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Down-conversion

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Georges Ringotte F6DFZ

Down-conversion

Hi to the group,

Here is a post I made on the TS-590 Yahoo Group about down or up conversion, close spaced signals dynamic range, and the
way receivers are classed in a few charts (Sherwood or W8JI for exemple).

I ordered very recently a K3, and it is on its way to France.
I must add that Elecraft took some measures to limit the problems associated
with down-conversion ; for exemple, transmit low pass filters are used to improve image rejection. There is also a complex
netwoork in the receiver path with notches on image frequencies of the upper ham bands. For remote signals IMD2 and IMD3,
relays are used to limit them.

Down-conversion is not a fatality, only a technical choise aimed to get the maximun performance on the ham bands.
If it's well implemented (as on the K3, I do hope !), all is fine. If not, you will have problems much more important than
limited 2 kHz dynamic range.

Professional receiver must cover the whole HF spectrum, and up-cpnversion is the only choise to do it.
It's possible to have a wide (aka 15 kHz) VHF roofing filter, and still very good 2 kHz dynamic range, but it comes
at a cost.

All receiver are compromises, between technical choise, cost, and even marketting. You must understand this to
make the better choise with the money you want to spend.

I have a lot of respect for the work done by Rob Sherwood. But
its chart, and the way receivers are classed, describes only a
very small part of receivers performances, even if it has the
merit to provoque the interest of prospective buyers and by way
of consequence the interest of marketing departments of
ham equipment manufacturers.

2kHz spacing IMD3 describes the behavior of a receiver in presence
of close spaced signals, mainly CW and data. A good figure
can be had by a tight selectivity near the frontend obtained
by a narrow bandwidth roofing filter (as narrow as 200 Hz for
the K3), and/or by a wideband roofing filter and judicious
gain distribution (as the RS XK2100 does).

As Rob pointed it, 2 kHz IMD3 has little to mean for SSB because
IMD generated by nearby transmitters is more prevalent than
IMD generated by the receiver itself ; there, selectivity obtained
by the DSP is the main factor.

The chart of Rob Sherwood is very incomplete ; for exemple,
he could add image rejection, and IMD2 and IMD3 generated by
remote signals.

Image rejection is not a problem with a good up-conversion
design, figures well over 100dB are mesured (FT-2000). But it's
not the same case with downconversion designs (K2, K3, high in
the Rob chart, Eagle, FT-5000, TS-590). With a 9MHz high IF,
it's difficult and costly to reject images over 70dB on the
higher ham bands, even more difficult with a lower IF (K2).
One must understand that these low figures are a real threat
when cycle 24 will peak. Can you imagine your "chart topping"
$5k+ receiver with the upper ham bands full of BC and utilities
image signals ?

Remote signals IMD2 and IMD3 are even more prevalent.
These measurements describe the behavior of a receiver in
presence of signals for which the sum and/or difference
(F1+F2 or F1-F2 for IMD2, 2xF1-F2 or 2xF2-F1 for IMD3) falls
were you want to receive.
Take real life exemples, described in CQDL magazine in the 80's
by the late DL1BU. Imagine for IMD2 large BC signals from
the 31m band combining with large BC signals from the 25m band.
If your receiver has poor IMD2 (see ARRL review of some
"chart topping" receivers), your 15m ham band will be full
of strong combs of BC signals each 5kHz.
The same problem exists for 31m and 19m BC bands falling inside
the 12m ham band, and for 21m and 19m BC bands falling inside
the 10m ham band, but there are a lot of over combinaisons doing
the same.
For IMD3, the 41, 31 and 21 m BC bands can generate
tremendous spurious signals on the 40, 30 and 20 m ham bands.

