I have the ubitx. I have moved the BFO frequency to get the best
carrier suppression without leaving the transmitted audio tinny.
tim ab0wr
On Sun, 29 Apr 2018 21:30:11 +0530
"Satish Chandorkar" <satish.vu2snk1@...> wrote:
By the way you have not stated witch BITX board you are testing. If
it is a SMD version from Ashar Farahan
then you should do the carrier shifting mod suggested by VK3YE Peter
Parker by witch you will move the carrier
to place it correctly and improving the audio quality of the BITX.
Replacing the carrier oscillator capacitor C102
with a lower value from 47pF to some thing like 22pF or 27pF and then
again re aligning the carrier oscillator
so achieve correct side band suppression by tuning C103 This applies
to almost all BITX boards and not uBITX
Satish
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On Sun, Apr 29, 2018 at 9:06 PM, Arv Evans <arvid.evans@...>
wrote:
Tim
Just disconnecting the microphone sometimes does not reduce
incidental noise
in the audio spectrum enough for a good measurement of carrier
balance. This is
especially true if using a microphone preamplifier. Shorting the
microphone input
to ground kills any possibility of external AF, or external RF,
getting into the audio
input and causing modulation.
It should be possible to do some minimal modification of the BFO
balanced modulator
so you can have manual control of the balance. This should not be
necessary so you
probably want to do this in a non-destructive manner to allow
restoration of the original
circuit if that does not solve the problem.
Your situation is interesting. It has been several years since I
worked on the BITX20A
design, but there we always obtained at least 45 db of carrier
suppression, and sometimes
in the range of 50 to 65 db. This was with 1N4148 diodes which are
supposed to be
inferior to those in the BITX-40 and uBITX.
I think you have already addressed possibility of hum or noise on
the power supply, so
that is probably not the problem.
Arv
_._
On Sun, Apr 29, 2018 at 8:37 AM, Tim Gorman <tgorman2@...>
wrote:
Arv,
I get the same carrier level in the spectrum analyzer with a tone
or without a tone, just the two-tone generator output impedance as
a termination.
When you say to terminate the mike input what impedance are you
thinking of? I've attached a png of the output circuit of the
two-tone. I use the line output which basically offers a 33K
impedance to the mic input.
I don't think it is the diodes themselves. If one or both of them
were bad I wouldn't expect even 25db of suppression.
I've never been a big fan of this kind of balanced modulator. There
isn't much you can do to maximize carrier balance. I thought
perhaps it might be unequal inter-winding capacitance in T7 so I
did the best I could to equally space all the windings around the
toroid and the wire length to the circuit board but it made no
difference at all.
I haven't tried yet but I've thought about lifting C63 to isolate
the mic pre-amp to see if that makes the carrier suppression
better. I've got everything torn down now to move the circuit
board closer to the back so I can use the box as a heat sink. When
I get it all back together I will try liftin C63 to see what
happens.
tim ab0wr
On Sat, 28 Apr 2018 17:10:53 -0600
"Arv Evans" <arvid.evans@...> wrote:
Tim
Interesting. How much carrier suppression do you get without
tone insertion?
Just terminate the mike input and balance the modulator without
any modulation
being inserted. If that dip is too shallow then it could point
to the modulator diodes
themselves.
Arv
_._
