开云体育

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
> _._
>
>