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.
Sent from my Samsung Galaxy smartphone.
Original message From: Mike Zabel Date:
2017-12-22 1:30 PM (GMT-05:00) To: donw...@embarqmail.com Cc: Elecraft
Reflector 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
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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