[Elecraft] K3 and SDR Integration - Almost completed!!!

[Jack Smith]

Don:

Kenwood 930, 940 and 870 derive the 8830 KHz IF output port from a 
dedicated mixer that duplicates the main mixer, but without the roofing 
filter, if I recall the schematic correctly. Hence there are parallel 
signal paths, one for the main signal and a second path for the IF sample.

Therefore, the rather high local oscillator leakage signal generated by 
a typical Softrock receiver (I measured mine at -40 dBm output, 10 MHz 
version) that is fed back into a Kenwood IF output port is unlikely to 
cause a problem, as there is a fairly complicated path for the signal to 
get back into the main signal chain.

In contrast, the K3, as I understand it, take an IF sample ahead of the 
IF filter, with a source follower. Hence, a signal injected into the 
source follower's output will leak back into the main IF path and be 
heard in the receiver, reduced by the reverse gain of the source 
follower.  It would be unusual, in my experience, to find a source 
follower to have much more than 40 or 50 dB  reverse isolation at 8 MHz. 
If we assume 50 dB reverse isolation, then the -40 dBm signal leaking 
out of the Softrock would be fed back into the K3's IF chain at around 
-90 dBm, which is clearly a problem.  One solution would be a second 
amplifier between the K3's IF output port and the Softrock. This 
amplifier should be designed to have high isolation and about 0 dB gain. 
Perhaps a grounded gate amplifier would  be a good starting point.

Jack K8ZOA


Don Rasmussen wrote:
> Scott,
>
> Did your TS940 require any kind of a special buffer
> circuit to keep the softrock lite from injecting noise
> into the IF, as the K3 might require?
>
> ----------------------------------------
> [Elecraft] K3 and SDR Integration - Almost
> completed!!!
> Scott McClements kc2pih at gmail.com 
> Tue Jul 24 16:48:49 EDT 2007 
>  
> I have been using it with my Kenwood TS-940S with a
> Softrock lite kit for the last two months.
>
> RIght now I am fixing up new ("IF Stage") setup menus
> so that one
>
> -Scott, WU2X
>
>
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