Re: signal offset during transmission with Panadapter?

[tomb18]

Hi . I was wondering if anyone could explain something to me please. If one has a Panadapter like lppan or sdrplay attached to if out the k3, and you transmit, you see the transmitted signal displaced a couple of kHz higher. Why exactly would this happen? I have some ideas but I am not quite sure. Thanks. 


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-------- Original message --------From: Mike Zabel <[hidden email]> Date: 2017-12-22  1:30 PM  (GMT-05:00) To: [hidden email] Cc: Elecraft Reflector <[hidden email]> Subject: Re: [Elecraft] 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!
>>
>

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[Don Wilhelm]

Tom,

That is the result of the transmit signal leakage into the receive IF
output.  If it is bothersome, a relay operated with PTT to break the IF
output would be in order.

It is not a true representation on the final transmit signal because of
shaping circuits in the K3 transmit chain following the leakage point.
So it cannot validly be used to analyze the final transmit signal.

73,
Don W3FPR

On 12/22/2017 1:45 PM, tomb18 wrote:
> Hi . I was wondering if anyone could explain something to me please. If one has a Panadapter like lppan or sdrplay attached to if out the k3, and you transmit, you see the transmitted signal displaced a couple of kHz higher. Why exactly would this happen? I have some ideas but I am not quite sure. Thanks.
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[tomb18]

HiOK thanks! Just as I thought but wasn't sure 73 Tom 


Sent from my Samsung Galaxy smartphone.
-------- Original message --------From: Don Wilhelm <[hidden email]> Date: 2017-12-22  2:54 PM  (GMT-05:00) To: tomb18 <[hidden email]>, Elecraft Reflector <[hidden email]> Subject: Re: [Elecraft] signal offset during transmission with Panadapter?
Tom,

That is the result of the transmit signal leakage into the receive IF
output.  If it is bothersome, a relay operated with PTT to break the IF
output would be in order.

It is not a true representation on the final transmit signal because of
shaping circuits in the K3 transmit chain following the leakage point.
So it cannot validly be used to analyze the final transmit signal.

73,
Don W3FPR

On 12/22/2017 1:45 PM, tomb18 wrote:
> Hi . I was wondering if anyone could explain something to me please. If one has a Panadapter like lppan or sdrplay attached to if out the k3, and you transmit, you see the transmitted signal displaced a couple of kHz higher. Why exactly would this happen? I have some ideas but I am not quite sure. Thanks.
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