Speaking of real life, I bought in the mid 80's a brand new
TS-830s, a very good reveiver, with down-conversion and
a preselector. This receiver is quite high into Rob chart, and
highly regarded (see eHam.net). When, for the first time
I connected it to my Sommer XP507 antenna (a kind of
log periodic wide band beam), the receiver was useless during
certain parts of the day when the propagation was good on the
BC bands. I could solve the problem from IMD2 with 2 switchable
high-pass filters (18 MHz high-pass for 17 and 15 m ham bands, and
24 MHz for 12 and 10 m ham bands). For 40, 30 and 20 m ham bands,
the only solution was to use the 20 dB attenuator to reduce IMD3.
I could also have used an outboard preselector.

I can attest that with the peak of cycle 24, if you have a
large and/or broadband antenna, these same problems will occur
if your receiver has poor IMD2 and remote signals IMD3. I can
also certify that 2kHz IMD3 will have very little interest
because statisticaly you will have a few ham signals inside
your roofing filter, and thousands large BC and utilities
signals outside your roofing filters. RF selectivity
(VRF or Digisel), relays switching of RF band-pass filters
will be most important, not 2 kHz IMD3, even if it has its
own relative importance.

Think about your trafic, about your antenna, make your
own jugement and enjoy your rig, no one is perfect. Think,
it's just a hobby.

Best regards to the group.

Georges F6DFZ

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Johnny Siu

Re： Down-conversion

Hello Georges,

Thanks for your informative message and I like to read it.

You are right. Pofessional radios employ up conversion scheme and wide roofing
filter.
Of course, they have other expensive measures to ensure good 2Khz dynamic range.

For the insitutional users, they would use both digital and voice mode. CW is
not their main interest.

cheers,

Johnny VR2XMC

----- 郵件原件 ----
寄件人﹕ Georges Ringotte F6DFZ <[hidden email]>
收件人﹕ [hidden email]
傳送日期﹕ 2010/11/18 (四) 3:05:42 PM
主題： [Elecraft] Down-conversion

Hi to the group,

Here is a post I made on the TS-590 Yahoo Group about down or up conversion,
close spaced signals dynamic range, and the

way receivers are classed in a few charts (Sherwood or W8JI for exemple).

I ordered very recently a K3, and it is on its way to France.
I must add that Elecraft took some measures to limit the problems associated
with down-conversion ; for exemple, transmit low pass filters are used to
improve image rejection. There is also a complex

netwoork in the receiver path with notches on image frequencies of the upper ham
bands. For remote signals IMD2 and IMD3,
relays are used to limit them.

Down-conversion is not a fatality, only a technical choise aimed to get the
maximun performance on the ham bands.
If it's well implemented (as on the K3, I do hope !), all is fine. If not, you
will have problems much more important than

limited 2 kHz dynamic range.

Professional receiver must cover the whole HF spectrum, and up-cpnversion is the
only choise to do it.
It's possible to have a wide (aka 15 kHz) VHF roofing filter, and still very
good 2 kHz dynamic range, but it comes
at a cost.

All receiver are compromises, between technical choise, cost, and even
marketting. You must understand this to
make the better choise with the money you want to spend.

I have a lot of respect for the work done by Rob Sherwood. But
its chart, and the way receivers are classed, describes only a
very small part of receivers performances, even if it has the
merit to provoque the interest of prospective buyers and by way
of consequence the interest of marketing departments of
ham equipment manufacturers.

2kHz spacing IMD3 describes the behavior of a receiver in presence
of close spaced signals, mainly CW and data. A good figure
can be had by a tight selectivity near the frontend obtained
by a narrow bandwidth roofing filter (as narrow as 200 Hz for
the K3), and/or by a wideband roofing filter and judicious
gain distribution (as the RS XK2100 does).

As Rob pointed it, 2 kHz IMD3 has little to mean for SSB because
IMD generated by nearby transmitters is more prevalent than
IMD generated by the receiver itself ; there, selectivity obtained
by the DSP is the main factor.

The chart of Rob Sherwood is very incomplete ; for exemple,
he could add image rejection, and IMD2 and IMD3 generated by
remote signals.

