RF Mixers  www.radiopharos.it
    IN SEARCH OF THE IDEAL RF MIXER
Vincent Italia

 


 

 
 

The passion and interest for the receivers, primarily of super heterodyne type, took me over the years to focus my attention in the mixer stage which is undoubtedly the most critical. The design of the mixer was, and is, a pain and a challenge among the various designers. The supremacy is being fought, in particular, with ever higher IP3 or TOIP values! In the mid / late eighties I tried different types of mixers, a few unpublished, with results that I would say interesting, I resumed the "research", although occasionally, in 2003. Below is described one of those mixers.

 

 

DOUBLE TRIODE PASSIVE RF MIXER


Vincent Italia


 

 
 

Ever since it was invented, the superheterodyne radio teased the imagination of radio designers in trying to get more and more efficient mixers and with better performance. Soon it was realized that the mixing action not only produced the trigonometric product of the RF signal with that of the local oscillator, and therefore their sum or difference, but also the generation of spurious originated mainly from strong signals in antenna.
In the sixties, a talented protagonist in the world of RF mixer design was undoubtedly William K. Squires, chief designer and vice president of Squires-Sanders, Inc. As is known to those who, like me, for many years now hang out in the world of Radio, he designed a switching balanced mixer with an electrostatic beam deflection tube that was used successfully in the SS-1R and SS-IBS receivers of the aforementioned company. In 1967 he designed a two FETs balanced passive mixer with excellent characteristics with regard to the intermodulation and noise figure (Ref.1 and 2), but perhaps little is known, to most people, that he also designed a mixer with a dual triode, with the same principle as the one with the two FET. This mixer was used in the AN/URR-58 receiver for the U.S. Coast Guard. The characteristics of this receiver were and are exceptional (Ref. 3), and this aroused in me curiosity and interest. Unfortunately I have not managed yet to get hold of a circuit diagram of the receiver but thanks to a draft of the principle of the mixer circuit, which appeared on Ham Radio 1973 (Fig.1 and Ref.4), I was able to build an experimental version , this in the mid-eighties.

 


The mixer employs a dual triode tube type 6DJ8 (ECC88 equivalent) in a singular balanced configuration. The RF signal is sent to the center tap of the primary of the first intermediate frequency transformer where the two out of phase signals of the local oscillator are, theoretically, canceled. The tube is biased for a Class C operation, this allows it to act as a switch and then to commutate alternately to ground the RF signal through the two cathodes, according to the polarity of the RF voltage generated by the local oscillator. The IF signal is picked up in the intermediate frequency transformer secondary. In the AN/URR-58 this circuit was preceded by a tube RF amplifier stage.
The schematic that I have reconstructed is that of Figure 2 where it can be noted that the sinusoidal signal of the LO is sent to the two grids in Push-Pull, via a broadband unbalanced / balanced transformer. The fixed negative Bias voltage is sent to the grids through VR1 which serves to balance it. The circuit can be simplified by eliminating VR1 and a resistor, as well as the four capacitors (see Fig.1). In my case these components were necessary because I used, at first, a Push-Pull tube oscillator. Then I added the broadband transformer, useful not only to have a balanced LO signal with any oscillator (or test generator), but also to raise its level. The tests and assessment measurements brought the results that appear in Table A and as you can see they are excellent! To be noted that in the original Squires version the oscillator has a square waveform and therefore the compression point would be even better!
 

FIGURE 2


 

   Ca  500 pF/250V.       R1  470 kΩ/1/2W
   Cb  0.01 μF/250V.       R2  470 kΩ/1/2W
   C1  820 pF       T1  IF TRANSFORMER W/CENTER 
 TAP PRIMARY
   C2  820 pF       T2  UNBAL./BAL. WIDEBAND 
 TRANSFORMER
   C3  0.01 μF        V1  6DJ8/ECC88 or 6922/E88CC   
   C4  0.01 μF      VR1  100 kΩ LINEAR
 P
OTENTIOMETER
   C5  0.01 μF       Z1  2.5 mH RFC
    R  4.7 kΩ/2W    

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PARTS LIST

 

 

  TEST FREQUENCY                                   7 MHZ
  INTERMEDIATE FREQUENCY                                  500kHz
  BIAS (NEGATIVE)                                  -12Volts D.C.
  LO LEVEL                                            4 Volts RMS (each grid)

  INSERTION LOSS*                   

  4dB (0dB with 0V. Bias/
  Grounded)


  1dB COMPRESSION POINT
  (INPUT-50
Ω)                              

 > +23dBm***


  MINIMUM DISCERNIBLE SIGNAL
  M.D.S. /50 Ω **(SSB 2kHz B.W.)

