[DSP-10] 1296 RSU -- more lessons learned

[Bob Larkin]

A couple more thoughts on transverters.  As many know, Paul calls this RSU 
or "Right Side Up" since the LO frequency is lower than the RF 
frequency.  This contrasts with his great little series of  "Multiband 
Microwave Transverters," that, for the 1296 case,  is Up Side Down.  This 
makes everything tune backwards, sidebands are backwards, etc, but the RF 
gear is simpler.  In case some are not aware, the DSP-10 has provision for 
this in the configuration file, and to the user, everything is returned to 
"Right  Side Up."  Tuning is normal, even though the I-F is going backwards 
and sideband selection is correct.  This is determined by the TVTR lines in 
the UHFA.CFG configuration file.

Also, when the DSP-10 goes into transmit, there is a brief moment of 
carrier insertion, due to capacitor charge producing DC.  The transmit 
power can be turned down to the needed level in 1 dB steps, but I don't 
think that stops the milliseconds of carrier.  Thre is probably not enough 
power to damage the mixer, but might be worth some investigation.  If there 
is a problem, one quick fix might be a pair of 1N4148 diodes from the 
transverter mixer 'I' port to ground.  I don't think these silicon diodes 
would conduct at transmit signal levels. I have not played with any of 
this, but wanted everyone to be aware.

Another possibility is to improvise a 10 dB pad as part of the transmit RF 
jumper on the outside of the DSP-10.  This is at 144 MHz, and ordinary 1/4 
Watt resistors with short leads should be OK.  This 50-Ohm coax jumper 
connects J212 to J213 (p 38 of Oct 1999 QST).  This would ensure that the 
mixer could not see more than about 5 mW of RF under any condition.

BTW, Another place where Up Side Down is beneficial, is for converting 432 
to 144, where a low side LO at 288 MHz is problematic for spur's.  Using a 
high side LO at 576 MHz works fine.

73, Bob