ALC and alternatives for controlling your linear output stage

Ashhar Farhan VU2ESE observes that ALC is just one way of controlling output power from a linear amplifier stage and that an easier approach is to do it in software.

This needs software that can control the ‘mic volume’.  You could set the value differently for each band.  There is another pay-off with software mic gain, it can make a major difference to the transmit IMD.  At voice peaks, the tx linear chain compresses. The gain is not constant between low and high levels of modulation. This is the cause of in-channel IMD.   Now, if we have a look up table that amplifies the peaks more than the lows, we can ‘correct’ the gain back to being linear. This simple concept goes by the name of ‘pre-distortion’ in the SDR world.


A power amp to go with your uBITx

Mark, DC3MS has completed his little PA to match his µBITx using eBay modules ( MRF186 + LPF Board ) and a NANO connected via i2c to control it.

The PA board is available on eBay from different sellers. The boards are also used in the MX-P50M.

He added a tandem match from, 4 arduino relays for PTT and ATT (-6dB) and an optocoupler board for switching the LPFs.

The Nano is connected to the ubitx via i2c.  Mark had to insert some lines of code to send the frequency and PTT info to his amplifier.  It was a tight fit in Ian KD8CEC’s software,  as there was not much memory left for additional code.

I tend to forget to switch antennas when changing fom 40/80m to higher bands and blew up a few finals in the past, so the code in the amplifier demands a low power tune after band changes.

When transmitting and high SWR occurs, PTT is disabled.

ATT is enabled on 20-80m as Mark’s µBITx puts out more than 5W on these bands.

Mark will gladly share all details if anybody is interested,

Link to the low pass filter used in the linear amplifier by Mark.



Plot of the fall-off in gain from Q90 through to the finals

The following diagram from Henning DK5LV provides an insight into the final four stages of the µBITx PA chain.  This stretches from Q90 through to the finals.

Warren WA8TOD has measured the gain and drive requirements of the transmit chain at operating frequency from Q90 through the finals.  A sweep generator was inserted at C80 and  the level adjusted first for a nominal 1 watt indicated on the wattmeter (Purple trace) and then five watts indicated (Blue).

– Gain of the chain varies from 56 dB at 3.7 MHz to 52 dB at 28.2
– Input levels at C80 were -24 dBm (purple) and -15 dBm (blue); note 9 dB input increase generates 7 dB output increase….. 2 dB com

pression and resulting unacceptable IMD (not shown)


Cleaning up the transformers in the output stage may fix harmonics

Jim AB7VF suggests that much of the harmonic cruft in the µBITx is from DC current flowing through the L7 and L8 ferrites that effectively lowers the inductance as current increases and allows RF to go everywhere.

Jim replaced the electrolytics as can be seen from the picture ..

He suggests that the transformer is effectively a centre tap.  He wanted tighter coupling between the two windings ….  The electromagnetic field set up by the DC passing through the coil will bias the ferrite “magnetic domains” causing a shift in the B-H field resulting in loss of inductance and the generation of spurs .

When feeding the center tap – current flows up toward the “dot” or start of the top winding “left hand rule” will give you the polarity of the magnetic field around the top coil … current also flows through the bottom coil away from the dot or start of the bottom winding creating a magnetic field opposing the one created by the top coil. The net result is no magnetic field to bias the little bitty magnets in the ferrite allowing the inductance to remain the same as without the current flow.

The following photo shows the 80 meter output of Jim’s unit after doing the L7,L8 mod and the output transformer mod.

Jim suggests putting a proper inductor on the IRF510’s that is NOT affected by DC current flowing through it and you will get legal output on 80m CW. would be interested in whether this approach works for others in cleaning up the harmonics, because it will be a lot cheaper and easier than sorting the LPFs.


A model build with PA mods, AGC and anti-pop mods

Mike N6CMY enjoyed his first build of the µBITx so much he built another one! This one avoided all of the mistakes of the first build.



1. Choke in series with R86.
2. All emitter resistors in buffer, predriver and driver bypassed.
3. Feedback resistor in PA increased to 600 Ohms.
4. Output Xformer replace with 2:4 on BN43-202.
5. Adafruit audio compressor in mic line with chokes to eliminate RF feedback.
6. PA powered by buck boost at 13.8V the rest of the board by 12V battery.


30W on 80 and 40, 20W on 20. RV1 adjusted to reduce output to 15W.


1. ND6T AGC installed (underneath main board) at K3 powered by 5V regulator.

2. Additional stage of audio gain installed between Q70 and U1 to make up for loss due to AGC.


Plenty of audio!! and good AGC action.
3. VE1BWV audio pop mod (similar to the one standard on the new ubitx) installed underneath the main board.

4. To further enhance the pop suppression PIN 1 at K3 is grounded.

RESULTS: Sounds fine to me.


A red/green LED TX/RX LED is installed on front and a TX/RX relay to key an external PA is installed on the back.


