Why high Q image reject filters are lossy in heterodyne receiver?

Why is high Q image reject filters in heterodyne receiver are lossy compared to low Q channel select filters?. Can anyone explain in detail. Thanks in advance

Rgds

loaded q and unloaded q relation

Hi,

It may depend on how far the image frequency is from the needed frequency: the closer the image to the receive frequency the more demanding it becomes to meet image rejection vs receive pass band frequency, isn't it?

Please be more specific on the receive channel frequency and bandwidth AND the IF frequency with a practical example, where you do experience or "cannot" avoid lossy high Q image reject filters, ok?

unkarc

Re: Q Factor

Q is a parameter of a single tuned circuit. In modern receivers the filters are multiple poles to get a flat gain in the passband and high rejection in the stop bands.

The circulating currents in a single tuned circuit are proportional to Q and the loss in the tuned circuit is proportional to the circulating current squared and the equivalent resistance of the coil and capacitor.

Re: Q Factor

Hai

Unkarc

The question arises as a result of a journal review paper, where the author had specified in order to supress the image signal in heterodyne receiver the bandwidth of the desired channel and the image signal should be far apart resulting in high frequency spectrum which in turn requires a high Q lossy filter.

flatulent

Thanks for your reply, now I could observe the relationship between the Q factor of a filter and the losses.

Rgds

Re: Q Factor

Hi, all. Talking about Filter Q, I think Randy Rhea from Eagleware had a very clear explanation. He give the expression of I.L for Cheby BPF, which is independent on topology. It is related to Qloaded, Qunloaded, and gs constant (from conventional filter cookbook).

His forum is also very informative, especially on filter design(printed, ceramic resonator type etc). I just love it.

www.eagleware.com






aaqiao



Joined: 22 Oct 2003
Posts: 3

Posted: Mon Dec 01, 2003 8:47 am Post subject: Insertion loss of the narrow band hairpin bandpass filter

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I am a graduate from China, and I am trying to design a hairpin bandpass filter with the following parameters:
2856MHz center frequency
60MHz bandwidth
30 dB stopband attenuation and no special limits for the transition bands

I designed it with M/FILTER and finally got a 7 degree hairpin microstrip filter. Its stopband attenuation seemed pretty good but its insertion loss of the passband was very bad, about 7db. I know that the insertion loss of the passband is inversely proportional to the unloaded Q of the resonator, but I have no ideas what to do next. Shall I select other kinds of transmission lines, such as coaxial line or waveguide? Waveguide bandpass filter seems attractive but I don't know how to design it. And can GENESYS do the work? I am eager for some suggestions.
I will be very thankful to your kindness.

Geng Zheqiao

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Randy Rhea
Eagleware Staff


Joined: 12 Jan 2003
Posts: 91

Posted: Mon Dec 01, 2003 1:42 pm Post subject:

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Yes, you are correct, insertion loss is inversely proportional to resonator unloaded Q. More specifically, the IL at the passband center is given approximately by

IL = 4.34*Qloaded/Qunloaded*(summation of the G values)

Qloaded is the filter center frequency divided by the filter bandwidth, in your case, 2856/60 = 47.6. This is a high loaded Q for lumped element and printed filters. Notice that insertion loss is directly proportional to loaded Q.

4.34 is one half the neper constant. The (sumation of the G values) is the sum of the reactive G values for the lowpass prototype used to design the filter. These do not include the termination G values G(0) and G(N+1). The G values are given in the G Values tab of the M/FILTER property box. The summation of the G values is given in the Summary tab once you have entered the passband ripple and the filter order.

The best way to reduce loss is to use resonators with higher unloaded Q. In microstrip, this means as thick a substarte as possible, because unloaded Q in both lumped and distributed resonators increases with increasing physical volume. If loss is a primary concern, I would use a substrate thickness up to 0.125" or 3.2 mm at 2.8 GHz. At higher frequencies thinner substrates must be used or radiation and additional operating modes become a problem. With thick substrates, evanescent modes exist which are not simulated by circuit theory simulators. Therefore, for your application I highly recommend using the EMPOWER electromagnetic simulator to improve accuarcy. I also recommend considering an interdigital filter with the thick substrate. It is degraded less by evanescent modes.

Thick substrates are expensive. That is why machined filters are often used for applications where the ground plane spacing must be large to reduce loss. An example filter type is slabline which is round rods between two flat ground planes with air as the dielectric. These are often done in interdigital or combline. There is an example slabline combline with photographs in my book HF Filter Design and Computer Simulation.

You might find that it helps a little to use a wider bandwidth and then increase the order to achieve the stopbands. It all depends on whether the increased summation of the G values or the wider bandwidth has the most influence. You can use your GENESYS simulator to quickly compare the loss of different bandwidths. This is a little easier in the latest versions of the software which allow you to interactively use synthesis and simulation.

Waveguide filters would have low loss but they would be extremely large. GENESYS will simulate coaxial processes, but M/FILTER synthesizes only coaxial in the stepped impedance bandpass which is probably not suitable for your application. M/FILTER handles slabline very well, but 2% bandwidth with large ground plane spacing will suffer greatly from evanescent modes.

My final recommendation: thick microstrip or suspended microstrip as an interdigital with M/FILTER to start the design and EMPOWER to refine the design.

Clear skies and high Q