1st Mixer and 45MHz IF Filter

    

 

1st Mixer

     For the first mixer, I will use a passive type.  This approach means that we will have no gain during the first RF frequency conversion process.  This isn’t necessarily bad, since we won’t gain up any unwanted signals or noise.  The TUF-1+ mixer from minicircuits will work here, it has good isolation and the conversion loss is as expected for a passive mixer (5 – 9dB of loss). 

    The input impedance is 50 ohms as long as the output (IF) pin is terminated into a 50 ohm load.  Sometimes, designers will add a 3dB attenuation pad to the output of the mixer to provide constant impedance for the mixer output load.  This is because the mixer output is usually connected to an IF filter that probably won’t have 50 ohm impedance at both the sum and difference frequencies produced by the mixer. 

 

     This mixer will require a LO power of +7dBm.        

                    +7dBm = 10*log(Pmw) = 10*log(5mW) = 10*log(mV2 / 50) = 500mVrms or 1.4Vp-p          

 

     The mixer has two switchable input paths, the first being from the RX bandpass filter which is used for receive mode and the second would be the transmit path from the IF filter.

 

 

45MHz IF Filters

     ECS makes a small SMD crystal filter in a 5 x 7 mm package.  They can be a bit hard to solder (watch for solder shorts to the top of the package).  The filter is a 2 pole filter but you can put two parts in series and achieve a 4 pole response.  When doing this, the dots on the package should share the common connection point.  I read this in their app notes but they didn’t explain why.

     Impedance termination:  In receive mode, the one side of the filter will be connected to the 1st mixer’s IF pin (50 ohm) and the other side of the filter will connect to the 2nd mixer (high impedance).  This will require two different LC networks for termination.  The 45MHz filter’s terminating impedance is 560Ω // 6pF.  Both networks are configured as low pass filters to help improve the high frequency rejection (above 45MHz).

 

 

 

References

1.      Filter Application Notes, ECS International, Inc.  Apno-0017

2.      ECS96SMF45 Datasheet, ECS International, Inc.

 

 

 

 

 

 

 

 

References:

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