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I'm designing a simple diode ring mixer for down-conversion from 1.5G to 200k. I want a band-pass LC pi-filter on the mixer IF output.

I've designed the filter for 50R source and load impedance by using a low-pass butterworth prototype and used the usual equations to scale and transform into a band-pass at my center frequency of 200k.

enter image description here

My LTSpice simulation shows the input impedance is only 50R at the center frequency, dropping to 0R in the stop band.

LTSpiceSimPlot

Should I be concerned that the filter wont absorb mixer spurs and harmonics in the stop band and will instead reflect them back into the mixer?

What is best practice for terminating an IF output with a narrow bandwidth?

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  • \$\begingroup\$ Probably worth adding the filter schematic to the question. It's certainly possible to design constant-impedance L-C filters, though they may have slightly more passband loss, which brings you back to the age-old compromise between sensitivity and strong signal overload performance. \$\endgroup\$
    – user16324
    Commented Mar 17, 2017 at 12:18
  • \$\begingroup\$ As a side issue, assuming the bandwidth of your 200 kHz filter is 10 kHz, then the stability of your 1.5 GHz signal needs to be on the order of 1 ppm or better. \$\endgroup\$
    – Barry
    Commented Mar 17, 2017 at 13:03
  • \$\begingroup\$ I recall Rabbit RF links, to transmitter 900MHz video around the house. The transmitter used 3dB pads before and after the up-conversion mixer. Before any transmit filters. \$\endgroup\$ Commented Mar 17, 2017 at 14:28

3 Answers 3

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Here is one way to do it:

Terminate the mixer into a common-base (or common-gate) amplifier, using a resistor in series with the input to increase the input impedance to 50 Ohms. The output of this amplifier can drive your filter. Increase the bias current if you need less distortion.

Instead of terminating L1 and C1 to ground, connect them to the positive supply voltage. Add a 50 Ohm resistor in parallel with L1 and C1 to provide a 50 Ohm output impedance. The current to feed the collector of the common-base amplifier will go through the inductor. If you increase the filter input impedance (and the collector resistor) to a value above 50 Ohms, there will be gain. The value of this gain can be adjusted to maximize dynamic range, improve sensitivity, or reduce distortion. The advantage of having L1 and C1 in parallel with the resistor is that the resistor value can be made larger without reducing the collector voltage.

Usually, the worst problem with an amplifier in this position is that the second-harmonic distortion of the amplifier will cause a signal at 100k to be converted to 200k, which is in your passband. A 100kHz notch filter on the input side of the amplifier will help get rid of this.

To stabilize the amplifier, put a ferrite bead or 10 Ohm resistor between the base terminal and ground.

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This is unfortunately an unavoidable problem. The best course of action is to build a mixer with decent port to port isolation and terminations. Have something like a 3dB pad at all outputs (between the mixer and filter etc) to offer some attenuation to the reflections. This way repeated reflections will be avoided as the signal incurs additional attentuation with each round trip.

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I think what I'm going to do is pass the IF output through a low-pass zobel bridged tee and terminate in 50R at an opamp input. The low-pass zobel will block RF from reaching the opamp while presenting a nice wideband 50R impedance to the mixer. Then use a MFB filter to bandpass at 200k.

enter image description here

My LO and RF inputs are already driven by well matched RF amplifier outputs, so I'm hoping no 3dB pads will be required here.

Thanks for the comments.

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