[HBR] HR-67 / balanced 7360

[hbr@mailman.qth.net]

In a message dated 4/12/02 10:41:14 PM Eastern Daylight Time, 
john_finigan@yahoo.com writes:

> > The 7360 module I have is setup as a real balanced 
>  > mixer and is a bear to keep balanced.  Using this
>  > tube single ended which most designers do seems to >
>  be defeating the purpose of using it. 
>    
>  
>  I think that Squiers-Sanders used it this way, but
>  I've never heard of someone doing it in a homebrew
>  design.  I'm very curious as to what sort of coils,
>  etc it takes??
>  
Squires Sanders used two 7360s as balanced mixers in the SS-1R. The schematic
and manual are on the BAMA site. They used special IF transformers and a 
variety
of balancing adjustments.

They used balanced mixers because of concerns about LO radiation and IF 
feedthrough. IF feedthrough can be a real headache because most single ended 
mixers also amplify any signals at the IF, and the gain at IF is typically 
four or more times the conversion gain. If there is no RF stage, the 
sensitivity of the IF amplifer must be very high because the signal from the 
mixer can be at a very low level.

For example, suppose I have a nice set of 2215 kHz filters (thanks, Kees!!) 
and frontend tuned circuits that reject 2215 kHz by 40 dB relative to the 
signal frequency. If I use a typical single-ended mixer with 20 dB conversion 
gain, the
its gain at 2215 can be 26 dB or more. This extra gain effectively degrades 
the 
RF coils' rejection by 6 dB, so I really have only 34 dB of IF rejection.

Now if I am trying to listen to a 1 uV signal and an unwanted signal 100 uV 
signal appears on 2215, it will bury the desired 1 uV signal by 6 dB! So I 
need LOTS of IF rejection.

The SS-1R uses a tunable first IF of 5 - 5.5 MHz on all bands except 40 
meters, 
so IF rejection is very important, and a simple trap won't help much. 

My 4th edition of the RSGB Handbook shows both single ended and balanced 7360 
mixer circuits. It also shows the Pullen mixer but does not identify it by 
that name. A receiver using a balanced 7360 mixer, 1682 kHz IF and no RF 
stage is also shown in the SSB chapter. This RX uses a double-tuned input 
circuit and a modified British first-IF transformer.

The Southgate Type 7 receiver section has a 1400 kHz IF, a single-ended 7360 
mixer and 6EH7 RF amp. Also a 5 pole highpass filter to help eliminate BC 
band signals. IF rejection problems were simply insurmountable without the RF 
stage. The 1400 kHz IF filters were hamfest specials and simply too good to 
not use.

73 de Jim, N2EY