Re: Are these output watt rating crazy?

Jerry Gaffke
 

Mark,

Interesting.   

You might be right that the filter with the 900nH inductors has an incorrectly implemented corner frequency.
Worth exploring further.
Your simulation shows a corner at around 7mhz, not the expected 5mhz.
That would let a 3.5mhz second harmonic leak by.
The second harmonic will be quite weak due to the push-pull topology of the IRF510 finals,
this 7mhz corner may not matter.

However, the stock uBitx definitely does not have a low pass filter appropriate for 160m !!!!!
We've had a number of folks here who assumed it did, since the firmware allowed them to spin down and transmit on 1.8mhz.
The third harmonic of 1.8mhz at 5.4mhz will go out on the air almost as strong as the fundamental.

The filter with the 900nH inductors that you simulated is definitely the lowest LPF on the uBitx.
When I said it had a 5mhz corner, I was going by Farhan's circuit description notes.
I had assumed this had been gone over thoroughly, since lots of folks in the forum
were trying to figure out how to reduce the problematic v3 harmonics from leaking past the relays.

I don't think the apparent 7mhz corner you found has anything to do with lack of power on 40m as seen by some.
I think it is more likely that some of the uBitx boards have incorrectly wound 750nH inductors.
When operating on 40m, both the v3 and v5 versions of the uBitx are using the LPF with the 750nH inductors.
At least, when operating with Farhan's code as archived on github.
There are other firmware releases out there, but I suspect they mostly copied
the code to control the LPF relays from Farhan.



Here's the v3 schematic:
  https://github.com/afarhan/ubitx/blob/master/ubitxv3.pdf

And here's the v3 code from
  https://github.com/afarhan/ubitx/blob/master/ubitx_20/ubitx_20.ino
that switches the relays around to select an appropriate LPF:

void setTXFilters(unsigned long freq){

  if (freq > 21000000L){  // the default filter is with 35 MHz cut-off
    digitalWrite(TX_LPF_A, 0);
    digitalWrite(TX_LPF_B, 0);
    digitalWrite(TX_LPF_C, 0);
  }
  else if (freq >= 14000000L){ //thrown the KT1 relay on, the 30 MHz LPF is bypassed and the 14-18 MHz LPF is allowd to go through
    digitalWrite(TX_LPF_A, 1);
    digitalWrite(TX_LPF_B, 0);
    digitalWrite(TX_LPF_C, 0);
  }
  else if (freq > 7000000L){
    digitalWrite(TX_LPF_A, 1);
    digitalWrite(TX_LPF_B, 1);
    digitalWrite(TX_LPF_C, 0);
  }
  else {
    digitalWrite(TX_LPF_A, 1);
    digitalWrite(TX_LPF_B, 1);
    digitalWrite(TX_LPF_C, 1);
  }
}
 
So when operating on 40m (unless you are tuned to be exactly on the lower band edge, don't do that!),
the signals A,B,C  (driving the KT1,KT2,KT3 relays) are driven with 1,1,0 respectively.
KT1 and KT2 are in bypass mode (relay on), and KT3 (off) selects the 7mhz LPF (has 750nH inductors).
So no, on a v3 uBitx, the LPF with the 900nH inductors is only used when operating below (or exactly on) 7mhz.

The v5 uBitx wires up the relays much differently to avoid having harmonics leak around the relays.
    file:///home/chronos/u-ba688dba5e13d507cb8c8d472169a2b5e212d72b/MyFiles/Downloads/ubitx_v5.pdf
The code for v5
    https://github.com/afarhan/ubitx_v5/blob/master/ubitx_v5.1_code.ino
drives the LPF relays with A,B,C == 0,1,0  when operating in the 40m band:

  else if (freq > 7000000L){
    digitalWrite(TX_LPF_A, 0);
    digitalWrite(TX_LPF_B, 1);
    digitalWrite(TX_LPF_C, 0);
  }
This powers the middle relay on the v5 to select the filter with the 750nH inductors,
the other two relays are left unpowered in bypass mode.
Note that the highest freq LPF with the 330nH inductors (for 15m,12m,10m) is always inline on the v5.

Jerry, KE7ER




On Sun, Nov 10, 2019 at 09:08 PM, Mark - N7EKU wrote:
I used the AADE filter simulation/design program.  I posted because I had seem some comments on the power outputs and was curious to simulate the low filter to see if that might impact some of the power levels.  I simulated the lowest filter I saw on the ubitx v3 schematic.  Did I miss a lower one?  This one seems to be the lowest and used for both 40m and 80m with a cutoff very close to 7MHz (actually showed -2dB at 7.3MHz).

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