Hope this helps

73
Evsn
AC9TU

Can someone give me an idea what typically is the V6 current draw in SSB mode with no modulation??
Thanks

Well, I haven't done it yet to the uBitX, but here is Evan's picture of how he did it: /g/BITX20/message/83186
and that is what I plan to do when I remove the boards to make the repair.

If you are referring to the BitX40, this is how I did it there

BTW, I think I found out? why this happened. This afternoon, driving out to weld a split hitch on a hay rake, I saw another large ash limb on the ground. Sure enough, it had taken down the long leg of the 160m OCFD that I was using atypically with the amp to snag the DX station (I have another one that is used more often, but that one was set up for the uBitX)...so that is likely why I had RF in the shack and why it blew the Q90 in the uBitX. Just goes to show...you never know...

=Vic=

Thanks, John. Do you have any recommendation for leaded parts?

- Jerry KF6VB

Jerry, for surface mount parts I mostly use the ATC100B 500V series. For larger capacitance values parallel lower values to keep your voltage rating high as they de-rate with capacitance ....but always be mindful of current sharing so try to make them as equal as possible. For higher power such as my legal limit amps and filters I use the extended range parts. For silver micas, any 5% 300V part will do with ample of overhead @70W or even 100w .



John

On 2021-06-07 09:24, John Cunliffe W7ZQ wrote:

Jerry,

current. I always recommend RF caps like ATC or Johanson .

*** Do you have an exact series to recommend? Starting from scratch
with a Digikey search is daunting. This particular LPF probably likes
leaded parts - the lands are too far apart for SMD.

I might be doing an LPF board of my own - if I can't get the Chinese linear
clean enough, might have to go with 7-pole filters. In that case, would
definitely use SMD caps.

- Jerry KF6VB

Jerry,
3kv caps for 70W? I use 500V caps for 600w and they just loaf along.Filter caps need to be able to take lots of circulating current. If you use those small caps from Hilitchi you should use several in parallel to accommodate for the current since those caps have a? quite lousy dielectric i.e. use 3x62pf for a 180pf or 3x82 for a 240pf cap). Those capacitors can handle a lot of voltage but hardly any RF current. I always recommend RF caps like ATC or Johanson . That is for several reasons, one, as mentioned the RF current capability but the other, the real issue are the resonances. Those cheap capacitors have unwanted resonances that don't always affect the fundamental? but present high impedance's to harmonics causing filter performance to lack on higher frequencies i e. lousy harmonic suppression. The lousy dielectric will cause a higher ESR at higher frequencies and thus will impact filter performance. I have seen differences as much as 30db improvement in harmonic suppression? just by swapping out leaded capacitors with surface mount capacitors, or cheap ceramic capacitors with mica capacitors. If you need to use leaded capacitors because of the layout, use silver mica's (at 70W you can get away with 300V ones) and keep the lead length as short as possible.? A nice write up on silver mica capacitors can be found here https://eepower.com/capacitor-guide/types/mica-capacitor/#

John

Vic, where did you install the back to back?
--
Frank, KG9H

Yeah, it doesn't make much sense. I was thinking about RF in the shack - RF sometimes
does strange things. I put the amp on the bench just to look at it and measure the quiescent, which is 228 mA. The amp is called "XDT-PA70" and it has a pair of
IRF530N transistors.

I just looked up the IRF530N - it doesn't look like a super linear device. The
constant-current part of the curves is short.

When I have some more time, I might just stick an oscilloscope on it.
I did use a step attenuator - after the initial 30dB Narda. As I flipped
the switches, the peaks went up & down in a sensible manner. And the harmonic
& fundamental went up & down the same amount.

I also tried sticking using the tinySA with its antenna. My vertical is about a hundred feet up the hill from the shack. Same difference.


- Jerry

That seems hard to achieve. Maybe you are over driving the amp and possibly also the tinysa. If you have an attenuator available, see if the harmonic drops more than the attenuation.?

Curt

Vic

Interesting misfortune. A 500w amp remains on my wish list.?

Switching diodes and maybe even transistors are often even more vulnerable when not biased.?

I am thinking try using the leaded parts you have of course with short leads. I built a rig with hundreds of smd parts the nc2030 let's just say it now has a few leaded parts, and not because I dropped any in assembly.?

Best to keep the ubitx unhooked from antennas except when shack is only receiving. Somehow my v4 is alive and well, modified maybe never finished.?

Curt wb8yyy?

Thanks for reminding me, Arv. I used to know this stuff pretty well,
but that was over 40 years ago. Time to review.

In the meantime, I took the uBITX and the amp off the desk, lest I be
tempted to actually operate with them :).

- Jerry KF6VB