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KX2 Offset I.F.

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Mike Zabel

KX2 Offset I.F.

Hello group,

I received a new KX2 from Elecraft a few days ago and I am really enjoying
the little GEM!

As many of you already must know, the receiver performs brilliantly. I have
been comparing with a couple of SDR receivers in the shack and the KX2 is
preferred for weak signal reception in any mode.

I plan to use it portable of course, but for now have it set up in the
shack with an SDR receiver tracking with it as a panadapter.

I have noticed that the KX2 produces about a -74dBm signal into the SDR
that is connected in parallel to the same Rx antenna (through a T/R switch).

The signal is offset about 8 kHz from the VFO , so the KX2 must use the
offset full time where it was a menu option on the KX3 I had?

For anyone using a similar setup I wonder if you see the signal at about
the same power level?

Am I correct that the nulling or supression is done at the audio stage
within the KX2 (because I can't hear it when I tune the 2nd VFO to it in
Dual Rx)?

Thanks Elecraft for another great transceiver and thank you all on the
group for being there as a great resource!

KV4TT - Mike

KX2 s/n 2300
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Don Wilhelm

Re: KX2 Offset I.F.

Mike,

The KX2 is fully an SDR with the "computer" in the enclosure. Yes, it
has a permanent 8kHz shift, so all the DSP operation is conducted at
8kHz (not audio), including the nulling and suppression.

You are hearing the actual VFO freuency - it is 8kHz from the operating
frequency of the KX2.

If you turn on the preamp, the locally heard VFO signal level should be
reduced. The KX2 is designed to operate normally with the preamp on.

If your KX2 and your other SDR receiver are close to each other, you
could be hearing some leakage from the KX2 circuitry rather than from
the antenna alone.

73,
Don W3FPR

On 12/22/2017 9:38 AM, Michael Zabel wrote:

> Hello group,
>
> I received a new KX2 from Elecraft a few days ago and I am really enjoying
> the little GEM!
>
> As many of you already must know, the receiver performs brilliantly. I have
> been comparing with a couple of SDR receivers in the shack and the KX2 is
> preferred for weak signal reception in any mode.
>
> I plan to use it portable of course, but for now have it set up in the
> shack with an SDR receiver tracking with it as a panadapter.
>
> I have noticed that the KX2 produces about a -74dBm signal into the SDR
> that is connected in parallel to the same Rx antenna (through a T/R switch).
>
> The signal is offset about 8 kHz from the VFO , so the KX2 must use the
> offset full time where it was a menu option on the KX3 I had?
>
> For anyone using a similar setup I wonder if you see the signal at about
> the same power level?
>
> Am I correct that the nulling or supression is done at the audio stage
> within the KX2 (because I can't hear it when I tune the 2nd VFO to it in
> Dual Rx)?
>
> Thanks Elecraft for another great transceiver and thank you all on the
> group for being there as a great resource!
>
> KV4TT - Mike
>
> KX2 s/n 2300
> ______________________________________________________________
> 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
> Message delivered to [hidden email]
>

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Mike Zabel

Re: KX2 Offset I.F.

Hi Don,

Thank you for the great explanation.

I must have had the ATT on when I did the original power measurement
(rough) on the SDR

I tested again and this time I get:

-77dBm - Preamp On
-55dBm - Preamp Off
-74dBm - ATT on

Just relative differences with the absolutes unique to my setup of course.

The VFO signal is in the noise with the antenna disconnected from both KX2
and SDR.
The VFO signal is lower when using separate antennas for each as would be
expected.

I was just not used to seeing a signal like that when using the SDR to
track superhet's before, so it had me scratching my head for a bit until I
considered the Rx architecture of the KX2(3).

Thank you for helping me get everything organized in my thinking, sometimes
that is a monumental task!

73 de KV4TT - Mike

KX2 s/n 2300

On Fri, Dec 22, 2017 at 11:14 AM, Don Wilhelm <[hidden email]>
wrote:

