Henning,
I put the case together on the ubitx for now. I'll have to wait till I
open it up again to check on the filter bandwidth and the bfo frequency.
I tried pulling the windings on T7 away from the very bottom of the
core in order to minimize stray capacitive coupling. When I get a
chance I'll look at the carrier suppression again and let you know if
it changes.
The only asymmetry I can see in the balanced modulator is that perhaps
the diode pair is not loaded symmetrically by the stray capacitance of
the 50ohm, 4db pad. I haven't looked at the circuit board to see if
this might be an issue.
I wonder if the 50ohm, pi-network 4db pad is needed to control the level
into the filter or if a single 50ohm resistor would do?
I'm sure a double balanced mixer would provide better carrier
suppression but it's also more complicated.
When I get some spare time I might try putting together a single
balanced mixer like this and see what kind of suppression I can get.
tim ab0wr
On Wed, 21 Mar 2018 11:32:53 +0100
"Henning Weddig via Groups.Io" <hweddig@...> wrote:
Tim,
thanks for Your reply! For the overall carrier suppression two things
are to be considered:
1)? the balance of the single balanced modulator
2) the additional suppression done by the slope of the xtal filter
For 2) normally the so called -20 dB point of the filter attenaution
is chosen. This point is a function of the BFO frequency.
Regarding my unmodified BITX40 I noticed that I could hear the normal
tone but it seemed that I also get a response from the "upper
sideband". Further investigation showed that the BFO (measured to be
11.998 051 MHz) was within the passband of the xtal-filter. The 3 dB
points of the filter result in a bandwidth of 2 kHz, see the attached
plots.
The "resulting bandwidth" under these conditions is far too small,
resulting in a "muffled sound" already reported by other users. A
remedy of broadening the filter bandwidth is not the solution,
althogh a bandwidth of a bit larger than only 2 kHz is advisable.
The BFO frequency should be at the location of the marker. Replacing
the 47 pF cap with a 20 pF trimmer (as originally forseen) shifted
the? BFO frequency upwards.
For the uBITX the BFO is generated within the SI5351, so it should be
easy the adjust this frequency to the correct point. To do this the
filter center frequencxy, 3 dB points and 20 dB point must be known
or individually determined, not easy to be done without test
equipment.
For the balance of the modulator the following points have to be met:
??? the exact symmetry of the windings including their center tap
??? the exact balancing of capacitive (or inductive i.e. complex)
impedances
??? the exact equal performance of the two diodes (switches) in
their on-resistance, off resistance and capacitances-- ok a double
diode on a single wafer may guarantee this requirement.
Please have a look back on the "old" discrete double balanced mixers
used as modualtors, they used:
??? selected diode quads (even within a common housing e.g? offered
as a quad? for instance OA154 Q to match the thermal behavoiour)
??? a potentiometer to individually balance resitive component and
exact symmetry fof the two center tapped windings of the modulator
???? two caps (one fixed, one trimmer) to capacitively balance the
modulator
Of course the carrier suppression is also a function of the wanted
signal levels and IM performance of the modulator which is also a
function on how large the wanted signals can be made and how "good"
the IM performance has to be.
I have seen on other schematics using a single balanced mixer with a
potentiometer (as formerly used by Ashar) AND the capacitive
balancing has been implemented.
My guess is that a carrier suppression of at least 20 dB can be
reached. With the additional carrier suppression done by the fitler a
carreir suppresion of 40 dB should be possible.
I have built a double balanced mixer using two BN43-2402 cores with
trifilar windings and four non selected 1N4148 diodes. Without
further adjustments I got a carrier suppression of 26 dB, related to
a singel tone, but I did not measure the IM performance with a two
tone input.
I am not sure if the equal spacing of the trifilar winding alone
guarantees a perfect carrier suppresison.
So pelase determine the 12 MH x-tal filter bandwidth and -20 dB
point(s) and then adjust the BFO frequency to this -20 dB point. But
make sure that the lowest audio frequency to be passed through is
still in the range of 300 Hz or so.
One problem exists: the xtal filter skirts are not equal (due to the
ladder filter topology).? If the other sideband is to be used (for
the BITX 40 in USB mode) by not shifting the VFO? the -20 dB point
can result in a higher "lowest audio frequency".
Henning Weddig
DK5LV
Am 21.03.2018 um 02:54 schrieb Tim Gorman:
I forgot the attachments!
On Tue, 20 Mar 2018 20:40:15 -0500
"Tim Gorman" <tgorman2@...> wrote:
Henning,
I've looked at the balanced modulator in the ubitx. It is set up
differently than what I am used to but it should work.
The arrangement I am used to is shown in the first attachment. The
ubitx is in the 2nd attachment.
In looking at T7 the windings are not very well spaced on the core.
They are crammed together at the bottom near the circuit board. I
don't know if this could cause some stray capacitance that would
upset the balance of the circuit.
I might see if I can take a probe and separate the windings and
get a little better spacing and see if that helps to balance the
carrier out.
I have to say that I am a little surprised that no one else has any
suggestions on how to better balance the carrier.
tim ab0wr
Tue, 20 Mar 2018 05:29:59 -0400 "Henning Weddig
via Groups.Io" <hweddig@...> wrote:
Tim, this is a nice measurement-
At which output power did You take the measurement? What is the
attenauation of the power attenuator between the output of the
uBITX and the Input of the spectrum analysser? Your measaurement
shows a thrid intermodulation of 34 dB/ related to a tone, in
relation to PEP please add 6 dB results in 40 dB of d3! BUT:
marker 1 shows the supressed carrier is only 20 dB below the
carrier, or 26 dB to PEP. Commercial rigs suppress by at least 40
dB or better. This result is a"proof" of my worries concerning
the unsuffiecient carrier Suppression of the balanced modulator
and xtal-filter..
Henning Weddig
DK5LV.
-----Urspr¨¹ngliche Mitteilung-----
Von: Tim Gorman <tgorman2@...>
An: BITX20 <[email protected]>
Verschickt: Mo, 19. Mrz 2018 22:46
Betreff: [BITX20] two-tone test on ubitx
Attached is a screen save from my spectrum analyzer for a two-one
test of my new ubitx.
The tones were driving the finals to about 10watts output (not
quite, but close). I had a 30db external pad and a 10db internal
pad in the SA.
The carrier was only down about 20db (marker 1). That's not as
good as I expected. I'll have to look at that and see what can be
done to improve it.
The 3rd order IMD (marker 4) was down about 34.5db which is
adequate.
The heat sinks get *hot* while running a key down measurement like
this. If you are going to use the rig for a digital mode it
probably needs to be set for about half-power out of the rig.
That or a significant increase in heat-sink capability should be
installed.
tim ab0wr
