Mixer design choosing proper frequency range for circuit

[Hetfield]

Hey i came across an example on the radio communication handbook by mike dennison as it seen below, and i wonder that how do we decide the capacitor and inductor values on the collector side and in what frequency range this circuit works?
thanks in advance

 

[betwixt]

The LC circuit should resonate at the frequency difference or the frequency sum of the two inputs. The choice of value for C or L is up to you, within reason you can adapt the value of one to suit the other. f = 1 / (2 * pi * sqrt( L * C )). I can't give you a starting value without knowing what your frequencies are.

A simple mixer like that isn't good at rejecting the input frequencies at the output so it would be wise to keep them far enough apart that the LC circuit has a good chance of rejecting them.

Should be good to about 200MHz but performance will tail off as the frequency gets higher.

I think I have a copy of the book somewhere, I wrote part of the VHF/UHF manual !

Brian.

 

[BigBoss]

This one cannot work beyond 50-100 MHz.( may be less )

 

[Hetfield]

The LC circuit should resonate at the frequency difference or the frequency sum of the two inputs. The choice of value for C or L is up to you, within reason you can adapt the value of one to suit the other. f = 1 / (2 * pi * sqrt( L * C )). I can't give you a starting value without knowing what your frequencies are.

A simple mixer like that isn't good at rejecting the input frequencies at the output so it would be wise to keep them far enough apart that the LC circuit has a good chance of rejecting them.

Should be good to about 200MHz but performance will tail off as the frequency gets higher.

I think I have a copy of the book somewhere, I wrote part of the VHF/UHF manual !

Brian.

wow its very interesting to get replied by someone who is one of the co-writers of the book. thanks for your reply again.
i design the circuit on the AWR by setting the resonant freq as 28 Mhz but i cannot any result at the AWR. it says there are no items to show even though i set the spectrum on the graph section, do you know why?

 

[vfone]

This is the IF output (collector) spectrum, when injecting in the transistor base -5dBm (48MHz) and +7dBm in the emitter (20MHz).
Depending by the transistor used, the LC circuit in the collector may need some tuning. I used 330nH inductor and 83pF capacitor.

 

[betwixt]

You can see from Vfone's picture that the two input frequencies are not rejected by that circuit. Only the tuned circuit in the collector can select the wanted (28MHz or 68MHz) product. All the other peaks are unwanted products or the fundamental input signals themselves.

A balanced mixer will work far better, it can reject both inputs, giving you a much cleaner output spectrum before applying the filtering. Note that the other outputs in the spectrum are twice the fundamentals added to the equation.

Brian.

 

[Hetfield]

You can see from Vfone's picture that the two input frequencies are not rejected by that circuit. Only the tuned circuit in the collector can select the wanted (28MHz or 68MHz) product. All the other peaks are unwanted products or the fundamental input signals themselves.

A balanced mixer will work far better, it can reject both inputs, giving you a much cleaner output spectrum before applying the filtering. Note that the other outputs in the spectrum are twice the fundamentals added to the equation.

Brian.

thanks for your generous replies also i want to ask a question about a mixer designing after i research many mixer topologies i decided to design a mixer by using double balanced gilbert cell which have 5 Ghz LO and and a 4.9 Ghz RF signals. which parameters do you think should i consider i mean i want to use BJT, should i consider the bandwidths of the BJTs that i will use and i want to know what do you think the current on the cell should be for better linearity.