Jerry and pals,
I forgot to mention that I switched off the VFO by shorting out the 2pin
connector in the middle of the board.
Then I fed my sig-gen at +17dbm into the mixer toroid. And all the ghost
images where gone. It was quiet like
a commercial rig. So a sine wave it is.
So before I get out the Si's and Radionos etc, I thought I will try to
tame the VFO to produce a sine wave.
First thoughts is to isolate the VFO Q9 and couple it to a buffer as
lightly as possible. Q9 may need some
work to JUST break into oscillation to produce a clean sine wave. Then
couple lightly and bring up the
power level required to the mixer.
Maybe I am too obsessed with this BITX40 analog VFO, thanks
Farhan.
73
Raj, vu2zap
At 14-03-2017, you wrote:
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Raj,
As in post??
/g/BITX20/message/21997
Best bet is to move the VFO up to 19mhz, one of Allison's primary
criticisms of the Bitx.? Would be tough to get an analog VFO stable
enough at 19mhz, though the Raduino could do it easily.
Transporting the VFO to the mixer as a sine wave might help a
bit.?? A clean square wave doesn't have any even harmonics,
only odd harmonics.? I'd bet the fourth harmonic is likely due
mostly to second order effects in the diode ring mixer, not due to what's
coming out of? the vfo, but I could be wrong.? Worth
trying.? But no matter how perfect your sine wave is when going into
that diode ring transformer, since the current through the diodes is an
exponential function of the voltage across the diodes that current will
be nothing like a sine wave.? It is exactly there in the diode ring
that the image is getting down converted to 12mhz.by the fourth harmonic
of the vfo.? A square wave is preferred around the diode
mixers:?
/g/BITX20/message/22538
On the other hand, going to a sine wave on your 4.8mhz vfo would likely
help on the birdie at 7.2mhz.? There the 5'th harmonic of the vfo at
4.8*5=24, and that beats with the second harmonic of the bfo.? With
proper shielding around the BFO end of the filter and care taken to keep
the 5'th harmonic of the vfo from radiating in from wires between your
vfo and the Bitx40 mixer, the birdie should have nowhere to mix.
>? stick to a sine wave from the VFO or add a filter between the
VFO and mixer.
Pretty much the same thing.? A 4.8mhz bandpass filter on the VFO
output would permit only a sinewave through to the diode ring
mixer.? The mixer is better off with a square wave.
You must be trying to get a rise out of me.? All favorite topics to
set me off into a rant.??? ;-)
> Rahul:? I feel that the mixer is being bit overdriven in
transmit, this might result in some spurii..?
Could be, I haven't looked.? But that's a separate subject from
Raj's images on receive.
Jerry, KE7ER
On Tue, Mar 14, 2017 at 05:34 am, Raj vu2zap wrote:
- So the 4th harmonic receives tunes 7.2 to 8 Mhz while my dial goes
from 7 to 7.2 !
- I guess the only way to fix this is stick to a sine wave from the VFO
or add a filter
- between the VFO and mixer.
Raj vu2zap