[NLRS] More on sequencers

[Dr. Gerald N. Johnson]

On Mon, 2006-03-27 at 18:53 -0500, W0ZQ at aol.com wrote:
> 
>  
> 
> In a message dated 3/27/2006 4:52:17 P.M. Central Standard Time,  
> chrise at pobox.com writes:
> 
> These  sequencers are all cool and everything but there is still one
> fundamental  problem I don't see getting solved--  namely, how do you
> keep the RF  from coming out of the IF rig until all of the other stuff
> has been  sequenced?
> 
> 
> 
> 
> I have wondered this from time to time myself.   I suspect that  the answer 
> is that it usually doesn't matter. 

Oh, IT DOES MATTER. 

Perhaps the transverter with TIS circuit is well protected, but the
ordinary PA/preamp, especially the preamp is what gets fried when the
alternate keying happens, like from the front panel "call" button. Or
using KOX on CW.
>  
> The current modification to the DEM TIS circuit that I am looking at may  
> make this worse .... I am looking to use the IF rigs (FT-817) PTT output line to  
> key the SMA relay right away, but the circuit will hold off on keying the  
> transverter & external 8 watt amp for say 100 ms thus allowing the SMA  relay 
> time to switch.   This would prevent hot switching of the SMA  relay.  As you 
> point out, the FT-817, since its driving the bus here, has  started to send 144 
> MHz RF out its IF line not too far behind when its PTT went  active (low), yet 
> the transverter is being held out of transmit mode for 100 ms  by my control 
> circuit.   In this case I think the receiver mixer may  see whatever the drive 
> level is that is typically being used at the transmit  mixer (for those 100 
> msec or a bit less).   Question is, can the  receiver mixer take this power ?   
> If so, than it doesn't matter and  this approach should work.   If they 
> can't, I blow the receiver  mixer.   
>  
> So that's the question ; can the receiver mixer withstand the same 144 RF  
> power level that is typically directed to the transmit mixer ?    According to 
> the DEM parts list, the transmit mixer and the receive mixer are  the same type 
> of device, a MA4E2054B.   I suspect the answer is yes IF  you have a well 
> controlled (ie; no spikes) IF source.   Any RF  power spikes coming from your IF 
> rig would most likely blow the receiver  mixer.
>  
> Comments ?
>  
> 73, Jon
> W0ZQ
>  
The mixer will take the same power for receive as for transmit, but the
transmit side likely has lots of attenuation. The best protected receive
side has excess gain after the mixer followed by an attenuator to make
the system gain only what is needed for the transverter receiver front
end to set the system noise figure which also serves to protect that IF
chip from transmit power in the receive side. But we don't do that for
2m and 432 preamps, perhaps we should. Follow the preamp with a 006
followed by a MAV-011 and then 20 dB of attenuation to protect the
preamp.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer