KX1 improvements

[Karl Larsen]

    I have been downloading PDF files explaining the way H-mode Mixers
work, and then the data sheet for the Texas Instruments SN74CBT3125
large scale integrated chip in surface mount size. The H-mode Mixer is a
double balanced Mixer that uses a Quad-FET bus switch as the non-linear
part of the Mixer.

    The TI chip uses 5 volts at 18 mA in a Mixer configuration. I need
to study all the information I have now on my computer and then write up
an actual proposed receiver mixer for the KX1 that will knock out nearly
all third order image signals.

    This is not needed in the USA, but it is critical to KX1 use on 40
meters in Europe.


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[Geoffrey Mackenzie-Kennedy-2]

----- Original Message -----
Karl Larsen <[hidden email]> wrote:

<snip>


>    The TI chip uses 5 volts at 18 mA in a Mixer configuration. I need to
> study all the information I have now on my computer and then write up an
> actual proposed receiver mixer for the KX1 that will knock out nearly all
> third order image signals.

--------------------------------------------------------------------------

In 2005 a simple modification was made to the H-mode mixer using the
FST3125, as used in the CDG2000 receiver, a mod I have yet to find on any
website, which involved adding a potentiometer to adjust the switching
thresholds between the two pairs of FETs. According to one of the designers
of the receiver this mod raised its IIP3 on 40m to +50dbm (do not know
offset) for an input noise floor of -130dbm ( do not know the bandwidth),
thus achieving on 40m a third order dynamic range of  close to120db - not
phase noise limited. Apparently the IIP3 of the CDG2000's preselector and
first 9MHz IF filters 'bottom out' at the same point. Can send you details
off list if of interest.

The four transformer version seems to work best for me using Mini-Circuits
transformers, although hand wound transformers on binocular cores work well
also.

73,
Geoff
GM4ESD




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[wayne burdick]

There are potential issues with using any type of mixer besides the
present SA612 in the KX1.

First is PCB space. The present mixer, which requires nearly no
additional parts, barely fits. I seriously doubt that there would be
enough room for any type of multi-transformer bus-switching mixer, even
if were all SMD and replaced the 30 or 30/80 meter module. The problem
with the transformers is vertical space, which there is also very
little of. The alternative would be an active mixer with a much higher
IP3 than the SA612, but those that I'm familiar with also draw a lot of
current and have a very poor noise figure.

Second is conversion loss. The present mixer has about 15 to 18 dB
gain. If you use a passive mixer that has 5 dB loss, you'll have to
make up 20 dB or so gain somewhere, or the rig's audio output would be
way down. Increasing gain at audio is not recommended because it would
likely lead to microphonics, so the gain would have to be added at RF.
This suggests an MMIC RF amp after the lossy mixer -- but that's
another current hog. If you put an LNA (low noise preamp) ahead of the
mixer instead, you lose a lot of your IP advantage gained.

Finally, there's the LO. The present LO injection is on the order of
400 mVpp, but is working into a high impedance on pin 6 of the mixer
(about 2K ohms). If you use a different mixer that requires LO power of
say 0 dBm into 50 ohms, the LO will need a buffer amplifier, and you
also risk increases in RX and TX spurs.

The present KX1 mixer is an optimal solution *except* in the case where
you need more dynamic range. But this is why there's an RF gain control
on the rig. On 40 meters at night, you can probably adjust the RF gain
control for 10 to 20 dB of attenuation ahead of the mixer and still
copy 90% of the signals you can hear with it set for maximum gain.
That's an instant increase in IP3 of approximately this amount.

For rigs like the KX1, what the world needs is a Gilbert cell
multiplier (mixer) IC with gain, NF, LO drive, and current requirements
similar to the SA612, but with a 20-dB higher IP3. If there is such a
device available, please let me know  :)   For example, none of Analog
Devices' mixers meet these requirements.

73,
Wayne
N6KR


---

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[Darwin, Keith]

Boy Wayne, doesn't it stink to be constrained by the real world when
doing designs!

LOL!

- Keith KD1E -
- K2 5411 -

-----Original Message-----
From: wayne burdick

...

For rigs like the KX1, what the world needs is a Gilbert cell multiplier
(mixer) IC with gain, NF, LO drive, and current requirements similar to
the SA612, but with a 20-dB higher IP3. If there is such a
device available, please let me know  :)

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[k6dgw]

Engineering isn't science per se, it is really the art of trade-off
management in the real world.  Success is not perfection of the design
in every aspect, but rather perfection of the trade-offs to best meet
the goals.  I really like my KX1.  I wish it had more audio, my hearing
sucks, but I have a little headfone amplifier (a potential Elecraft
kitlet for those of us that live in a quiet world?).  It does a good job
and the batts last a long time (if I remember to turn it off).