> Mike,
>
> The KX2 is fully an SDR with the "computer" in the enclosure. Yes, it has
> a permanent 8kHz shift, so all the DSP operation is conducted at 8kHz (not
> audio), including the nulling and suppression.
>
> You are hearing the actual VFO freuency - it is 8kHz from the operating
> frequency of the KX2.
>
> If you turn on the preamp, the locally heard VFO signal level should be
> reduced. The KX2 is designed to operate normally with the preamp on.
>
> If your KX2 and your other SDR receiver are close to each other, you could
> be hearing some leakage from the KX2 circuitry rather than from the antenna
> alone.
>
> 73,
> Don W3FPR
>
>
> On 12/22/2017 9:38 AM, Michael Zabel wrote:
>
>> Hello group,
>>
>> I received a new KX2 from Elecraft a few days ago and I am really
>> enjoying
>> the little GEM!
>>
>> As many of you already must know, the receiver performs brilliantly. I
>> have
>> been comparing with a couple of SDR receivers in the shack and the KX2 is
>> preferred for weak signal reception in any mode.
>>
>> I plan to use it portable of course, but for now have it set up in the
>> shack with an SDR receiver tracking with it as a panadapter.
>>
>> I have noticed that the KX2 produces about a -74dBm signal into the SDR
>> that is connected in parallel to the same Rx antenna (through a T/R
>> switch).
>>
>> The signal is offset about 8 kHz from the VFO , so the KX2 must use the
>> offset full time where it was a menu option on the KX3 I had?
>>
>> For anyone using a similar setup I wonder if you see the signal at about
>> the same power level?
>>
>> Am I correct that the nulling or supression is done at the audio stage
>> within the KX2 (because I can't hear it when I tune the 2nd VFO to it in
>> Dual Rx)?
>>
>> Thanks Elecraft for another great transceiver and thank you all on the
>> group for being there as a great resource!
>>
>> KV4TT - Mike
>>
>> KX2 s/n 2300
>> ______________________________________________________________
>> 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
>> Message delivered to [hidden email]
>>
>>

Don Wilhelm

Re: KX2 Offset I.F.

Mike,

Think of the 8kHz shift similar to a low frequency (8kHz) IF if your
thinking is along the lines of a superhet receiver.

Of course, the KX2 is *not* a superhet, it is a full SDR. If the "IF"
were at a higher frequency, the VFO would be further away from the
signal frequency - by the frequency of the IF.

The SDR DSP processing is at 8kHz in the KX2, or selectable between
baseband (zero frequency) and 8kHz in the KX3.

Using baseband has some advantages (the roofing filters), but also some
downsides such as increased exposure to strong radio broadcast signals
leaking into the baseband stream. If you have ever experimented with
Direct Conversion receivers, you will know about that breakthrough.

An SDR in simplified form is two identical Direct Conversion receivers
operated 90 degrees out of phase with each other. That allows the
signal processing to be done mathematically (whether that be analog or
digital). A study into the Phasing Receivers of Rick Campbell KK7B
design along with op-amp mathematics will give you the analog version of
that signal processing.
The problem with analog implementation is that real components have
tolerances that must be dealt with - digital mathematics can be exact
calculations.

73,
Don W3FPR

On 12/22/2017 12:07 PM, Michael Zabel wrote:

> Hi Don,
>
> Thank you for the great explanation.
>
> I must have had the ATT on when I did the original power measurement
> (rough) on the SDR
>
> I tested again and this time I get:
>
> -77dBm - Preamp On
> -55dBm - Preamp Off
> -74dBm - ATT on
>
> Just relative differences with the absolutes unique to my setup of course.
>
> The VFO signal is in the noise with the antenna disconnected from both KX2
> and SDR.
> The VFO signal is lower when using separate antennas for each as would be
> expected.
>
> I was just not used to seeing a signal like that when using the SDR to
> track superhet's before, so it had me scratching my head for a bit until I
> considered the Rx architecture of the KX2(3).
>
> Thank you for helping me get everything organized in my thinking, sometimes
> that is a monumental task!

Mike Zabel

Re: KX2 Offset I.F.

This sounds like a fun area to study, Thanks for putting me on the path. I
will look up KK7B's work.

KV4TT - Mike

On Fri, Dec 22, 2017 at 12:37 PM, Don Wilhelm <[hidden email]>
wrote:

> Mike,
>
> Think of the 8kHz shift similar to a low frequency (8kHz) IF if your
> thinking is along the lines of a superhet receiver.
>
> Of course, the KX2 is *not* a superhet, it is a full SDR. If the "IF"
> were at a higher frequency, the VFO would be further away from the signal
> frequency - by the frequency of the IF.
>
> The SDR DSP processing is at 8kHz in the KX2, or selectable between
> baseband (zero frequency) and 8kHz in the KX3.
>
> Using baseband has some advantages (the roofing filters), but also some
> downsides such as increased exposure to strong radio broadcast signals
> leaking into the baseband stream. If you have ever experimented with
> Direct Conversion receivers, you will know about that breakthrough.
>
> An SDR in simplified form is two identical Direct Conversion receivers
> operated 90 degrees out of phase with each other. That allows the signal
> processing to be done mathematically (whether that be analog or digital).
> A study into the Phasing Receivers of Rick Campbell KK7B design along with
> op-amp mathematics will give you the analog version of that signal
> processing.
> The problem with analog implementation is that real components have
> tolerances that must be dealt with - digital mathematics can be exact
> calculations.
>
> 73,
> Don W3FPR
>
>
> On 12/22/2017 12:07 PM, Michael Zabel wrote:
>
>> Hi Don,
>>
>> Thank you for the great explanation.
>>
>> I must have had the ATT on when I did the original power measurement
>> (rough) on the SDR
>>
>> I tested again and this time I get:
>>
>> -77dBm - Preamp On
>> -55dBm - Preamp Off
>> -74dBm - ATT on
>>
>> Just relative differences with the absolutes unique to my setup of course.
>>
>> The VFO signal is in the noise with the antenna disconnected from both KX2
>> and SDR.
>> The VFO signal is lower when using separate antennas for each as would be
>> expected.
>>
>> I was just not used to seeing a signal like that when using the SDR to
>> track superhet's before, so it had me scratching my head for a bit until I
>> considered the Rx architecture of the KX2(3).
>>
>> Thank you for helping me get everything organized in my thinking,
>> sometimes
>> that is a monumental task!
>>
>

