|
Hi all,?
Unfortunately, I don't have any equipment except my ears to measure the power levels.??
After calibration, during receive, my radio has what seems (to my ears) to be relatively poor suppression of the unwanted sideband.? Adjusting the USB BFO frequency hasn't made any dramatic improvement.? I'm running KD8CEC firmware and IF Shift isn't giving me any real joy either.?
Is anyone else seeing the same?? Is there some adjustment that I'm missing to improve this???
Thank you & 72
Dave Beal
AE6RQ
|
With your Raduino running + / - of the crystal filter's center frequency in say 100 Hz step and connecting it to the input of the crystal filter and
measuring the output of the crystal filter with a RF probe you will be able to plot the actual crystal filter response on the paper. With this plotted performance you will actually know the center frequency of your crystal filter and also both the total pass bands slopes. Now you can accurately determine your BFO carrier frequency for allowing correct pass band? thru the crystal filter and placing BFO to that frequency.
So a RF probe you should have or construct to help you other than your ears
Satish
VU2SNK
On Sat, Apr 28, 2018 at 2:19 AM, David Beal <aardvarq@...> wrote: Hi all,?
Unfortunately, I don't have any equipment except my ears to measure the power levels.??
After calibration, during receive, my radio has what seems (to my ears) to be relatively poor suppression of the unwanted sideband.? Adjusting the USB BFO frequency hasn't made any dramatic improvement.? I'm running KD8CEC firmware and IF Shift isn't giving me any real joy either.?
Is anyone else seeing the same?? Is there some adjustment that I'm missing to improve this???
Thank you & 72
Dave Beal
AE6RQ
|
Hi !
I am also interested in seeing filter plots measured on a professional test instruments.Not much information is available on topic. Dishal predicts totally different values for capacitors and impedance for same xtal parameters than the software from EMRFD.?
Pete N6QW has also put up on his blog having some frustrating time with the filters calculated in Dishal.
I too conducted some experiments with my homebrewed SNA, yes indeed pass band ripple was low but the was not satisfied with the filter skirt. Possibly my parameter measurements were not that precise.
Lately I have started experimenting with a different xtal filter . The half bridge lattice by OM? Rolf Heine DL6ZB
?An 8 xtal dual cascade appears to be an attractive option for following reason:
Simple in design , highly compatible with uBitx architecture and TIA's strong Z's. Xtal matching need not necessarily required and more poles of filtering for same number of? xtals, Easy bandwidth change same filter can perform dual function for SSB as well as CW? and yes the bifilar coils can be the existing matching transformer.?
In his discussions on Facebook groups OM Rolf has posted couple of multi pole designs ,need to look around.
Anyone for this hack....
73
Rahul VU3WJM
|
Two things result in carrier...
One is the BFO too close to or in the filter passband, it should be 6-10 db down the edge of the filter.
Blaming the filter will not fix that, unless its seriously broken.? ? NOTE: Each unit will vary some on the
best BFO setting this happens with commercial filters too.??
The other is imbalance in the modulator circuit.? Solution is fix it.
There is a remote possibility of a tone at higher than you can here present in the tx due to a circuit or wiring issue.?
Its not rocket science to measure the power out and the residual carrier.? a 50 ohm load, a diode detector
is all that is needed plus a voltmeter.? Compare power out at full power and with no audio at all the ratio in
DB should be better than 40DB (10,000:1) or for 10W out less than 1mW.
Allison
|
Another member of the group gave me this advice. Tune to 7074 using LSB. You should hear nothing. Then change to USB and you should hear all kinds of psk and other digital modes. If you hear these signals in LSB then your BFO frequency is not correct. My BFO was way off after I first tried to calibrate it. I then started over with mine at 996.4 and ended up at 996.7. If yours is a long way off from that then I'll bet you hear psk at 7074 when in LSB. tim ab0wr On Fri, 27 Apr 2018 13:49:51 -0700 "David Beal" <aardvarq@...> wrote: Hi all,?
Unfortunately, I don't have any equipment except my ears to measure
the power levels.??
After calibration, during receive, my radio has what seems (to my
ears) to be relatively poor suppression of the unwanted sideband.
Adjusting the USB BFO frequency hasn't made any dramatic
improvement.? I'm running KD8CEC firmware and IF Shift isn't giving
me any real joy either.?
Is anyone else seeing the same?? Is there some adjustment that I'm
missing to improve this???
