[hpsdr] Munin revisited.

[Kjell Karlsen]

Hi all.

Some years ago I did a layout for a 100W PA using a pair of RD100HHF1  
MOSFET transistors and called it Munin and an updated version, Munin II.  
As far as I know more than 250 of them were built around the world.

Munin was more or less a copy of the 100W PA´s used by ICOM, Kenwood and  
Yaesu. Apache Labs also have used a variant of this PA in all the 100W  
units sold until now. It is extremely robust and I have only managed to  
destroy 1 pair of them when I forgot to install the heat sink and used  
them for a long time at 100W out.

In the ANAN-7000, Apache Labs has removed the Push Pull Driver stage that  
uses 2 pcs. RD16HHF1 and drive the RD100HHF1 directly from the Orion MKII  
board that can deliver maximum 3W out.

I was very surprised when I saw this so I decided to test with one of my  
Munin boards. Using the same values as in the -7000 for the input circuit  
and a 1:1 transformer, I got 100W OUT with <3W IN on all bands except 6  
meter. I measured the input Return Loss and found that by removing a 200  
pF capacitor across the secondary of the transformer, the output on 6  
meter could be reach with 2,5W IN.

Then I tried to reduce the feedback in the PA. The feedback resistors were  
2 pc. 22 Ohm in parallel on each transistor and by increasing them to 39  
OHM each, I can get 100W out with <1W drive on all bands! That means that  
the gain is >20 dB! And the linearity is not worse using less feedback.

According to the data from Mitsubishi, the gain on 30 MHz is 11,5 dB for  
one devise so for 2 in Push Pull the gain is 3 dB higher, 14,5 dB. So were  
does all this extra gain come from?

The gates are purely capacitive (Ciss=260 pF for each gate) so no power is  
lost in the input. So by just using a 1:1 transformer and load it with 50  
Ohm on the secondary, the driving voltage is 20V P-P. There is a mismatch  
on the highest frequencies so one should expect that the gain should drop  
a lot of dB but still the gain is above 20 dB. It might be possible to  
optimize the input RL but I see no point in that as we have the Gain By  
Band adjustments in PSDR and PiHPSDR.

Another positive observation I did was that the efficiency is better when  
using this solution.

When using the original Munin, the current draw at 100W OUT was between 18  
and 20A on all bands up to 10 meter and 22A on 6 meter. But now the  
current is <15A up to 10 meter and just a bit higher on 6 meter. I have no  
explanation for this!

May be it is time to do a new layout for a MUNIN III? It can be really  
small.

73, Kjell

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