 

 Better than -119dBm
  LO/RF REJECTION (Pin 2 of T1)   >25dB
  LO/IF REJECTION (Pin 5 of T1)   >75dB
  RF(500kHz)/IF REJECTION (Pin 5 of T1)   35dB (45dB with 0V. Bias/
  Grounded)
 

     * Between pins 2 and 5 of T1

   ** RF tuned circuit with x10 voltage gain factor

 
*** Measurement limited by available generator

 

 

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TABLE A

 

Subsequently I made a simple but substantial change to the circuit, the removal of the B voltage! The FET, as we know, can function as a mixer without any power supply, but here it is a thermionic tube! It is true that in the past tube diodes have been used as mixers (some of a special kind like the double diodes RD12 or LG12) but in the concerned circuit we are dealing with a double triode!
In fact we have to do with a tube that functions as a switch (more specifically as a variable resistor) and not as an amplifier. In help are the Bias and the RF local oscillator voltages. The modified version appears in Figure 3 where it can be seen that the intermediate tap of the IF transformer primary (and therefore the tube plates) is brought to ground via the antenna circuit.

 

FIGURE 3

 

By varying the Bias voltage change the performance of the mixer, this makes us understand that the tube continues to work as a triode (double). It can be groped to explain the operation of this unusual mixer, and with these particular working conditions, in the following way: the plate being connected to the ground, is at the same polarity as the cathode. The electrons, which always have a negative charge, are expelled from the cathode, which is also of negative polarity. As usual, the grid functions as a flow regulator of the electrons, and hence to their journey toward the plate (or anode), which being negatively charged are attracted by the grid because in this case has a fixed positive bias, and leaves them to pass on their way toward the anode that, however, tends to repel them being of the same negative polarity. When the grid becomes negative (negative half-wave of the oscillator voltage), it is a barrier for the electrons that cannot overcome it in order to reach the plate (which in any case would reject them, being of the same polarity). Electrons flow is forced back to the cathode. Things are reversed again when the grid is back with positive polarity. The fact is that this rhythmic change of state varies the anode current of the tube and thus also its internal resistance, therefore the ends of the primary IF transformer are alternately connected to ground by the two triodes. It is obvious that a circuit of this type can only produce an insertion loss but being a high impedance circuit the raising of the RF signal, thanks to the antenna filter step-up, amply compensates for the loss and results in an excellent sensitivity!
The sensitivity in the modified circuit is better than in the original one, probably the absence of the anode voltage, and thus a very low current in the plates, significantly decreased the noise. With a good voltage gain factor of the radiofrequency filter an RF amplifier stage is almost superfluous. Differences in performance between the two circuits, the one with B power and that without are evident by comparing the results in Table A and Table B.

 

  TEST FREQUENCY                   7 MHZ
  INTERMEDIATE FREQUENCY                   500kHz
  BIAS (POSITIVE)  +12Volts D.C.
  LO LEVEL                                            4 Volts RMS (each grid)

  INSERTION LOSS*                   

  4dB (6db with 0V. Bias/
  Grounded)

 
  1dB COMPRESSION POINT
  (INPUT-50
Ω)
                              

 

  +23dBm

 
  MINIMUM DISCERNIBLE SIGNAL
  M.D.S. /50 Ω **(SSB 2kHz B.W.)

 

  Better than -124dBm
  LO/RF REJECTION (Pin 2 of T1)  >40dB
  LO/IF REJECTION (Pin 5 of T1)  >90dB
  RF(500kHz)/IF REJECTION (Pin 5 of T1)    45dB
   

     * Between pins 2 and 5 of T1

   ** RF tuned circuit with x10 voltage gain factor

 

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TABLE B

 


Therefore we have an excellent mixer, with a dual-triode tube, that has been modified in a passive mixer with superior performance. But....if we also remove the fixed positive grid voltage, what happens? It continues to work but there is an increase in the insertion loss of 2dB which then takes from 4 to about 6 dB total. The RF local oscillator voltage is sufficient to operate the tube!
What? If we remove also the voltage to the filaments?! No, this is not possible!!
Unless we find another system to heat the filament and then emit electrons ......

 

 

 

NOTE: 

See Addendum:   http://www.radiopharos.one/addendum-mixer.htm


 

 

 

 

 

References:
 

1-  William K. Squires - W2PUL 
     Mixer circuit employing linear resistive elements  
     Patent N° 3,383,601.

2-  Ulrich L. Rohde DJ2LR 
    
a - Communications receivers for the year 2000 
     Ham Radio magazine - November 1981
     b - Performance capability of active mixers
     Ham Radio magazine - March 1982

3-  www.radiopharos.it
     Squires-Sanders, Inc. - AN/URR-58 Receiver

4-  Ray Moore - K1DBR
    
Designing communications receivers for good strong signals performance
    
Ham Radio magazine - February 1973

5-  www.radiopharos.it
     SEM - An inedited passive FET Mixer with conversion gain

6-  www.radiopharos.it
     Conversion gain in passive mixer
    

 

 

 

 

 

 

 

 

 
 

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