Thank you & 72
Dave Beal
AE6RQ
|
I've never been able to get over 25db of carrier rejection according to my Rigol DSA815. Not unless I move the BFO so far that signals are unintelligible. If you adjust by 6db to allow for PEP equivalence this is still only 31db of rejection. In looking at the modulator circuit I don't see much of a way to increase carrier rejection without major butchery, i.e. totally replacing the modulator with a ring mixer which would allow providing for a carrier balance adjustment. tim ab0wr On Fri, 27 Apr 2018 18:22:21 -0700 "ajparent1/KB1GMX" <kb1gmx@...> wrote: Two things result in carrier...
One is the BFO too close to or in the filter passband, it should be
6-10 db down the edge of the filter. Blaming the filter will not fix
that, unless its seriously broken.? ? NOTE: Each unit will vary some
on the best BFO setting this happens with commercial filters too.??
The other is imbalance in the modulator circuit.? Solution is fix it.
There is a remote possibility of a tone at higher than you can here
present in the tx due to a circuit or wiring issue.?
Its not rocket science to measure the power out and the residual
carrier.? a 50 ohm load, a diode detector is all that is needed plus
a voltmeter.? Compare power out at full power and with no audio at
all the ratio in DB should be better than 40DB (10,000:1) or for 10W
out less than 1mW.
Allison
|
Thank you very much.?
This is very helpful, easy, and it works well.?
--
Dave Beal
AE6RQ
|
Tim? AB0WR
Some earlier BITX transceivers did use a ring mixer but the carrier suppression was
not noticeably better than with the present design.? Allison's comments are valid and
reflect a good place to start looking.?
Placing the BFO down the edge of the? crystal filter passband helps improve carrier
rejection and improves audio by filtering off unneeded lower frequency voice products.?
You can tailor the lower frequency speech response by how far down the filter skirt
you place the BFO.?
If you have means to do spectrum analysis you can measure crystal filter response
and use that information to determine where the BFO should be set.? If you do not
have spectrum analysis tools, it is still possible to do a manual sweep by adjusting the
BFO in small (20 to 100 Hz) steps across the filter passband and plot the filter output
using a diode detector and graph paper.? Note that linear graph paper will show an
expanded curve where log graph paper will show a more conventional decibel curve.
If noise is being injected into the balanced modulator via the microphone or microphone
amplifier it will not be possible to get a deep null in carrier balance.? It is conventional
practice to short the microphone input while doing carrier balance adjustments and
measurement.
Allison's comment regarding possibility of undetected ultra-sonic tones present that
can cause unwanted carrier insertion is something that we do not usually look for.?
A quick look at modulator AF input with an oscilloscope would probably show any
such problem.
Typical crystal ladder filters usually show a steeper curve on its lower sideband than
on the upper sideband.? Some of the high-IF designs ignore that and as a result will
show less carrier rejection on either USB or LSB, depending on the design.?
Arv K7HKL
_._
On Fri, Apr 27, 2018 at 8:50 PM, Tim Gorman <tgorman2@...> wrote: I've never been able to get over 25db of carrier rejection according to
my Rigol DSA815. Not unless I move the BFO so far that signals are
unintelligible. If you adjust by 6db to allow for PEP equivalence this
is still only 31db of rejection.
In looking at the modulator circuit I don't see much of a way to
increase carrier rejection without major butchery, i.e. totally
replacing the modulator with a ring mixer which would allow providing
for a carrier balance adjustment.
tim ab0wr
On Fri, 27 Apr 2018 18:22:21 -0700
"ajparent1/KB1GMX" <kb1gmx@...> wrote:
> Two things result in carrier...
>
> One is the BFO too close to or in the filter passband, it should be
> 6-10 db down the edge of the filter. Blaming the filter will not fix
> that, unless its seriously broken.? ? NOTE: Each unit will vary some
> on the best BFO setting this happens with commercial filters too.??
>
> The other is imbalance in the modulator circuit.? Solution is fix it.
>
> There is a remote possibility of a tone at higher than you can here
> present in the tx due to a circuit or wiring issue.?
>
> Its not rocket science to measure the power out and the residual
> carrier.? a 50 ohm load, a diode detector is all that is needed plus
> a voltmeter.? Compare power out at full power and with no audio at
> all the ratio in DB should be better than 40DB (10,000:1) or for 10W
> out less than 1mW.
>
> Allison
|
Arv, I have a spectrum analyzer. I use a two-tone generator. There is no issue with spurious microphone response. While the two-tone gen is just a standard kit, I can't see any distortion on my oscilloscope in either the time domain or in the frequency domain using the scopes FFT math function. I'm sure there is some but not enough to cause this carrier suppression figure. Nor are there any ultra-sonic tones showing up on the spectrum scan, either at audio freq's or rf freqs. Just a single tone from the two-tone generator and the carrier. 25db is the *best* I can get by adjusting the BFO. Standard adjustment of the BFO gave about 18db to 22db. Moving the BFO down the filter skirt gave me another 3db. tim ab0wr On Sat, 28 Apr 2018 11:13:53 -0600 "Arv Evans" <arvid.evans@...> wrote: Tim AB0WR
Some earlier BITX transceivers did use a ring mixer but the carrier
suppression was
not noticeably better than with the present design. Allison's
comments are valid and
reflect a good place to start looking.