I'm not real anxious to see the KX2 show up anytime soon.  I'd have to
buy it ... like you wouldn't know that? ... and right now, I have some
other plans for my "Hobby Account."  Andrea has been hitting the
needlepoint stores fairly hard these last few weeks, and my account is
thus moderately full.

GARY:  I haven't forgotten my KPA100 and all your advice, just haven't
had time to get to it.

Fred K6DGW
Auburn CA CM98lw

"One should strive to become at least as good a person as your dog
already believes you are."
     (seen on a T-shirt at OMSI in Portland OR)

wayne burdick wrote:

> There are potential issues with using any type of mixer besides the
> present SA612 in the KX1.
>
> First is PCB space. The present mixer, which requires nearly no
> additional parts, barely fits. I seriously doubt that there would be
> enough room for any type of multi-transformer bus-switching mixer,
> even if were all SMD and replaced the 30 or 30/80 meter module. The
> problem with the transformers is vertical space, which there is also
> very little of. The alternative would be an active mixer with a much
> higher IP3 than the SA612, but those that I'm familiar with also draw
> a lot of current and have a very poor noise figure.
>
> Second is conversion loss. The present mixer has about 15 to 18 dB
> gain. If you use a passive mixer that has 5 dB loss, you'll have to
> make up 20 dB or so gain somewhere, or the rig's audio output would be
> way down. Increasing gain at audio is not recommended because it would
> likely lead to microphonics, so the gain would have to be added at RF.
> This suggests an MMIC RF amp after the lossy mixer -- but that's
> another current hog. If you put an LNA (low noise preamp) ahead of the
> mixer instead, you lose a lot of your IP advantage gained.
>
> Finally, there's the LO. The present LO injection is on the order of
> 400 mVpp, but is working into a high impedance on pin 6 of the mixer
> (about 2K ohms). If you use a different mixer that requires LO power
> of say 0 dBm into 50 ohms, the LO will need a buffer amplifier, and
> you also risk increases in RX and TX spurs.
>
> The present KX1 mixer is an optimal solution *except* in the case
> where you need more dynamic range. But this is why there's an RF gain
> control on the rig. On 40 meters at night, you can probably adjust the
> RF gain control for 10 to 20 dB of attenuation ahead of the mixer and
> still copy 90% of the signals you can hear with it set for maximum
> gain. That's an instant increase in IP3 of approximately this amount.
>
> For rigs like the KX1, what the world needs is a Gilbert cell
> multiplier (mixer) IC with gain, NF, LO drive, and current
> requirements similar to the SA612, but with a 20-dB higher IP3. If
> there is such a device available, please let me know  :)   For
> example, none of Analog Devices' mixers meet these requirements.
>
> 73,
> Wayne
> N6KR
>

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[Geoffrey Mackenzie-Kennedy-2]

Wayne Burdick <[hidden email]> wrote:


> There are potential issues with using any type of mixer besides the
> present SA612 in the KX1.
>
> First is PCB space. The present mixer, which requires nearly no additional
> parts, barely fits. I seriously doubt that there would be enough room for
> any type of multi-transformer bus-switching mixer, even if were all SMD
> and replaced the 30 or 30/80 meter module.

<snip>

------------------------------------------------------------------------------

Where I have gone astray is that I had thought that the discussion was about
a new design of a rig similar to the KX1 but with higher IIP3, and not a
retrofit modification. I agree that it would be difficult if not impossible
to fit in a H-mode mixer in place of the KX1's existing mixer. In addition
it probably would be difficult to find space for the IF and LO modifications
required to take full advantage of the proven high performance of a H-mode
mixer.

With regard to the front end mixer's conversion gain / loss, I would suggest
that in high performance HF receivers a mixer exhibiting conversion loss
rather than gain is more useful *provided* that the noise figure of the
following IF system is held suitably low. Obviously the overall noise figure
is that of the IF system increased by approximately the conversion loss of
the mixer and insertion loss of the input filters and switching, but the
overall IIP3 would be approximately that of the IF increased by these
losses. While conversion gain improves noise figure, assuming that the noise
figure of the mixer itself is of the right order, the IIP3 of the overall
receiver will be less than that of the IF system by approximately the gain
of the mixer minus the loss of the front end filters. Also the increased
level of unwanted signals coming out of the mixer places greater stress on
any IF stage that might be before the IF filters, and the filters
themselves.