Image rejection is not a problem with a good up-conversion
design, figures well over 100dB are mesured (FT-2000). But it's
not the same case with downconversion designs (K2, K3, high in
the Rob chart, Eagle, FT-5000, TS-590). With a 9MHz high IF,
it's difficult and costly to reject images over 70dB on the
higher ham bands, even more difficult with a lower IF (K2).
One must understand that these low figures are a real threat
when cycle 24 will peak. Can you imagine your "chart topping"
$5k+ receiver with the upper ham bands full of BC and utilities
image signals ?

Remote signals IMD2 and IMD3 are even more prevalent.
These measurements describe the behavior of a receiver in
presence of signals for which the sum and/or difference
(F1+F2 or F1-F2 for IMD2, 2xF1-F2 or 2xF2-F1 for IMD3) falls
were you want to receive.
Take real life exemples, described in CQDL magazine in the 80's
by the late DL1BU. Imagine for IMD2 large BC signals from
the 31m band combining with large BC signals from the 25m band.
If your receiver has poor IMD2 (see ARRL review of some
"chart topping" receivers), your 15m ham band will be full
of strong combs of BC signals each 5kHz.
The same problem exists for 31m and 19m BC bands falling inside
the 12m ham band, and for 21m and 19m BC bands falling inside
the 10m ham band, but there are a lot of over combinaisons doing
the same.
For IMD3, the 41, 31 and 21 m BC bands can generate
tremendous spurious signals on the 40, 30 and 20 m ham bands.

Speaking of real life, I bought in the mid 80's a brand new
TS-830s, a very good reveiver, with down-conversion and
a preselector. This receiver is quite high into Rob chart, and
highly regarded (see eHam.net). When, for the first time
I connected it to my Sommer XP507 antenna (a kind of
log periodic wide band beam), the receiver was useless during
certain parts of the day when the propagation was good on the
BC bands. I could solve the problem from IMD2 with 2 switchable
high-pass filters (18 MHz high-pass for 17 and 15 m ham bands, and
24 MHz for 12 and 10 m ham bands). For 40, 30 and 20 m ham bands,
the only solution was to use the 20 dB attenuator to reduce IMD3.
I could also have used an outboard preselector.

I can attest that with the peak of cycle 24, if you have a
large and/or broadband antenna, these same problems will occur
if your receiver has poor IMD2 and remote signals IMD3. I can
also certify that 2kHz IMD3 will have very little interest
because statisticaly you will have a few ham signals inside
your roofing filter, and thousands large BC and utilities
signals outside your roofing filters. RF selectivity
(VRF or Digisel), relays switching of RF band-pass filters
will be most important, not 2 kHz IMD3, even if it has its
own relative importance.

Think about your trafic, about your antenna, make your
own jugement and enjoy your rig, no one is perfect. Think,
it's just a hobby.

Best regards to the group.

Georges F6DFZ

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Arie Kleingeld PA3A-2

Re: Down-conversion

In reply to this post by Georges Ringotte F6DFZ

Thanks Georges,

Interesting thoughts.

73,
Arie PA3A

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Scott Ellington

Re: Re： Down-conversion

In reply to this post by Johnny Siu

I wonder if ANY commercial/military HF users require the kind of close-in dynamic range that we do. If you have a choice, why would you design a communications system that allows signals within 2 kHz of each other with a 140 dB difference in amplitude? Only the relative anarchy of the ham bands allows that! I'm not complaining, as long as we have receivers like the K3.

73,

Scott K9MA

On Nov 18, 2010, at 1:25 AM, Johnny Siu wrote:

> Pofessional radios employ up conversion scheme and wide roofing
> filter.
> Of course, they have other expensive measures to ensure good 2Khz dynamic range.

Scott Ellington
Madison, Wisconsin
USA

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Georges Ringotte F6DFZ

K2 PA Failure

In reply to this post by Georges Ringotte F6DFZ

Hi to the group,

When I had my K2/10, I had multiple failures of the PA transistors.
Close investigation showed that the problem came from my switching power
supply (a professional 5A one from Friwo).
When switching on the K2, the power supply emited large spikes that
destroyed the PA transistors.
If the K2 was first switched on its internal battery, and then the power
supply was switch on, no problem as the capacitors of the K2 were already
charged.