Placing the BFO down the edge of the crystal filter passband helps
improve carrier
rejection and improves audio by filtering off unneeded lower frequency
voice products.
You can tailor the lower frequency speech response by how far down the
filter skirt
you place the BFO.
If you have means to do spectrum analysis you can measure crystal
filter response
and use that information to determine where the BFO should be set.
If you do not
have spectrum analysis tools, it is still possible to do a manual
sweep by adjusting the
BFO in small (20 to 100 Hz) steps across the filter passband and plot
the filter output
using a diode detector and graph paper. Note that linear graph paper
will show an
expanded curve where log graph paper will show a more conventional
decibel curve.
If noise is being injected into the balanced modulator via the
microphone or microphone
amplifier it will not be possible to get a deep null in carrier
balance. It is conventional
practice to short the microphone input while doing carrier balance
adjustments and
measurement.
Allison's comment regarding possibility of undetected ultra-sonic
tones present that
can cause unwanted carrier insertion is something that we do not
usually look for.
A quick look at modulator AF input with an oscilloscope would
probably show any
such problem.
Typical crystal ladder filters usually show a steeper curve on its
lower sideband than
on the upper sideband. Some of the high-IF designs ignore that and
as a result will
show less carrier rejection on either USB or LSB, depending on the
design.
<goog_592175929>
Arv K7HKL
_._
On Fri, Apr 27, 2018 at 8:50 PM, Tim Gorman <tgorman2@...> wrote:
I've never been able to get over 25db of carrier rejection
according to my Rigol DSA815. Not unless I move the BFO so far that
signals are unintelligible. If you adjust by 6db to allow for PEP
equivalence this is still only 31db of rejection.
In looking at the modulator circuit I don't see much of a way to
increase carrier rejection without major butchery, i.e. totally
replacing the modulator with a ring mixer which would allow
providing for a carrier balance adjustment.
tim ab0wr
On Fri, 27 Apr 2018 18:22:21 -0700
"ajparent1/KB1GMX" <kb1gmx@...> wrote:
Two things result in carrier...
One is the BFO too close to or in the filter passband, it should
be 6-10 db down the edge of the filter. Blaming the filter will
not fix that, unless its seriously broken. NOTE: Each unit
will vary some on the best BFO setting this happens with
commercial filters too.
The other is imbalance in the modulator circuit. Solution is fix
it.
There is a remote possibility of a tone at higher than you can
here present in the tx due to a circuit or wiring issue.
Its not rocket science to measure the power out and the residual
carrier. a 50 ohm load, a diode detector is all that is needed
plus a voltmeter. Compare power out at full power and with no
audio at all the ratio in DB should be better than 40DB
(10,000:1) or for 10W out less than 1mW.
Allison
|
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
_._
On Sat, Apr 28, 2018 at 3:33 PM, Tim Gorman <tgorman2@...> wrote: Arv,
I have a spectrum analyzer. I use a two-tone generator. There is no
issue with spurious microphone response. While the two-tone gen is
just a standard kit, I can't see any distortion on my oscilloscope in
either the time domain or in the frequency domain using the scopes
FFT math function. I'm sure there is some but not enough to cause
this carrier suppression figure. Nor are there any ultra-sonic tones
showing up on the spectrum scan, either at audio freq's or rf freqs.
Just a single tone from the two-tone generator and the carrier.
25db is the *best* I can get by adjusting the BFO. Standard adjustment
of the BFO gave about 18db to 22db. Moving the BFO down the filter
skirt gave me another 3db.
tim ab0wr
On Sat, 28 Apr 2018 11:13:53 -0600
"Arv Evans" <arvid.evans@...> wrote:
> Tim? AB0WR
>
> Some earlier BITX transceivers did use a ring mixer but the carrier
> suppression was
> not noticeably better than with the present design.? Allison's
> comments are valid and
> reflect a good place to start looking.
>
> Placing the BFO down the edge of the? crystal filter passband helps
> improve carrier
> rejection and improves audio by filtering off unneeded lower frequency
> voice products.
> You can tailor the lower frequency speech response by how far down the
> filter skirt
> you place the BFO.
>
> If you have means to do spectrum analysis you can measure crystal
> filter response
> and use that information to determine where the BFO should be set.
