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uBITX Ver. 5 Completion Report

Phil Karn
 

On 2/15/20 08:55, Ora Smith wrote:

Calibration: The calibration instructions mystified me, so I ended up
tuning the carrier frequency to 10 MHz on my Flex, which is pretty
accurate.  BFO adjustment was far off initially. I found that it also
affected carrier suppression. Tuning the BFO by the "sounds good"
technique resulting initially in only 25 db of suppression. QST
reported 49 db suppression, so I suppose I could tweak some more, but
at 35 db it's OK, about what I see with my S-line. Opposite sideband
suppression looks very good on the Flex scope - I can't see anything. 
Spectral purity (carrier spectrum) is not as good as expensive
commercial rigs, but I have had no complaints on the air.
In my case it would help if I had an *exact* description of how the
various generated clocks change with control adjustments (USB/LSB/CW,
tuning knob, calibration, BFO, etc). I know this depends on the firmware
running on the Arduino.

I don't see any carrier suppression adjustments on my v6, just a
balanced modulator that in theory (but not practice) should remove it
entirely. So I suspect the actual carrier suppression depends on where
you park the carrier relative to the crystal filter passband. That's why
it changes with the BFO frequency setting. Better suppression involves
moving the carrier farther away from the crystal passband, which means
cutting off the low frequency response.

This is where I'm really spoiled by SDRs and their razor-sharp filters.
A 'next generation bitx' really ought to go digital here. The crystal
filter, BFO and balanced modulator/product detector would move into
software.

(Has anybody noticed just how *good* SSB can sound when generated and
received by an SDR with 100 Hz cutoffs instead of 300 Hz, and with good
frequency references?)

The lack of AGC on the receiver is bothersome. I have to ride the
volume control constantly when there is QSB or a roundtable with
greatly varying signal strengths.
Yeah, but I'm actually surprised at how usable it is without one.

I do get the impression that there's at least one opinion per ham on how
an AGC *should* work, so by omitting an AGC altogether Ashhar cleverly
avoided that argument. :-)

73, Phil

Ora Smith
 

I recently finished (? - is it ever done?) a uBITX version 5.1 kit with the nice blue case that I received as a gift.  I must admit that when I opened the box and realized it was mostly a bag of parts with no instructions I nearly quit before I started - really glad I kept going. Though occasionally frustrating (it's not a Heathkit!), I now have a very capable 80-10 meter SSB/CW transceiver. It brings back the magic of old time radio.  I find myself using it to the exclusion of a bunch of other fancy radios. I discovered a few things along the way I thought I would share. Perhaps none of this is new, but here goes anyway.

Calibration: The calibration instructions mystified me, so I ended up tuning the carrier frequency to 10 MHz on my Flex, which is pretty accurate.  BFO adjustment was far off initially. I found that it also affected carrier suppression. Tuning the BFO by the "sounds good" technique resulting initially in only 25 db of suppression. QST reported 49 db suppression, so I suppose I could tweak some more, but at 35 db it's OK, about what I see with my S-line. Opposite sideband suppression looks very good on the Flex scope - I can't see anything.  Spectral purity (carrier spectrum) is not as good as expensive commercial rigs, but I have had no complaints on the air. 

I have made a number of barefoot contacts (first contact was with Curacao on 20 meters) but the rig also will drive my Ten Tec Titan (pair of 3CX-800's) to 1000 watts on 80, about 500 on 40 and 20, and 300 on 17 meters. Have not tried other bands.  The trick is how to key the amp. I installed a keying relay inside the case - $1 for a relay, 6 cents for diodes, and a couple of bucks for a piece of solderable breadboard.  However, the keying relay I installed is activated by PTT instead of by the radio T/R switch.  In order to key the amp on CW, I Rube Goldberged a hand PTT switch in parallel with the amp keying line. That way I can key the amp in CW mode to tune it. Operating CW this way would be cumbersome, but mostly I am on SSB. I do see a noticeable power output spike on first key down, but have not investigated, and have not looked at the keying characteristics.

I get great audio reports using the included mic element.  It doesn't seem to have polarity marked, so if it doesn't work, reverse the wires. That happened to me. I put the little element in a D-104 mic head with a grip-to-talk stand. Now the mic is almost as big as the radio.

The radio came with a few defective parts that gave me some grief: the shaft encoder was bad, but I found an exact replacement at Digikey for about $1.60. Also, the fuse holder was defective (may have been my fault), and the fuse that came with the radio was high resistance and intermittent. I ended up bypassing the case's fuse holder and putting an automotive type in-line fuse cartridge in the power lead. The on/off switch that came with the case was defective, but I ended using the switch in one of the included volume controls.

I found the internal speaker to be tinny sounding, so I just plugged an amplified speaker I already had into the ear phone jack.

The lack of AGC on the receiver is bothersome. I have to ride the volume control constantly when there is QSB or a roundtable with greatly varying signal strengths.

By the time I got to installing the T/R LED it was too much trouble to pull the main board out of the case, so I just skipped it.  

Overall it's an amazingly good radio. Wish I had this in 1961 instead of an S-38 and DX-35.  The radio is a great QSO conversation topic.  I've only been harassed by one group (75 meters, of course), HI!

73,

Ora
W9DJ