A basic block diagram for FMCW radar I will be using, modified MIT radar project: MIT block diagram diagram I My transmit frequency is 2200-2600 MHz, my receive frequency is ~2200-2600. I want my IF frequency to be a constant 1 MHz (or 100 MHz; I haven't decided on bandwidth yet). The only way I can think of doing it is is introducing another VCO2 and having it produce constant 1 MHz RF signal, so it will mix with first filter, giving me LO 2201-2601 MHz. The LO is then mixed with receive frequency and I get a constant IF of 1 MHz.

Is there a better way to approach it? Introducing second VCO, second mixer and filter seems like too much.

EDIT: my specific question is about keeping IF constant, I put the whole block diagram just for illustration.

  • \$\begingroup\$ According To the Figure It looks Like You are Using A VCO To generate a 2200-2600MHZ as a Carrier Signal??? Is it A model For Gain/Directivity Measurement Model?? Because You have Directly Fed the S/G from the Splitter To Receiver. Do You have A Information Source as a Triangular Wave?? That You Mean To Receive At the Receiving End,You have Also used a Splitter, if So then Why You Use A ADC at Receiving end. As Your Motive is Quite Not Defined in Question, I Suggest That you use a Superheterodyne Reciever You Will Need A Additional Oscillator. But It Quite Gives A Constant IF. \$\endgroup\$
    – MaMba
    Mar 6, 2015 at 19:03
  • \$\begingroup\$ Is the TX waveform going to be a short pulse of constant frequency? Or are you planning to do a frequency chirp? And what is the radar for? \$\endgroup\$
    – user57037
    Mar 6, 2015 at 19:14
  • \$\begingroup\$ Yes, I'm using VCO to generat 2200-2600MHz signal. This model if FMCW radar. Triangular wave is used to oscillate VCO. I needs ADC at receiving end to process the signal. \$\endgroup\$ Mar 6, 2015 at 19:16
  • \$\begingroup\$ It's FMCW radar which will be used to determine speed and distance \$\endgroup\$ Mar 6, 2015 at 19:18
  • \$\begingroup\$ FMCW is used to detect Surrounding Objects (Rest/Moving) uses dropler effect,as the Object Position is Varied The If will Vary Why Do You Want A Contant If ..A Superheterodyne Reciever Might Help, \$\endgroup\$
    – MaMba
    Mar 6, 2015 at 19:21

1 Answer 1


To answer your question: No, you can't generate your local oscillator that way.

The output of the first mixer will have both sum and difference frequencies in it. In other words, in addition to a signal that runs from 2201 to 2601 MHz, you'll also have a signal that runs from 2199 to 2599 MHz, and there's no way to eliminate just one of those signals.

It isn't clear to me why you want an additional frequency offset anyway. The usual idea with this kind of radar is that the reflected signal already has a frequency offset from the current transmitter frequency that's based on the round-trip delay between the radar and the reflecting object. The beat frequency gives you a direct measurement of the distance to the object. And if it has a radial velocity, by observing how the beat frequency shifts when the transmit frequency switches between rising and falling, you can measure that as well.

  • \$\begingroup\$ Agree. I have experience with FMCW (FM homodyne). It is called homodyne because you mix RX directly with TX. (Of course TX must be attenuated first). The system I am familiar with is an imaging radar with high sensitivity (for low RCS targets). Depending on the rate of change of frequency and the range to target, the signal that reaches the ADC may be in the audio frequency range. Determining distance is pretty easy. Determining downrange or uprange component of speed is not too difficult. But cross range speed cannot be determined from a stationary radar. \$\endgroup\$
    – user57037
    Mar 6, 2015 at 22:01
  • \$\begingroup\$ True, but the problem with is that the shift in frequency will be in order of 1 mHz, almost DC. At that frequency, 1/f noise will dominate my signal, as well as DC offset and other factors. I will place a bandpass filter between first and second mixers to filter the image. \$\endgroup\$ Mar 6, 2015 at 23:01
  • \$\begingroup\$ It will require an exceedingly narrow bandpass. Impractical. Why don't you ramp your VCO faster to achieve a higher signal frequency? \$\endgroup\$
    – user57037
    Mar 6, 2015 at 23:55
  • \$\begingroup\$ If you use homodyne detection, the frequency at the ADC from a point scatterer will be df/dt * 2r/(C), where df/dt is the ramp rate of the chirp (Hz/s), C is the speed of light, and r is the range to target. So if df/dt is larger, the frequncy at your ADC will be higher. \$\endgroup\$
    – user57037
    Mar 7, 2015 at 1:21

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