Bandpass filter before downconversion

Question

Hi, 
 We are in the process of designing a radar receiver with a fixed center frequency of about 10.5 GHz and and bandwidth of 50 MHz. It is a cost conscious design based on COTS components. 
 Following the LNA (e.g. ADL5723) is usually a bandpass filter (BPF) and then the mixer (HMC220B). Can we make the receiver work with a BPF after the mixer and skip the BPF before the mixer? 
 The reason is that a BPF in the 10 GHz range is expensive (100+ USD) and not easily available, where a BPF with a center frequency of 2.45 GHz or in the lower 1-2 GHz range is readily available from multiple sources at around 1 USD. 
 Is this a feasible approach? 
 Thanks. 
 Kind regards, 
 Claus

huangzeyan · Accepted Answer

Hi Claus, 
 This is part of your system design and the answer really relies on the performance you need from your application. 
 Regards.)

viktor_vojta · Answer

Hello, 
 Usually the input filter (e.g. BPF) is introduced in the system to suppress unwanted signal at image frequency. And also potential strong interferers, which fall into IF frequency due to different mixing products. 
 But you can have good BPF almost for free. Try to design it! I would recommend you edge-coupled microstrip filter. At this frequency it could be small.

viktor_vojta · Answer

Hello. 
 Usual way. Define the limits for the pass-band and attenuation in the rejection band. Based on the requirements you need to determine order of the filter. If you know the PCB stack-up, you can start the design. The theory you can find on the internet and perform the synthesis (or buy any filter design tool). Or, second posibility is to do raw estimation of dimension limits (instead of full synthesis) of u-strip lines and optimize them in any RF simulator using any build-in optimizer. 
 Or course manufacture a filter sample will be very helpful and recommend, you can verify parameters on the VNA.

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Bandpass filter before downconversion

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