With a mixer exhibiting conversion loss, the place to make up the gain is, I
believe, in the IF *after* the narrowband IF filters, not as you point out
before the mixer nor at audio.

Forgive me for "old news" which is not relevant to a simple modification to
the KX1, simply comment on one of your points.

The H-mode mixer requires approximately 0dbm LO power if properly driven,
but because of the space problem in the KX1 not relevant. FWIW I have had
fewer spur problems with this mixer than with a Level 7 or a Level 27 diode
ring with proper LO injection, and properly terminated at all three ports.

To be fair to the KX1, I suspect that you did not design it to be able to
cope with the BC stations on 40m as we hear them here, while still providing
copy when the desired signal is riding the noise level at S1 or so, a
situation where attenuators can be counter-productive. Depending on the time
of day, the level of signal here from some BC stations above 7.100 MHz can
reach -10 dbm  carrier or higher, with 'enthusiastic' modulation in some
cases.

73,
Geoff
GM4ESD








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[Karl Larsen]

Geoffrey Mackenzie-Kennedy wrote:

> Wayne Burdick <[hidden email]> wrote:
>
>
>> There are potential issues with using any type of mixer besides the
>> present SA612 in the KX1.
>>
>> First is PCB space. The present mixer, which requires nearly no
>> additional parts, barely fits. I seriously doubt that there would be
>> enough room for any type of multi-transformer bus-switching mixer,
>> even if were all SMD and replaced the 30 or 30/80 meter module.

    This is correct. The SA612A is a mixer designed to make life easy.
It has a oscillator you can add a crystal or L-C to and provide a LO. Or
this can be used as an amplifier for an external LO. It is a Gilbert
Cell design but not well done. It has gain.
>
> <snip>
>
> ------------------------------------------------------------------------------
>
>
> Where I have gone astray is that I had thought that the discussion was
> about a new design of a rig similar to the KX1 but with higher IIP3,
> and not a retrofit modification.
    Well some have wondered if it could be a retrofit. It can't.
> I agree that it would be difficult if not impossible to fit in a
> H-mode mixer in place of the KX1's existing mixer. In addition it
> probably would be difficult to find space for the IF and LO
> modifications required to take full advantage of the proven high
> performance of a H-mode mixer.
    Yes. The LO has to beefed up from 400 mV into 2k-ohms to something
else yet to be determined since the LO has to switch the FETs on and
off. The space problem is caused by the transformer, and the gain
problem is due to the H-mode having a loss of gain like -5DB.


>
>
> With regard to the front end mixer's conversion gain / loss, I would
> suggest that in high performance HF receivers a mixer exhibiting
> conversion loss rather than gain is more useful *provided* that the
> noise figure of the following IF system is held suitably low.
    Actually the noise figure of the IF stages is reduced by the loss
through the mixer. This is also a problem.

> Obviously the overall noise figure is that of the IF system increased
> by approximately the conversion loss of the mixer and insertion loss
> of the input filters and switching, but the overall IIP3 would be
> approximately that of the IF increased by these losses. While
> conversion gain improves noise figure, assuming that the noise figure
> of the mixer itself is of the right order, the IIP3 of the overall
> receiver will be less than that of the IF system by approximately the
> gain of the mixer minus the loss of the front end filters. Also the
> increased level of unwanted signals coming out of the mixer places
> greater stress on any IF stage that might be before the IF filters,
> and the filters themselves.

    Yes this is all true. I have heard the H-mode mixer has a poor noise
figure but it is the loss of gain that causes this.

    Nothing can be done without a lot of design work and breadboard
building. I don't see this happening.

73 Karl K5DI

>
>
> With a mixer exhibiting conversion loss, the place to make up the gain
> is, I believe, in the IF *after* the narrowband IF filters, not as you
> point out before the mixer nor at audio.
>
> Forgive me for "old news" which is not relevant to a simple
> modification to the KX1, simply comment on one of your points.
>
> The H-mode mixer requires approximately 0dbm LO power if properly
> driven, but because of the space problem in the KX1 not relevant. FWIW
> I have had fewer spur problems with this mixer than with a Level 7 or
> a Level 27 diode ring with proper LO injection, and properly
> terminated at all three ports.
>
> To be fair to the KX1, I suspect that you did not design it to be able
> to cope with the BC stations on 40m as we hear them here, while still
> providing copy when the desired signal is riding the noise level at S1
> or so, a situation where attenuators can be counter-productive.
> Depending on the time of day, the level of signal here from some BC
> stations above 7.100 MHz can reach -10 dbm  carrier or higher, with
> 'enthusiastic' modulation in some cases.
>
> 73,
> Geoff
> GM4ESD
>
>
>
>
>
>
>
>
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>

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[Geoffrey Mackenzie-Kennedy-2]

Karl Larsen <[hidden email]> wrote:

>    Yes. The LO has to beefed up from 400 mV into 2k-ohms to something else
> yet to be determined since the LO has to switch the FETs on and off. The
> space problem is caused by the transformer, and the gain problem is due to
> the H-mode having a loss of gain like -5DB.

