How will this affect the lower freqs?

Farhan what power out are you seeing with this modification at 28 MHz? 
I have installed Howard’s mods to the driver and in seeing 4 watts with RV1 at 2/3 ccw. 

Skip Davis, NC9O

On May 5, 2018, at 08:13, Ashhar Farhan <farhanbox@...> wrote:

peeps,

28 Mhz is unusually open for the last one week. I realized that our ubitx output is woefully low. Here is a fix that I made today, if some of you can try it out, let me know if it works as well.

Step #1 Increase the predriver gain towards the higher frequencies

The predriver Q90 has a emitter degeneration capacitor C81 (0.1uf) and and R83 (10 ohms). I replaced the C81 with a 470 pf and the R83 with 5 ohms. With this, the emitter reactance decreases with increasing frequency, yielding higher gain beyond 14 MHz.

Step #2 Take off the feedback from the IRF510s. 

Just remove the R261, R262 (both are 220 ohms) to open up the gain.

Let me know if this mod is replicable?

- f

Thanks Farhan, this is a great design and has been a easy platform to modify both hardware and firmware. 

I might try part of your mod to see what results I get too. 

Skip Davis, NC9O

On May 5, 2018, at 09:35, Ashhar Farhan <farhanbox@...> wrote:

4 watts, similar power as his mod.

- f

On Sat, 5 May 2018, 19:04 Skip Davis via Groups.Io, <skipnc9o=yahoo.com@groups.io> wrote:

Farhan what power out are you seeing with this modification at 28 MHz? 
I have installed Howard’s mods to the driver and in seeing 4 watts with RV1 at 2/3 ccw. 

Skip Davis, NC9O

On May 5, 2018, at 08:13, Ashhar Farhan <farhanbox@...> wrote:

peeps,

28 Mhz is unusually open for the last one week. I realized that our ubitx output is woefully low. Here is a fix that I made today, if some of you can try it out, let me know if it works as well.

Step #1 Increase the predriver gain towards the higher frequencies

The predriver Q90 has a emitter degeneration capacitor C81 (0.1uf) and and R83 (10 ohms). I replaced the C81 with a 470 pf and the R83 with 5 ohms. With this, the emitter reactance decreases with increasing frequency, yielding higher gain beyond 14 MHz.

Step #2 Take off the feedback from the IRF510s. 

Just remove the R261, R262 (both are 220 ohms) to open up the gain.

Let me know if this mod is replicable?

- f

Are you able to measure compression for linear operation (SSB) ? Harmonic content and levels ?

73 Kees K5BCQ

So, here are the final mods, 

Replace C81 from 0.1uf to 470pf

Replace R83 from 10 ohms to 2.2 ohm

(you can short R83 as well) 

Replace 97, R98 from 47 ohms to 220 ohms

Remove C261, C262. 

So, what happens is that removing the C261 and C262 increases the gain of the finals. They are run open. Hence greater gain at 28 mhz. 

However, the gain is very high at lower frequencies. So, in order to reduce the gain at the lower frequencies, the 0.1 uf cap is replaced by the 470 pf. As the frequency of the signal drops, less and less RF flows through the 470 pf, decreasing the gain of the predriver. 470 pf is not a magical value, 220 pf works almost as well. 

Here are the pictures at 14 mhz. The two tone test reveals 20 db IMDR, which is alright. If you want to see better, buy yourself two RD16HHF1s. 

- f

On Sat, 5 May 2018, 22:02 Kees T, <windy10605@...> wrote:

Are you able to measure compression for linear operation (SSB) ? Harmonic content and levels ?

73 Kees K5BCQ

Very cool, thanks for digging into this.
I tried various schemes in LT-Spice, but improving the final is more
difficult than it seems going in. 
I could seldom correctly predict the results I got.
Need to re-read a few chapters of EMRFD. 
And of course, real hardware is likely quite different than the simulation.

So is this still the previously reported  4W at 30m, 13W on 80m?

Would raising the PA-PWR voltage to 24v improve that IMDR?
Perhaps if keeping power down to something reasonable with RV1,
we could get by without up-sizing the heatsinks.

Jerry


On Sat, May 5, 2018 at 09:28 pm, Ashhar Farhan wrote:

So, here are the final mods, 

Replace C81 from 0.1uf to 470pf

Replace R83 from 10 ohms to 2.2 ohm

(you can short R83 as well) 

Replace 97, R98 from 47 ohms to 220 ohms

Remove C261, C262. 

 

So, what happens is that removing the C261 and C262 increases the gain of the finals. They are run open. Hence greater gain at 28 mhz. 

However, the gain is very high at lower frequencies. So, in order to reduce the gain at the lower frequencies, the 0.1 uf cap is replaced by the 470 pf. As the frequency of the signal drops, less and less RF flows through the 470 pf, decreasing the gain of the predriver. 470 pf is not a magical value, 220 pf works almost as well. 

Here are the pictures at 14 mhz. The two tone test reveals 20 db IMDR, which is alright. If you want to see better, buy yourself two RD16HHF1s. 

 

- f

Ashar,

Is this mod going to be implemented on new production?

Thanks!

73 Steve

G1KQH

That's interesting.
Perhaps the bias wants to be higher than 100ma.
Though as I recall, wa2eby was recommending 10ma of bias, and is quite competent.
As is Allison.

Perhaps there's IMD contributions from earlier stages?
Perhaps a fault in the uBitx design, I'm starting to suspect the use of the two chokes to feed the drains.

Jerry



On Sun, May 6, 2018 at 07:53 am, Tim Gorman wrote:

I tried raising the voltage on my final to 24v. It didn't help the 3rd
order IMD at all. I suspect most of the IMD is coming from crossover
distortion, not flat-topping.