Evening up power output – interim suggestions from Allison KB1GMX

Allison KB1GMX has provided an update on her experimentation with flattening out the power curve on the µBITx.

Gain and power line up with a few possible solutions listed:

Mixer – output at full tilt about -10 dbm. Go higher and the spur gets bigger so lower output here is better.

Q90 – no mods at 80m about 16db at 10M about 10-11db and not more than 3mW power. Replace it with a really hot transistor. BFR106 has a FT of 5ghz and can do over 100mW. This should keep the stage gain at about 16-18db from 80 through 10M and deliver the 3mw! It is the easiest stage to make flat and high gain as this get more difficult at higher power out.

Pre-driver – takes that 1mw and boost it to about 40mw, We hope. What she saw was about 45mW at 80m and barely 8mW at 10m . Note the gain at 80m is about 16db and at 10m maybe 9db.

We need the gain again to be more stable and higher as well. Transistors tried that worked better included the TO18 2n2222A, 2n3866, and 2n5109. The 5109 was best even though she used only one. Second best was tie between a single 3866 and two 2n2222. MPSH10 was disappointing.

Allison plans to try an oddball 2n6661A, [at 14$ each most will choke] a VMOS fet has potential and requires many circuit changes[Bipolar bias to MOSFET]. They worked really well in another project at VHF.

Driver – takes that 40mw and make about .4W at 80 and sinks to barely .08W at 10m. Gain for that stage is supposed to be about 16db and likely is at 80m but at 10m measured for different 3904s from around 8 to 11db.

This is where the 2n3904 really fails. Its bandwidth at high currents actually does down. The 2n2222A has the reverse, the gain and bandwidth increased with increasing current. While not an ideal part it does work better. The ideal parts here would be two devices in push pull [not 4] and with higher FT.  Allions hasn’t yet tried 5109 parts here but its a solid bet. She wishes 2n3553s, 2SC2166, 2SC799, 2sc1306 and others were still around.

She plans to try an oddball 2n6661A here too as its good for up to 6W.

Finals IRF510 at 80m approaches 15-20W with a gain of over 17db, at 10m its about 13db, but the drive is so low that yields maybe 2W.

If there is enough power at the gate it does the job we ask. RD16HHF does not offer more gain or bandwidth as MOSFETS do not have the equivalent of FT.  These parts do offer a handy tab that can be grounded.

To go from -10 dbm[.0001W] to +40 dbm[10W] we need 50 db of gain over all. That includes losses to transformers and other circuit features. So we need more gain than that available.  Maybe 6 to 10DB more gain would allow some latitude on the gain adjustment pot.

So with four stages and the last being limited to the IRF510s. We set a few rules.

  1. We never run a transistor wide open as one may exceed and another fall short.
    For 2n3904s that happens and the lowest common denominator for them is around 10db. Use better transistors and lots of feedback. More likely to work for everyone than a bet on Monte Carlo.
  2. We only have four stages! The board is laid out that way. Reality sucks.
  3.  IRF510 or RD16HHF you get about 13-16db of gain, period. More
    is wishful thinking or running wide open and risking stability. Hint IRF510s blown cost about US$2, RD16HHFs blown cost US$10. Going to IRF520 and 530 are not better [for 10W output] as the internal capacitances are significantly higher and these parts don’t make the job easier.
  4. Based on total gain needed and what the finals can be reasonably expected to do: The three prior stages must deliver 45 to 47 db of gain. Its also a lot so attention to stability is everything.
  5.  The result must be stable. Oscillation will kill the finals.

As it turns out, item four is the killer as its basically asking for 16db from all three stages. You need good parts for that. Also the interstage coupling must be up to the job while not introducing uncontrolled losses. This is a tall order. So far I have only partial answers.

Answers so far:

Dump the 3904s and use 2n22222(TO18), for those interested in 80-17M it works remarkably well and give a boost up high too. If you go to 2n5109s you may need one as pre-driver and 2 in the driver stage.  They are big and the space is small.  Use the SHORTEST leads possible.

Replace or parallel R941,R911, R96,R942 to get 11 ohms each (I paralleled 22ohms across them).  Lower emitter resistance helps  the gain and power out to finals.

Replace Q90 with BFR106.  Note R81 has to be increased to between
2K and 2.7K for this part. (for those making suggestions I tried 2n2369 in
SMT, it was better but not great). Mouser has the BFR106 for a whopping 38 cents each.

Change C81 to 470 pf, This flattens the 80 and 40M runaway power and helps the higher bands.

There are many changes to transformers possible but for the moment the above are best bang for the buck. Also changing the transformers might be a handful for some.

Warning every time I tried the 2:4 turn transformer with any ferrite the finals were heating a lot and the stage efficiency was well under 40% [terrible with IRF or RD16]. The 2:3 was better at high power but below 50% efficent. This is still in the grinder…


Conclusions on how to eliminate spurs

Alison KB1GMX has advice to constructors on the spurs on the higher bands:

For bands below 20Mhz spurs are NOT an issue as the low pass filters catch it.

