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I have following circuits :

Rf Amplifier :

enter image description here

Mixer :

enter image description here

The amplifier is able to give me a good gain of around 200 to 300 for signals in 100 MHz range. The mixer, which follows the amplifier, is more of a copy paste job to which I have added a couple of resistors to simultaneously amplify the output a couple of times.

The amplifier is preceded by an RF filter. The mixer is followed by an IF filter ( 10.6 to 11 MHz). So the scheme is as follows :

Rf Filter -> Rf Amplifier -> Mixer -> IF Filter

The Rf filter and Amplifier taken together work as expected. The mixer and IF Filter together work as expected. But problem occurs when I connect the Amplifier to the Mixer.

My output suddenly goes pretty weak and low frequencies begin to appear superimposed on the intended 10.7 MHz ( and nearby) frequencies.

Here is the output for 100 MHZ input signal :

enter image description here

while ideally it should have been my intermediate frequency 10.7 MHz.

Can someone tell me where am I going wrong ?

Thank you.

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  • \$\begingroup\$ Can you provide more information on your application? It sounds like you are saying you a 100MHz input signal, and a desired 10.7 MHz output signal? Are these the center frequencies of the signals and they have some bandwidth, or are these signals supposed to be tones? Assuming that these are tones, this implies that you are mixing down 100 to 10.7 which would require a LO of either 89.3 or 110.7 MHz. Is this what you have? Without doing to much analysis on your schematics, are you certain that the signal level at the output of the RF amplifier is appropriate for input to the mixer? \$\endgroup\$
    – Jotorious
    Feb 7, 2015 at 17:45
  • \$\begingroup\$ Its actually a part of superhet receiver for FM . LO currently is modelled as a signal gen so no problem there. The main input is 100 uV which is amplified to around 30 mV by amplifier, which is decent I believe. 10.7 Mhz is intermediate frequency which I will feed to demodulator. ( I have a feeling that amplifier is somehow affecting the mixer, dunno how ) \$\endgroup\$ Feb 7, 2015 at 17:47

2 Answers 2

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I've just recently started studying RF circuit design, thereby my help might be of limited use. What we can infer from your problem description is essentially that connecting the mixer alters your gain; this makes sense as the latter is given by the gm times load product of the circuit. In your case the load is modified by the introduction of the common base stage of the mixer. This part essentially behaves as current follower (linearized by R19, but let's ignore that for the analysis), thereby your new gain is to expected to be somewhere around the gm of Q1 times the load of the mixer! As for the introduction of the undesired frequencies: I have to give that some more thought, without a rigorous mathematical approach I believe pinpointing the exact reason will be difficult.

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I'm not a RF guru and would love to hear from someone more experienced that I. But at first glance, it looks as if you need a buffer between your RF amplifier and the mixer RF input. The 2nd stage of your RF amplifier is in common-base configuration - this requires a large load impedance to achieve significant voltage gain. The input impedance of your mixer stage is approximately that of the emitter resistor: about 390 Ohms.

I'm also not sure about the values of R20 & R21 in the mixer section - these seem awful large compared to the emitter resistor, at least for a 2n3904.

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  • \$\begingroup\$ They seem to work separately in this config ; I mean filter and amplifer together, and mixer and IF filter together, so I am not sure if changing resistances of mixer would do. Are you sure about adding a buffer amplifer would solve the problem ? \$\endgroup\$ Feb 7, 2015 at 18:09
  • \$\begingroup\$ Nope - not at all sure. As I mentioned, I'm not a RF guru. But I have used common-base amplifiers in other applications and I do remember that they have fairly high output impedance. \$\endgroup\$ Feb 7, 2015 at 23:05
  • \$\begingroup\$ Ok. I ll try both approaches and update accordingly, though it will take time. \$\endgroup\$ Feb 8, 2015 at 1:11
  • \$\begingroup\$ A common base amplifier has low input impedance and high output impedance, it has no current gain. There is no additional gain by cascading two of them unless you have an impedance transformer such as a transformer. To test an individual stage you need to load the output with the impedance it will see when you connect it to the following stage. \$\endgroup\$ Apr 9, 2015 at 3:26

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