Morning Karl,

The easy way to do this, and I am not thinking about a retrofit with no
space, is to use a high speed CMOS divide-by-two flip-flop to drive the
mixer FETs, the LO driving the flip-flop. The flip-flop provides the 50%
duty cycle square wave outputs which the mixer wants as injection. A penalty
which is not difficult to overcome is that the flip-flop has to be driven at
twice the injection frequency required by the mixer. Colin G3SBI, who
invented the H-mode mixer, came up in 2005 with a very simple addition of a
pot and three resistors which allows fine tuning of the FET's switching
point (time). A bonus when using a flip-flop is that it can be inhibited by
noise pulses coming from a "noise receiver" tuned outside of the band when
you want noise blanking without having to put a noise gate in the main
signal path which might compromise dynamic range, and the blanker will not
be affected by in-band signals. Certainly it punches holes in the signal
like any other blanker, but after almost 10 years of use I am still happy
with the system.

> Actually the noise figure of the IF stages is reduced by the loss through
> the mixer. This is also a problem.

I think that we are saying the same thing!  Approximate overall noise figure
= IF noise figure + mixer loss + preselector loss, all in dbs. In a HF
receiver this is not a problem since IFs with a noise figure of 2db nominal
and an IIP3 of  +35dbm or better have been around for many years, but they
draw some power. At VHF a preamp would be required, or a mixer with gain if
you can find one that has a low noise figure together with a high IIP3 and
does not cost a fortune.

> Yes this is all true. I have heard the H-mode mixer has a poor noise
> figure but it is the loss of gain that causes this.

> Nothing can be done without a lot of design work and breadboard building.
> I don't see this happening.

It is true that the noise figure of the H-mode mixer is approximately equal
to its conversion loss, and this would apply to a diode ring as well.
Actually there has been a lot of work done with the mixer since it was first
described in Radcom in the very early 90's, and much of the work done in
Italy and Japan has been aimed at reducing the number of transformers and
developing LO squarers. ARRL published details of the 'Triad' receiver using
the mixer, and as far as I know 250 or more of the improved version named
the CDG2002 have been built by hams worldwide.

73,
Geoff
GM4ESD








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[Ron D'Eau Claire-2]

Geoff, GM4ESD, wrote:

A bonus when using a flip-flop is that it can be inhibited by
noise pulses coming from a "noise receiver" tuned outside of the band when
you want noise blanking without having to put a noise gate in the main
signal path which might compromise dynamic range, and the blanker will not
be affected by in-band signals. Certainly it punches holes in the signal
like any other blanker, but after almost 10 years of use I am still happy
with the system.

-------------------------------------------

An interesting idea for noise blanking, Geoff: turning the L.O. on and off.
But keying an oscillator like that always produces sidebands around the
oscillator frequency. That has its own issues when that oscillator is used
as an L.O.

I wasn't aware of any dynamic range issues when using a noise blanker. The
big issue I've noticed is the appearance of the inevitable mixing products.
They are not normally a bother, but I have noticed an odd signal from time
to time when using the noise blanker and, sure enough, it disappeared when I
turned the blanker off.

Ron AC7AC


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[Vic K2VCO]

Ron D'Eau Claire wrote:

> An interesting idea for noise blanking, Geoff: turning the L.O. on and off.
> But keying an oscillator like that always produces sidebands around the
> oscillator frequency. That has its own issues when that oscillator is used
> as an L.O.

Geoff is talking about keying a flip-flop which is between the LO and
the mixer.  It can be used to gate the LO signal (as well as to convert
the mixer signal to the 50% duty cycle square wave required for the type
of mixer that he recommends).  He's not recommending keying the LO itself.

> I wasn't aware of any dynamic range issues when using a noise blanker. The
> big issue I've noticed is the appearance of the inevitable mixing products.
> They are not normally a bother, but I have noticed an odd signal from time
> to time when using the noise blanker and, sure enough, it disappeared when I
> turned the blanker off.

