K3 and SDR Integration - Almost completed!!!

[Don Rasmussen]

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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[Jack Smith-6]

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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>  

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[Scott McClements]

Don,

I have my softrock kit mounted in its own cabinent. I was able to hook
the softrock kit right to the 940 IF output - the signal level from
the 940 was ok and I don't have any evidence of softrock osc.
artifacts getting back into the 940's IF stage. I actually even have a
preamp
inline between the 940 and the Softrock kit because the IF output
seems to tail off as
you move up in frequency. Since I can also switch off the AGC
completely with the 940, it works out very well as a high resolution
bandscope and as a receiver.

I didn't know if the 940 output was truly "buffered" or not, so I
thank Jack for the description of the Kenwood circuit.

-Scott, WU2X


> From: Jack Smith <[hidden email]>

>
> 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.
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[Geoffrey Mackenzie-Kennedy-2]

Jack Smith wrote:

> 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.

 ------------------------------------------------------------------------

PA3AKE was using two grounded gate J310 FETs in cascade after each generator
for improved isolation while running IMD tests on H-Mode mixers. He reported
a reverse isolation of 75db and about 4db forward gain for each pair of
J310s at HF and 6m.

73,
Geoff
GM4ESD


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