[Premium-Rx] Racal 1772 LSB position

[Danny Higgins]

Hi Enrique.

Is your 2.35 kHz filter a USB, LSB or symmetrical filter at 1.4 MHz?  If it
is symmetrical (i.e. 1.4 MHz +/- 1.175 kHz) then you cannot use it for LSB
or USB, only CW or AM.

The RA1772 converts HF signals (0 to 30 MHz) to the first IF of 35.4 MHz by
mixing with the first Local Oscillator (LO) that covers 35.4 to 65.4 MHz.
Because the LO is above the wanted signal, a USB signal at say 9 MHz would
be mixed with the LO at 44.4 MHz to give an LSB signal at 35.4 MHz.  This is
mixed with the second LO at 34 MHz to give the final IF of 1.4 MHz.  Because
the second LO is below the IF, the sideband is not inverted this time, so a
filter with an LSB characteristic (e.g. 1.397 MHz to 1.4 MHz) is required to
receive the original USB signal.

In theory the filter can be in any slot on the board as long as the
selection wires from the bandwidth switch are wired to the correct pins for
each filter.  It is possible to have both the 2.35 and 2.7 kHz filters
installed at the same time and have 2 USB bandwidths but you may have to
remove the stops on the bandwidth switch to allow extra positions to be
available.

If you want to check what the actual bandwidths are, you can connect the 1
MHz reference output to the antenna input and tune the receiver to 29 MHz to
pick up the harmonic which should be reasonably strong.  If you switch the
AGC OFF and adjust the IF gain so that the signal does not overload the
receiver, you can plot the audio output level against frequency as you tune
the receiver around 29 MHz.  An LSB filter will produce an audible output
above 29 MHz and USB will be heard below 29 MHz.

Hope this helps,

Danny, G3XVR

-----Original Message-----
From: premium-rx-bounces at mailman.qth.net
[mailto:premium-rx-bounces at mailman.qth.net] On Behalf Of Enrique Fernandez
García
Sent: 10 December 2010 13:14
To: Premium-Rx at mailman.qth.net
Subject: [Premium-Rx] Racal 1772 LSB position

 
Hello Gents,
 
 Recently, I have acquired an used 1,4  2,35 Khz used filter at a fair price
for my Racal 1772 receiver.
 
 I have some doubts in which filter slot I have to put it since there is a
note in the maintenance manual that says:
 
      Due to the inversion in the receiver, a USB filter at FL1 (filter slot
Nº 1) has LSB characterics, and an LSB filter at FL5 has USB characterics.
 
  I do not know understand what ' characterics' mean.   The Racal I have has
at the FL1 filter slot a 3Khz SSB filter and at the filter slot FL5 there is
a 2,7 Khz.
 
 Doing an ocular inspection I see that the 2,7 Khz filter output has a wire
that goes to the LSB mode switch directly.
 
 I think that when I choose the LSB mode I use the 2,7Khz filter with that
bandwidth. But according to the note I would be using a 3Khz due to the
inversion. Is that right? Really am I using a 2,7 Khz bw or a 3Khz ?
 
 Since I am mainly interested in picking up weak signals on tropical band,
broadcasting stations, LSB mode it is very appropriate and I would like to
swap the 2,7 Khz ( 3Khz ? ) for the narrower 2,35 Khz one.
 
My desire it is to use the 2,3 Khz filter on LSB mode but with that
bandwidth (characterics).
What slot do I have to put it in , FL1 or FL5?
 
 I would be very pleased if somebody could throw some light in this confused
issue.
 
 Thank you very much for your valuable help.


      
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