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LTC5594 as Image reject mixer and disabling/bypass the IF amplifier

We need a single-ended, image rejected output from this mixer, at 140 MHz (Low IF, no DC). The IF amplifier seem to use a lot of power for very low gain. So we can disable the IF amplifier (AMPD pulled high with 1K res) and use the "Mixer Differential Output Pins" (MQP, MQM, MIM, MIP) directly, but there doesn't seem to be too much information on this in the datasheet. We would really appreciate a simple schematic showing this.

1. According to a reply in another thread, there is a mention of 100 ohm pull-up to VCC (5V) of the output pins. Is this internal to device or should we add these resistors on all the output pins?

2. Converting MQP and MQM to a single-ended 50 ohm. Can we just terminate MQM using 100 ohms and use the other pin? Does not using a 180 degree combiner mean 3 dB more noise figure? If so, please recommend a SMD transformer for this. Something like TCM2-1T+?

3. We intend to use ADQ-180+ 90° hybrid to combine the single-ended MQ and MI channels, and then low pass filter the output. Would low pass filtering on both channels, before the combination, help in any way?

4. In the noise figure and the gain graph shown for mixer-only, is the gain for a single channel or the combiner output (which I am assuming adds 3 dB gain)?

5. And what would be the OIP3 of this configuration? Looking for a rough estimate.

In essence:

MQP/MQM -> Pull-up both to VCC? -> DC Block -> Diff-to-single-ended (terminate or transformer) -> Low-pass filter? -> Combine with MIM/MIP channel using ADQ-180+

Thank you

Added TCM2-1T+
[edited by: mallets at 9:28 AM (GMT -4) on 6 Sep 2021]
  • Hello,

    1. To use LTC5594 as an image reject mixer, the below schematic can be used :

    To use it as a differential mixer, MIF to be combined with MQF and MIP to be combined with MIM using 90 degree hybrids. 

    2. To convert the differential to single ended, a transformer or an op-amp can be used.

    3. Yes

    4. All gains are measured to single channel: RF to I or RF to Q

    5. The OIP3 will be as mentioned in the datasheet. These modifications in the circuit won't affect the linearity of the circuit. 

  • Hello, I'm interested in using the mixer section of the LTC5594 only and from there going to the ADRF6520.  Can you recommend a circuit for DC coupling from the LTC5594 Mixer outputs to the ADRF6520 inputs?

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