Before restauring your K2, check your power supply with a scope at switch
on.

Best regards.

Georges F6DFZ

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John Ragle

Re: K2 PA Failure

Am I missing something here? Seems to me that the rational thing to do
is turn on the power supply and let it stabilize (should only take a
second or two), then turn on the K2/K3? Turning a K2 or K3 on and off
using the power supply switch never gives the firmware/software a chance
to initialize or shut down...just like pulling the plug on your computer
might well cause at least mild consternation to the machine! There are
no enormous filter caps in either Kn machine that need to be serviced at
power on. Seems to me that using a non-matched pair of transistors is
much more likely to be the cause of failure, as someone has already
pointed out.

John Ragle -- W1ZI

=====

On 2/17/2011 2:31 AM, Georges Ringotte F6DFZ wrote:

> Hi to the group,
>
> When I had my K2/10, I had multiple failures of the PA transistors.
> Close investigation showed that the problem came from my switching power
> supply (a professional 5A one from Friwo).
> When switching on the K2, the power supply emited large spikes that
> destroyed the PA transistors.
> If the K2 was first switched on its internal battery, and then the power
> supply was switch on, no problem as the capacitors of the K2 were already
> charged.
>
> Before restauring your K2, check your power supply with a scope at switch
> on.
>
> Best regards.
>
> Georges F6DFZ
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>
>
>

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g3ymc

Re: K2 PA Failure

The K2 has a real on/off switch. So always using that to turn it on and
off is certainly a good idea.

I am certainly puzzled at these claims of pa blowups. My original pair
of transistors is going fine after 9 years and some 30k QSOs. I have
removed the heatsink several times while fitting mods, and the last
time I replaced the heatsink washers as well, and have never had any
problems. I agree that the mechanical arrangement is not the best, but
it does the job.

73 Dave G3YMC

On 17 Feb 2011 at 3:14, John Ragle wrote:

> Am I missing something here? Seems to me that the rational thing to do
> is turn on the power supply and let it stabilize (should only take a
> second or two), then turn on the K2/K3? Turning a K2 or K3 on and off
> using the power supply switch never gives the firmware/software a chance
> to initialize or shut down...just like pulling the plug on your computer
> might well cause at least mild consternation to the machine! There are
> no enormous filter caps in either Kn machine that need to be serviced at
> power on. Seems to me that using a non-matched pair of transistors is
> much more likely to be the cause of failure, as someone has already
> pointed out.

http://www.davesergeant.com

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Mike Harris

Re: K2 PA Failure

In reply to this post by John Ragle

You can't turn the K3 fully on with the PSU power switch otherwise
nearly everyone would be doing it all the time.

Regards,

Mike VP8NO

----- Original Message -----
From: "John Ragle" <[hidden email]>
To: <[hidden email]>
Sent: Thursday, February 17, 2011 5:14 AM
Subject: Re: [Elecraft] K2 PA Failure

> Am I missing something here? Seems to me that the rational thing
> to do
> is turn on the power supply and let it stabilize (should only take
> a
> second or two), then turn on the K2/K3? Turning a K2 or K3 on and
> off
> using the power supply switch never gives the firmware/software a
> chance
> to initialize or shut down...just like pulling the plug on your
> computer
> might well cause at least mild consternation to the machine! There
> are
> no enormous filter caps in either Kn machine that need to be
> serviced at
> power on. Seems to me that using a non-matched pair of transistors
> is
> much more likely to be the cause of failure, as someone has
> already
> pointed out.
>
> John Ragle -- W1ZI

Georges Ringotte F6DFZ

K2's Future

In reply to this post by Georges Ringotte F6DFZ

Hi to the group,

Grab a K2 kit and/or its options as soon as you can.
Through hole components become more and more scarce, and I believe will come
a time
when Elecraft will announce they can no more produce these kits.
OHR, Wilderness Radio and others have such difficulties.

Regards

Georges

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