[R-390] Multicouplers - active vs. passive

[Jim]

I use a AMECO PLF FET Preamp.  It has two outputs. One feeds a 75S-3B and 
the other a R-390A.
It is tunable and I usually set it for unity gain.
The R-390A W/ CV-591 makes a decent (not great) band scanner and split 
receive for the S-line

Jim

> Lester wrote:
>
>>The obvious reason not to use passive is the degradation in Noise Figure 
>>due
>>to the splitting loss. In situations where ambient noise is significantly
>>above the receivers front end noise(NF), then passives would be practical.
>>It clearly is difficult to build a wideband amplifier that is superior to
>>the TRF front of the 390s, in terms of IM/Distortion performance.
>
> If you build an RF amplifier like the one in a TenTec RX340 (not the 
> switchable BFG16A RF preamp, the RF amp comprising six J310 FETs in 
> push-pull parallel) [schematics are available on the web], you will get 6 
> dB of gain with input-referred noise approaching -130 dBm in a 3 kHz 
> bandwidth, harmonics around -80 dBc at 100 mV input (-7 dBm), and enough 
> headroom to shrug off nearly anything that doesn't melt your antenna.  Run 
> with a 12 V supply, the 1 dB compression point is greater than 4 Vrms 
> input (+25 dBm).  Follow this amplifier with a passive splitter, and you 
> will have a state-of-the-art multicoupler.  System gain will be ~ +3 dB 
> with a 2-way splitter, 0 dB with a 4-way, and -3 dB with an 8-way.
>
> I know of no commercially-available multicoupler, past or present, that 
> can match this performance (of course, I have not seen every commercial 
> multicoupler ever produced).
>
> For anyone who wants to build one:  I typically select FETs for Idss of 
> 45-50 mA at 12 V (this needs to be pulse-tested with a duty cycle ~30%, to 
> mimic in-circuit operating conditions and prevent overheating the FETs) --  
> but 6 unselected FETs should work fine unless you are very unlucky in the 
> draw.  L59 and L60 are unnecessary and can/should be omitted.  You will 
> need to wind or find three suitable ferrite transformers, one of which 
> (T3) must operate as a 1:1 transmission-line transformer ("current-mode 
> balun") at the lowest frequency of interest.  I replace C115 with a 100 uF 
> tantalum, and add another 100 uF tantalum in parallel with C113.  Make 
> sure all of the multicoupler outputs, including any unused outputs, are 
> terminated in 50 ohms to maintain the rated isolation.
>
> Best regards,
>
> Don
>
>
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