K9MA

Re: KX2 Offset I.F.

In reply to this post by Don Wilhelm

On 12/22/2017 11:37, Don Wilhelm wrote:
> Of course, the KX2 is *not* a superhet, it is a full SDR. If the "IF"
> were at a higher frequency, the VFO would be further away from the
> signal frequency - by the frequency of the IF.

I must disagree. All the digital processing takes place after the
quadrature mixers. Up to that point, it IS a single conversion
superhet, but with two mixers with quadrature outputs. The same is true
of the KX3, though it can be operated as a direct conversion receiver,
which is simply a superhet with a zero frequency IF.

This is in no way meant to disparage the design of the KX2 or KX3: They
are truly excellent radios.

73,

Scott K9MA

--
Scott K9MA

[hidden email]

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Don Wilhelm

Re: KX2 Offset I.F.

Scott,

By your definition of an SDR, that may be true.
Some SDRs today are direct sampling at the antenna, but think back to
SDRs in the recent past when high frequency direct sampling was only
achieved at great cost - the simplest example is the SoftRock 40 which
used a quadrature mixer to baseband.
I think most would say the Softrock is an SDR.

73,
Don W3FPR

On 12/22/2017 1:53 PM, K9MA wrote:

> On 12/22/2017 11:37, Don Wilhelm wrote:
>> Of course, the KX2 is *not* a superhet, it is a full SDR. If the "IF"
>> were at a higher frequency, the VFO would be further away from the
>> signal frequency - by the frequency of the IF.
>
> I must disagree. All the digital processing takes place after the
> quadrature mixers. Up to that point, it IS a single conversion
> superhet, but with two mixers with quadrature outputs. The same is true
> of the KX3, though it can be operated as a direct conversion receiver,
> which is simply a superhet with a zero frequency IF.
>
> This is in no way meant to disparage the design of the KX2 or KX3: They
> are truly excellent radios.
>
> 73,
>
> Scott K9MA
>

K9MA

Re: KX2 Offset I.F.

Don,

I'm not sure there IS an accepted definition of a "Full SDR" radio, but
it's just a matter of semantics, anyway. I just wanted to point out
that the KX2 and KX3 use local oscillators and mixers to convert the RF
to the IF, like any superhet. From there, it's pure SDR. (Perhaps
aside from the analog "roofing" filters in the KX3.)

Now, if a radio used a high speed ADC to digitize the incoming RF
directly at something greater than the Nyquist rate, I'd call that "Full
SDR". So far, high resolution ADC's really aren't fast enough for the
high end of the HF spectrum. Direct sampling is sort of a gray area, I
suppose, as the undersampled ADC really acts like a mixer, and needs to
be preceded by a bandpass filter to eliminate the undesired alias products.

73,
Scott K9MA

On 12/22/2017 14:01, Don Wilhelm wrote:

> Scott,
>
> By your definition of an SDR, that may be true.
> Some SDRs today are direct sampling at the antenna, but think back to
> SDRs in the recent past when high frequency direct sampling was only
> achieved at great cost - the simplest example is the SoftRock 40 which
> used a quadrature mixer to baseband.
> I think most would say the Softrock is an SDR.
>
> 73,
> Don W3FPR
>
> On 12/22/2017 1:53 PM, K9MA wrote:
>> On 12/22/2017 11:37, Don Wilhelm wrote:
>>> Of course, the KX2 is *not* a superhet, it is a full SDR. If the
>>> "IF" were at a higher frequency, the VFO would be further away from
>>> the signal frequency - by the frequency of the IF.
>>
>> I must disagree. All the digital processing takes place after the
>> quadrature mixers. Up to that point, it IS a single conversion
>> superhet, but with two mixers with quadrature outputs. The same is
>> true of the KX3, though it can be operated as a direct conversion
>> receiver, which is simply a superhet with a zero frequency IF.
>>
>> This is in no way meant to disparage the design of the KX2 or KX3:
>> They are truly excellent radios.
>>
>> 73,
>>
>> Scott K9MA
>>

--
Scott K9MA

[hidden email]

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Post: mailto:[hidden email]

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