Spurs are only a problem for SSB and frequencies greater than 20mhz.

NOTE: due to the way the uBTX does CW it is never an issue on any band.

The short form is when you mix two frequencies you get a third, in a perfect world.

The diode mixers used are handy but they can present conundrums.  If any of the three ports (IF, LO, and RF) are mismatched (think SWR) The signal can be reflected back in.  Since DBMs are omnivorous in that any port can be input or output and if mismatched both!  This does not include effects of distortion in the source signals.But in the real world things like this exist.

Double balanced mixers also suffer from overload, too many and too strong and you get a plethora of signals.   What that means for lots of simple and complex reasons you can get “spurs” or spurious  outputs that are undesired.

Basic math, addition and subtraction:

If you mix 45Mhz with 73mhz you get 28mhz.  We want that  and the radio needs that.   However if any of the 28 gets reflected back into the DBM where it originated it mixes with 45mhz and we get 17mhz.With those four signals you get mixtures of those like:

  • 73-17=56
  • 28-17=11

Those are “first order” as they do not involve harmonics.  They will be the strongest, but not always equal strength.

Both inputs can have harmonics like 90mhz and 146mhz and the 34 and 56 coming out can have harmonics too.  If you add and subtract all the possibles you get an increasing sea of signals some weak some stronger.  We will not cover the possibles as the first order ones are the most troublesome.

The solution traditionally applied is band pass filters or if it isn’t between 28 and 29.9999 the filter strongly attenuates it.  But you need a band pass filter for most every band… uBitx takes the path of below a certain frequency you only need low pass filters and fewer of them.  And it generally works well especially for 80 though 17M…

But at 20mhz and up the low pass filter passes everything below 30mhz and if you overdrive the rig slightly you get a spur on the tech window on 10m where the spur is 16.5 to 16.7mhz and there is no filter for that.  What makes this worse is some radios are very poor at 10M putting out maybe 2W so pushing the audio to get more invites the problem to be greatly worse.

There is no setting we can safely give that absolutely assures there will
be no problem that is consistent with maximum achievable power.

As a licensed amateur radio operators we are responsible for signal quality and also not generating signals outside our assigned bands.

There are two solutions one is bandpass the other is high pass filter.
Either way the rig must be modified to allow those and there are side effects.

One side effect is you need extra switching not provided.  The other is any filter has a loss though it and that would further reduce power out.

Short of that, keep the power right down on the higher bands and go for it…


Fan shroud to help cool your finals

Dave K0MBT has been working to keep his power transistors cooler.  Initally he added a 80mm fan to blow across the transistors and board.

When doing ft8 he would get significant heating of the power transistors running on a 13.8v linear power supply.  The transistors would heat up enough  to make them too hot to keep his fingers against the surface.

He has, therefore, added a shroud similar to the one shown covering the stock heatsinks. It did not help that much.  He had a fairly small but 1″ tall computer heat sink, cut it in half and tapped in mounting holes. The shroud had to be redesigned to fit them but now he can only feel a very slight degree of heating wile running the FT8 tuning feature. 

This worked so well, he has added a similar heatsink to his BITx40.


Upgrading the PA output transformer

JerryW0PWE notes that in the posts on boosting and/or leveling output power he saw pictures of a binocular core being used as the output transformer in place of the toroid that comes with the uBitx board.  He asks “What size is that binocular core?”

Allison plans to use the longer version of the BN43-6802 (or two BN43-302 cores end to end).

John VK2ETA says he went back to IRF510s in the finals and has used a BN43-3312 as the single 302 was getting warm on the low frequencies and he could not put more than 2/3 turns in the core.  He tried both 1/2T and 2/4T in the 3312 and settled on 2/4T due to better efficiency. The 1/2T drew too much current at 80M.

There is plenty of gain and power at 14MHz and below on 16.8V for the PA but has had to resort to a gain control for flattening the power curve despite 330pF caps in all 6 drivers emitter resistors, 22uH inductor in the base feedback of  the pre-driver,  330 pF across the primary of the finals transformer and 820ohms in the finals feedback resistors.

Nick VK4PLN uses the BN43-202 with a winding ratio of 2:4 using 0.64mm enameled CU.  His rig is outputing 20W+ on 80/40/20 using RD16HHF1.  Nick doesn’t detect any heating… Maybe it will at that power level on 10m?

His plan is to now try using:

– the MPSH10 in the pre-driver
– a single 10T bifilar wound FT50-43 for feeding the power to the RD16HHF1s
– removing the PA feedback loop
– and other pre-driver mods as per Farhans suggestions….

This appears to be the common approach on most of the RD16 amp designs out there…and Nick hopes he will get 20w+ output from 80-10m….

Bill K9HZ tried a BN61-002 and it seems to be a winner.  It doesn’t heat up at full power and it seems to be extremely wide band.