Well, these mixing products are exactly what is meant by a 'dynamic
range issue'.  Loud signals outside of the passband cause undesired
responses, which make it hard to hear weak signals.  I know that the K2
noise blanker and the one in my TS850 cannot be used in a contest
situation because the profusion of loud signals generates a tremendous
amount of junk.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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[Geoffrey Mackenzie-Kennedy-2]

Ron, AC7AC, wrote:

An interesting idea for noise blanking, Geoff: turning the L.O. on and off.
But keying an oscillator like that always produces sidebands around the
oscillator frequency. That has its own issues when that oscillator is used
as an L.O.

I wasn't aware of any dynamic range issues when using a noise blanker. The
big issue I've noticed is the appearance of the inevitable mixing products.
They are not normally a bother, but I have noticed an odd signal from time
to time when using the noise blanker and, sure enough, it disappeared when I
turned the blanker off.

------------------------------------------------------------------------
Ron,

You are quite correct about the 'key click' issue!! After 10 years I still
have not got around to proving the reason, but I think that the
characteristics of the mixer, the fact that its 'normal' LO injection is a
well defined square wave from a flip-flop, and that it is a flip-flop being
keyed rather than an amplifier or oscillator could be preventing the keying
sidebands becoming an issue. I suspect that there are sidebands far enough
down so as not to be noticeable to the ear nor cause operating problems, but
might be seen by a spectrum analyzer at the mixer's IF port.

The dynamic range issue can arise in an 'in-line' IF noise blanker if any
part of its circuit, such as an amplifier, forms part of the load presented
to a preceding stage such as a post-mixer amplifier. In this case such an
amplifier within the noise blanker would receive a large share of the
wideband output from the post-mixer amplifier, so its IIP3 must be
adequately high to avoid compromising the IIP3 of the post mixer amplifier,
which in turn could compromise the dynamic range of the entire receiver.

BTW I use IIP3, input third order intercept, as a measure until a more valid
measure of performance is agreed upon to take care of the cube law
'misbehaviour' exhibited by some devices.

73,
Geoff
GM4ESD




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[Ron D'Eau Claire-2]

Vic, K2VCO wrote:

Geoff is talking about keying a flip-flop which is between the LO and
the mixer.  It can be used to gate the LO signal (as well as to convert
the mixer signal to the 50% duty cycle square wave required for the type
of mixer that he recommends).  He's not recommending keying the LO itself.

----------------------------------------------------------------------------
---

It doesn't matter where in the path you switch the signal, turning it on and
off causes sidebands. Actually it causes mixing which produces sidebands -
sum and difference frequencies based on the modulation frequency (the rate
at which it's turned on and off) and the signal being switched.

Does putting the switch in the L.O. path avoid those? I wonder. The act of
switching the L.O. on and off means we're feeding three fundamental
frequencies into the mixer in the signal path instead of two, plus the
sidebands from switching the L.O. on and off. It sounds to me like we'll get
a huge range of additional mixer products that wouldn't be there otherwise.
It might result in better overall performance than switching the signal
path, but it's not a panacea.

Ron AC7AC


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[Ron D'Eau Claire-2]

Thanks for the feedback, Geoff!

I can see, intuitively, an advantage of doing the switching in the more
predictable environment of the L.O. chain instead of the signal path with
who-knows-what coming along the bandpass and feeding relatively high-gain
amplifiers after it! Especially when you're already providing a switching
square wave as the L.O. signal itself.

It reinforces my long-standing belief that mixers produce bizarre
environments, electrically, not much unlike what Alice found down the rabbit
hole!

Ron AC7AC


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[Karl Larsen]

    As you study the act of mixing, you discover it is non-linear.
Somewhere there needs to be a non-linear device with current flowing
through it usually caused by the LO power injected into it. That is why
the LO needs to be a large signal.

    The bad part of a mixer is it mixes with EVERYTHING! So we put band
pass filters in front of the mixer trying to keep unwanted signals out.
This is not very effective.

    A double balanced mixer is just one that keeps the LO signal out of
the mixer output, a good idea. But it doesn't effect other inputs except
to the extent it tends to attenuate even products.

    After you go down this path you discover a mixer is a nasty thing.
It is sure nice when you find a cheap chip that has everything you want
in a mixer. If your designing a receiver you use it.


73 Karl K5DI




Ron D'Eau Claire wrote:

> Thanks for the feedback, Geoff!
>
> I can see, intuitively, an advantage of doing the switching in the more
> predictable environment of the L.O. chain instead of the signal path with
> who-knows-what coming along the bandpass and feeding relatively high-gain
> amplifiers after it! Especially when you're already providing a switching
> square wave as the L.O. signal itself.
>
> It reinforces my long-standing belief that mixers produce bizarre
> environments, electrically, not much unlike what Alice found down the rabbit
> hole!
>
> Ron AC7AC
>
>
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