> If you do not
> have spectrum analysis tools, it is still possible to do a manual
> sweep by adjusting the
> BFO in small (20 to 100 Hz) steps across the filter passband and plot
> the filter output
> using a diode detector and graph paper.? Note that linear graph paper
> will show an
> expanded curve where log graph paper will show a more conventional
> decibel curve.
>
> If noise is being injected into the balanced modulator via the
> microphone or microphone
> amplifier it will not be possible to get a deep null in carrier
> balance. It is conventional
> practice to short the microphone input while doing carrier balance
> adjustments and
> measurement.
>
> Allison's comment regarding possibility of undetected ultra-sonic
> tones present that
> can cause unwanted carrier insertion is something that we do not
> usually look for.
> A quick look at modulator AF input with an oscilloscope would
> probably show any
> such problem.
>
> Typical crystal ladder filters usually show a steeper curve on its
> lower sideband than
> on the upper sideband.? Some of the high-IF designs ignore that and
> as a result will
> show less carrier rejection on either USB or LSB, depending on the
> design.
>
>
>
>
>
>
>
> <goog_592175929>
>
>
>
> Arv? K7HKL
> _._
>
>
>
> On Fri, Apr 27, 2018 at 8:50 PM, Tim Gorman <tgorman2@...> wrote:
>
> > I've never been able to get over 25db of carrier rejection
> > according to my Rigol DSA815. Not unless I move the BFO so far that
> > signals are unintelligible. If you adjust by 6db to allow for PEP
> > equivalence this is still only 31db of rejection.
> >
> > In looking at the modulator circuit I don't see much of a way to
> > increase carrier rejection without major butchery, i.e. totally
> > replacing the modulator with a ring mixer which would allow
> > providing for a carrier balance adjustment.
> >
> > tim ab0wr
> >
> > On Fri, 27 Apr 2018 18:22:21 -0700
> > "ajparent1/KB1GMX" <kb1gmx@...> wrote:
> >?
> > > Two things result in carrier...
> > >
> > > One is the BFO too close to or in the filter passband, it should
> > > be 6-10 db down the edge of the filter. Blaming the filter will
> > > not fix that, unless its seriously broken.? ? NOTE: Each unit
> > > will vary some on the best BFO setting this happens with
> > > commercial filters too.
> > >
> > > The other is imbalance in the modulator circuit.? Solution is fix
> > > it.
> > >
> > > There is a remote possibility of a tone at higher than you can
> > > here present in the tx due to a circuit or wiring issue.
> > >
> > > Its not rocket science to measure the power out and the residual
> > > carrier.? a 50 ohm load, a diode detector is all that is needed
> > > plus a voltmeter.? Compare power out at full power and with no
> > > audio at all the ratio in DB should be better than 40DB
> > > (10,000:1) or for 10W out less than 1mW.
> > >
> > > Allison?
> >
> >
> >
> >
> >?
|
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
_._
|
Remove your microphone and then check for any carrier leakage at the input of the crystal filter
and after the crystal filter. I do this with a RTL dongle. May be your electrat mic may be faulty, Just confirm. Happened with me.
Satish VU2SNK
On Sun, Apr 29, 2018 at 8:07 PM, 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
> _._
>
>
|
Satish, No mic. Just the two-tone generator. When I get the rig put back together I'll try isolating the mic pre-amp to see if it is generating some kind of noise. It doesn't look like noise on the spectrum analyzer but I'm willing to try anything. tim ab0wr On Sun, 29 Apr 2018 20:14:39 +0530 "Satish Chandorkar" <satish.vu2snk1@...> wrote: Remove your microphone and then check for any carrier leakage at the
input of the crystal filter
and after the crystal filter. I do this with a RTL dongle. May be your
electrat mic may be faulty,
Just confirm. Happened with me.
Satish
VU2SNK
<>
Virus-free.
www.avast.com
<>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
On Sun, Apr 29, 2018 at 8:07 PM, 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
_._
|
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
> _._
>
>
|
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 VU2SNK
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
_._
|
All good points, Arv. I'll short the mic lead to ground and check it again. I sure hope that is what the problem is! I haven't actually put the scope on the power supply leads but I don't hear any hum or see any on the spectrum analyzer. Just the carrier dead on the transmit frequency. But it won't hurt to check it. I'm not sure what mods you could do to this modulator which wouldn't make leak through worse than it is. I'll let you know what I find when I get the rig back together. tim ab0wr On Sun, 29 Apr 2018 09:36:28 -0600 "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
_._
|
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
VU2SNK
<>
Virus-free.
www.avast.com
<>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
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
_._
|
Useful tip - I was in the deep end and this helped me get out.
|
Satish Chandorkar
