Q: Many times you see designs with a 4-pole crystal IF filter followed by an IF ampfollowed by a single crystal filter.I understand the idea that the crystal after the IF amp is to filter out noiseintroduced by the amp however:
Would it be better to use a wide roofing type filter after the IF amp, or:
Split the 4-pole crystal filter into two 3-pole crystal filters, one before the IF amp andone after it, or:
Would a single tuned circuit after the IF amp be just as good as thesingle crystal filter?
What are the trade-offs between filtering before and after the IF amp?
A: You are delving into a fairly complex issue: distribution of gain andselectivity in a multistage receiver. A very good reference on this is WesHayward W7ZOI's "Introduction to Radio Frequency Design".
But the basic concepts are just that: basic.Reduce the bandwidth and control the signal amplitudes prior to the activestages which have a finite signal-handling capability.
This means that any mixer or amplifier can be driven into its non-linearoperating range by high enough amplitude signals. Thus, one must ensure thatsignal amplitudes are within the linear operating range of the first activedevice. This is done is two ways:
use filters to reduce the bandwidth of the preceding circuitry feedingthe active device to only that required for the desired signal; this keepsunwanted high-amplitude signals from affecting operation..
control signal amplitudes by AGC as needed.
Thus, we commonly see relatively wide r-f filters used ahead of the r-famplifier and/or first (or only) mixer stage to reduce the likelihood thatstrong signals can reach the mixer and over-drive it, causing it to produce spurioussignals (IMD).
In the early stages of the receiver before the conversion to IF, thesefilters are commonly called preselectors. They serve the added purpose ofminimizing the response of the receiver to image signals whichthe receiver is prone to receive as a result of the mixer operation. Example: If the signal frequency is 14 MHz, the L.O. frequency is 23 MHz and the IF is 9 MHz, the image frequency is 32 MHz.)
Assuming an LO of 23 MHz, the first-order response frequencies would be LO + IF = 32 MHz and and LO - IF = 14 MHz, the desired response. Thus, 32 MHz is the principal image frequency to be rejected, along with its accompanying noise in a bandwidth equal to the first IF filter bandwidth centered on 32 MHz. The rule is that the image is twice the IF away from the desired signal frequency. Thus the preselector filter(s) must have some minimum required stopband attenuation at 32 MHz to avoid image-response issues.
Once the signal has been converted to IF, gain must be used to make up forthe loss in the mixer. So here again are one or more stages vulnerable tobeing overdriven and creating distortion products. Therefore, their signal levelshave to be controlled closely by appropriate AGC circuitry. Usually, a filter whosepassband is somewhat wider than the widest main IF filter is placed at the IFoutput of the first (or only) mixer. This filter is often referred to as a roofing filter.
It is usual practice to set the operating bandwidth of the receiver by oneor more filters operating at the IF. Once sufficient amplification has beenobtained, the signal is applied to the main IF filter. Again, one must becareful not to drive the filter too hard, if it is a conventional crystalfilter, since these too have a signal range beyond which they do not performas desired.
The filter will introduce loss which must be again be made up for byamplifier stages and again these must have their signal amplitudescontrolled to prevent overdriving. Since the signal levels are now ratherlarge compared to those at the mixer input, amplifiers capable of handlinglarge signals must be used. Any amplifier stage will generate noise in addition to increasing signallevel. It is the usual practice to follow the IF amplifier(s) with yetanother filter to limit the signal- and the noise-bandwidth of the receiver.Note that the filter does nothing to the noise except limit its powerthrough limiting the bandwidth. It does not "filter out the noise" and leavethe signal.
This filter usually is not as complex as the main IF filter nearer the mixeroutput. That is, it may not have as small a shape factor or be as narrow inbandwidth as the main filter. On the other hand, the filter may indeed havea smaller bandwidth in order to further reduce the operating bandwidth ofthe receiver. For example, the early IF filter may be a 500 Hz filterfollowed later by a 250 Hz filter for added CW selectivity.
All these remarks concerning filters apply to conventional receivers withcrystal filters. In DSP IF receivers, such as the Icom PRO series or theORION and other recent radios, the stages preceding the analog/digitalconverter (ADC) stage are operated and controlled so as to ensure that theADC cannot be driven into saturation (maximum digital count). No particulareffort is made to secure a narrow operating bandwidth in the precedingstages since the DSP filtering will accomplish this much better than anyfeasible crystal filter(s) can.
This is a very complex topic, and I have not done much to reduce thecomplexity in trying to give you some idea of how to answer your questions.Like most questions in engineering, the answer is "it depends." Many factorsbear upon how a receiver is designed, some posed by the user requirements,some by the characteristics of the devices used and some by the laws ofNature.
Finally, for a look at a most elegant receiver design, you might examine the circuitryof the Elecraft K2. Its seemingly oversimplified circuit manages with veryfew parts and a very simple architecture to provide performance surpassingthat of much more complex and expensive radios. You will find there a fairlycomplex crystal IF filter following the post-mixer amplifier and feeding theIF amplifier which then feeds a very simple crystal filter to the finalmixer stage which produces the audio output signal.
Again, Wes Hayward's book, and his new one "Experimental Methods in Radio FrequencyDesign", are outstanding and I highly recommend their study.
Copyright © 2004, George T. Baker, W5YR
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