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How does center-tapped transformer work on the circuit given below?

In analog rectifier circuits, one of ports of the center-tapped transformer is connected to a sinusoidal source while others are driven with a DC source or ground. But in mixer circuits as shown below, the situation is a bit different.

In RF usage of the center-tapped transformers, #1 node is driven with a small signal, "RF"; #3 node is driven with a large signal, "LO"; and together they prensent in #2 and #4 ports.

center-tapped transformer

For example on a single balanced diode mixer center-tapped transformers are used as shown below. mixer

At high side of the LO, D1 conducts while at the lower side of the LO, D2 conducts.

What does transformer do to perform such a operation? How do center-tapped transformers work when there are two sinusoidal sources?

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  • \$\begingroup\$ They allow a balanced voltage on the (as drawn) secondary. In your application, the common mode voltage is VLO, the differential voltage is VRF(* turns ratio). Thus one diode sees VLO+VRF, the other sees VLO-VRF. \$\endgroup\$
    – user16324
    Jan 27, 2021 at 12:48
  • \$\begingroup\$ What do you mean by this: In analog rectifier circuits, one of ports of the center-tapped transformer is connected to a sinusoidal source while others are driven with a DC source or ground. - it doesn't tally with your diagrams. Please be clear. \$\endgroup\$
    – Andy aka
    Jan 27, 2021 at 12:56
  • \$\begingroup\$ Is it the second diode seeing VLO-VRF? Can you write an answer with a little more elaboration, so that, I can accept it as an answer and everyone else can see? Also, from your explanation, shouldn't be the LO and RF frequencies same? @BrianDrummond \$\endgroup\$ Jan 27, 2021 at 13:12

1 Answer 1

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By mixing low level currents of RF (uV) and LO levels to permit the exponential current of the diodes will create lots of distortion. It is found that the Intermediate Frequency (IF) products of LO-RF and LO+RF are their harmonics are quite useful.

If one chooses the lower difference frequency then chooses carefully designed specs for a low pass or bandpass with bandstops for the higher LO and product sum (LO+RF) , one can get a high-quality frequency mixer.

To demonstrate this I randomly chose RF= 200 MHz and LO = 300 MHz to produce an IF out = 100 MHz. For the filter I chose an unloaded Chebychev LPF with resonant notches at 200,300 MHz

Typical metrics for conversion efficiency are the ratio of amplitudes for IF/RF and LO suppression as well as dynamic range and noise threshold.

enter image description here

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  • \$\begingroup\$ This is not an optimal design but then it only took me 20 minutes using Falstad.com/afilter and then display it on the time domain site. \$\endgroup\$ Jan 27, 2021 at 14:39
  • \$\begingroup\$ It is interesting to note that a mixer is also a modulator although in that case the output is at the RF port. \$\endgroup\$ Jan 27, 2021 at 14:46
  • \$\begingroup\$ It could be done to modulate any 1 of 2 ports with an output at the 3rd port depending on levels and impedances used as the exponential conversion is reciprocal from voltage to current to voltage. \$\endgroup\$ Jan 27, 2021 at 15:11
  • \$\begingroup\$ Maybe better modulator this way tinyurl.com/y5xufzwx \$\endgroup\$ Jan 27, 2021 at 15:28

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