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How do you null the carrier for the uBITX?

Mine puts out 0.1W on 160M to 0.7W on 20 Meters.

Gain is a disapointing low from 14MHz up.

Michael n2zdb

I meant 0.07 on 20 Meters.

Michael,

The short answer is, you don't. The newer released uBitx's dont have a way to null the carrier because theoretically it's not necessary due to the diodes used in the mixers. It's near impossible to find two discrete diodes that are perfectly the same, but the SMD diodes, (I think BAT54SL),? Farhan used are two diodes in a single package that are as near the same characteristics as you can get, so eliminating the need for a null adjustment. Maybe yours are not perfect?

Joel
N6ALT

Usually carrier at that level may indicate the BFO needs to be set a bit further away.

The uBitx has issues with carrier leakage but usually not that bad, so check the carrier frequency first!

Joel,

While the diodes are well matched the transformer is still handmade though fairly good.
To assure balance a 50 ohm pot at the winding center tap pins 5 and 3 would help.
The RF would be on the wiper and the two wires to the ends.? Mount it under the board
with short leads.

Allison

Ok I lowered the BFO to 11996200 and it is much better as far as the high end.

I can see the difference in the transmission on my Elecraft's P3...in fact it may be too high.

Yes the carrier is also much further down.....

Thanks guys!

73's Michael
n2zdb

You folks are all saying "carrier leakage". I thought that as well when playing with my rig. It turned out, in my case but I expect yours as well, it is not the carrier but the unwanted sideband. This rig's scheme always uses the upper sideband generated by the balanced modulator in both USB and LSB transmission. You need to look in real close with a spectrum analyzer using 100 Hz resolution bandwidth to see the difference. I have found the sweet spot for the BFO to be 11.9962 MHz, just as many of you have already found. If you can measure the output of the balanced modulator, you should see the carrier being down at least 40 dB from the sideband output. You shouldn't see the carrier in the transmitted signal, especially after the modulator output passes through the crystal filter.
Larry N2AJX

Larry,

>>You folks are all saying "carrier leakage". I thought that as well when playing with my rig. It turned out, in
my case but I expect yours as well, it is not the carrier but the unwanted sideband. <<

First as this is not the first time dealing with this, experience is from many prior
radios and the adjustment of many other commercial and kits.? ?There must?be
NO audio or other input to the modulator other than the BFO.? If that is true then
there cannot be another side band. and one can proceed to dial in the BFO.? Also any
measurement of carrier leakage requires that as a start.? Then one can add single or
two?tone audio and look at opposing sideband and carrier with modulation.

First the BFO must be set correctly and about 300hz down the edge of the filter.? More
and the audio suffers (RX and TX) from a lack of lows.

What? I measured is with audio shorted at the modulator and that cannot create another side band.?
Its the exact frequency of the BFO and its there if I remove the balanced? mod transformer and
diodes.? ? Its a board level leakage path of BFO pas the filter into the 12mhz?IF amp.? Also based
further work the balanced mod is well behaved and the carrier null should be at least 40db or better.

The first thing done was to ground the crystal cans, that improved filter leakage a few DB
especially at the edges.

The second was the filter in mine was 1670hz at the 6db points, terrible for voice work.
Change the caps to 82PF and got a 2150hz wide filter, on the tight side but much better.?
The filter shape factor is a rather soft 2.5:1 at -60db, (5300hz) that is typical for this
type of filter.

Then I set the BFO based on the filter sweep for mine that ended up being 11.99675
for about 10db/300hz down the slope.? ? Most the 11.9962 would result in the lower
700-1000hz part of the vice band being seriously attenuated both R and TX.? However
your filter could be offset lower over all, not uncommon.? FYI the upper edge is just
under 12.00000mhz as that is not the center frequency.

However the carrier due to the board issues carrier will still be there.? I've seen this on 5
units to date.? I have not seen it less than -33dbc, one CW op never noticed save for he had?
signals on the other side of zero beat, moving the bfo helped that.

An experimental fix was put a 12mhz crystal osc dialed to 11.99675mhz (for mine) and the balanced
mod in a mint tin connecting it to the filter input with a few inches of coax, same for audio.? Result
good modulator behavior and carrier at least 48db down and audio band pass starting at about
250hz to 2450hz (the filter 2150hz wide).

Allison

Dear Mr Allison many thanks for the detailed info. The skert or shape of 12 MHz filter is too great as you mentioned. Thus mooving BFO higher will give you not only carrier?
but some ather side band products. What I did, having spectrum analiser- did some culculations for 4 crystal Chebyshev 1 dB filter and step by step with 8 crystals moved with vary cap 10/150 pF from central to the end trying to get good responce. Result: 2607 Hz filter/-6/-3.17 losses/not more 1 dB fluctuations in band. Shape is a little better 1.6? = 2607/4000 Hz? -6/-60.
By the way 4-pole 45MHz SMD filter is much better the original one. Transformes for this filter - 43-2402 binocular with 3t/10t. 12 kHz/-6 dB/2 dB losses.
Best regards George RX3ARG/UR4CRG.?

Madam George,? :)? ? HInt: MR Parent was my father, i am his daughter.

>>The skert or shape of 12 MHz filter is too great as you mentioned. Thus mooving BFO higher will give you not only carrier .<<

First moving it up applied to mine please read.? Also If I put it there I had the classic
tinny audio.? Any audio below 900hz would be at least 15db down or more, again for
my filter. The spectrum analyzer was used to measure and confirmation. I have both Rigol
and HP8568B as part of the instruments needed in my work.

I agree 2.5:1 (6:60db) shape factor is poor.? Part of it is the layout and grounding.
Actually carrier suppression is not a factor of the filter as it only adds to it.? The
assumption is balanced modulator works properly and is not compromised
by RF leakage paths.

I'm glad you did the work for yours.? You made a better filter.? ?I could easily built a
better filter using those 8 crystals as I was quite disappointed.? That was not the point.?
I repeat it was to measure and?characterize the existing filter on the board.

I have better 45 mhz 4 pole filters as well for my parts bin.? I did also redesign
and correct the?matching network to get the filter back on frequency as it
was also asymmetric and off frequency.

Many problems and many? small fixes.

Allison

O sorry for the mistake Allison. Good? understanding with good equipment(I have NWT for spectrum). By the way 8-crystal Chebyshev filter will have just the same input/ output impedunce and it is not need to change transformes as for 4-pole SMD 45 MHz filter(570 Om). Thanks George.?